Chap 6

Vectors• 2-D Force & Motion Problems

• Trig Applications

• Relative Velocities

• Free Body Diagrams

• Vector Operations

• Components

• Inclined Planes

• Equilibrium

Vector Addition

• Tip to tail method

• Parallelogram method

8 N4 N

3 N

Suppose 3 forces act on an object at the same time. Fnet is not 15 N because these forces aren’t working together. But they’re not completely opposing each either. So how do find Fnet ? The answer is to add the vectors ... not their magnitudes, but the vectors themselves. There are two basic ways to add vectors w/ pictures:

Tip to Tail Methodin-line examples

Place the tail of one vector at the tip of the other. The vector sum (also called the resultant) is shown in red. It starts where the black vector began and goes to the tip of the blue one. In these cases, the vector sum represents the net force. You can only add or subtract magnitudes when the vectors are in-line!

16 N

20 N

4 N

20 N16 N

12 N

9 N

9 N

12 N

21 N

Tip to Tail – 2 Vectors

5 m

2 m

To add the red and blue displacement vectors first note:

• Vectors can only be added if they are of the same quantity—in this case, displacement.

• The magnitude of the resultant must be less than 7 m (5 + 2 = 7) and greater than 3 m (5 - 2 = 3).

5 m

2 m

blue + red

Interpretation: Walking 5 m in the direction of the blue vector and then 2 m in the direction of the red one is equivalent to walking in the direction of the black vector. The distance walked this way is the black vector’s magnitude.

Place the vectors tip to tail and draw a vector from the tail of the first to the tip of the second.

Commutative Property

blue + redred + blue

As with scalars quantities and ordinary numbers, the order of addition is irrelevant with vectors. Note that the resultant (black vector) is the same magnitude and direction in each case.

(We’ll learn how to find the resultant’s magnitude soon.)

Tip to Tail – 3 Vectors

We can add 3 or more vectors by placing them tip to tail in any order, so long as they are of the same type (force, velocity, displacement, etc.).

blue + green + red

Parallelogram Method

This time we’ll add red & blue by placing the tails together and drawing a parallelogram with dotted lines. The resultant’s tail is at the same point as the other tails. It’s tip is at the intersection of the dotted lines.

Note: Opposite sides of a parallelogram are congruent.

Comparison of Methods

red + blue

Tip to tail method

Parallelogram method

The resultant has the same magnitude and direction regardless of the method used.

Opposite of a Vector

v

- v

If v is 17 m/s up and to the right, then -v is 17 m/s down and to the left. The directions are opposite; the magnitudes are the same.

Scalar Multiplication

x

-2x

3x

Scalar multiplication means multiplying a vector by a real number, such as 8.6. The result is a parallel vector of a different length. If the scalar is positive, the direction doesn’t change. If it’s negative, the direction is exactly opposite.

Blue is 3 times longer than red in the same direction. Black is half as long as red. Green is twice as long as red in the opposite direction.½ x

Vector Subtraction

red - blue

blue - red

Put vector tails together and complete the triangle, pointing to the vector that “comes first in the subtraction.”

Why it works: In the first diagram, blue and black are tip to tail, so

blue + black = red

red – blue = black.

Note that red - blue is the opposite of blue - red.

Other Operations• Vectors are not multiplied, at least not the

way numbers are, but there are two types of vector products that will be explained later.– Cross product– Dot product– These products are different than scalar mult.

• There is no such thing as division of vectors– Vectors can be divided by scalars.– Dividing by a scalar is the same as multiplying

by its reciprocal.

Comparison of Vectors

15 N

43 m

0.056 km

27 m/s

Which vector is bigger?

The question of size here doesn’t make sense. It’s like asking, “What’s bigger, an hour or a gallon?” You can only compare vectors if they are of the same quantity. Here, red’s magnitude is greater than blue’s, since 0.056 km = 56 m > 43 m, so red must be drawn longer than blue, but these are the only two we can compare.

Vector Components

150 N

Horizontal component

Ver

tical

co

mpo

nent

A 150 N force is exerted up and to the right. This force can be thought of as two separate forces working together, one to the right, and the other up. These components are perpendicular to each other. Note that the vector sum of the components is the original vector (green + red = black). The components can also be drawn like this:

Finding Components with Trig

v

v cos

v si

n

Multiply the magnitude of the original vector by the sine & cosine of the angle made with the given. The units of the components are the same as the units for the original vector.

Here’s the correspondence:

cosine adjacent side

sine opposite side

Note that 30.814 + 14.369 > 34. Adding up vector components gives the original vector (green + red = black), but adding up the magnitudes of the components is meaningless.

Component Example

34 m/s

30.814 m/s 2514.369 m/s

A helicopter is flying at 34 m/s at 25 S of W (south of west). The magnitude of the horizontal component is 34 cos 25 30.814 m/s. This is how fast the copter is traveling to the west. The magnitude of the vertical component is 34 sin 25 14.369 m/s. This is how fast it’s moving to the south.

Pythagorean Theorem

34 m/s

30.814 m/s 2514.369 m/s

Since components always form a right triangle, the Pythagorean theorem holds: (14.369)2 + (30.814)2 = (34)2.

Note that a component can be as long, but no longer than, the vector itself. This is because the sides of a right triangle can’t be longer than the hypotenuse.

Other component pairs

There are an infinite number of component pairs into which a vector can be split. Note that green + red = black in all 3 diagrams, and that green and red are always perpendicular. The angle is different in each diagram, as well as the lengths of the components, but the Pythagorean theorem holds for each. The pair of components used depends on the geometry of the problem.

v

v cos

v si

n

vv

v cos v

sin

v cos

v sin

Component FormInstead of a magnitude and an angle, vectors are often specified by listing their horizontal and vertical components. For example, consider this acceleration vector:

53.13

a = 10 m/s 2

3 m/s2

4 m

/s2

a = 10 m/s2 at 53.13 N of WIn component form:

a = -3, 4 m/s2

Some books use parentheses rather than angle brackets. The vector F = 2, -1, 3 N indicates a force that is a combination of 2 N to the east, 1 N south, and 3 N up. Its magnitude is found w/ the Pythag. theorem:F = [22 + (-1)2 + 32]1/2 = 3.742 N

Finding the direction of a vectorx = 5, -2 meters is clearly a position to the southeast of a given reference point. If the reference pt. is the origin, then x is in the 4th quadrant. The tangent of the angle relative to the east is given by:

5 m

2 m

tan = 2 m / 5 m = tan -1(0.4) = 21.801

The magnitude of x is (25 + 4)1/2 = 5.385 m. Thus, 5, -2 meters is equivalent to 5.385 m at 21.801 S of E.

Adding vectors in component form

If F1 = 3, 7 N and F2 = 2, -4 N, then the

F1

F2

net force is simply given by:

Fnet = 5, 3 N. Just add the

horizontal and vertical components separately.

Fnet

F1F2

3 N

7 N

2 N

4 N

Inclined Plane

m

mg

perpendicular component

parallel component

A crate of chop suey of mass m is setting on a ramp with angle of inclination . The weight vector is straight down. The parallel component (blue) acts parallel to the ramp and is the component of the weight pulling the crate down the ramp. The perpendicular component (red) acts perpendicular to the ramp and is the component of the weight that tries to crush the ramp.

Note: red + blue = black

continued on next slide

Inclined Plane (continued)

m

mgmg cos

mg sin

The diagram contains two right triangles. is the angle between black and blue. + = 90 since they are both angles of the right triangle on the right. Since blue and red are perpendicular, the angle between red and black must also be

. Imagine the parallel component sliding down (dotted blue) to form a right triangle. Being opposite , we use sine. Red is adjacent to , so we use cosine.

continued on next slidemg sin

Inclined Plane (continued)

m

mgmg cos

mg sin

The diagram does not represent 3 different forces are acting on the chop suey at the same time. All 3 acting together at one time would double the weight, since the components add up to another weight vector. Either work with mg alone or work with both components together.

How the incline affects the components

m

mg

mg cos

mg sin

The steeper the incline, the greater is, and the greater sin is. Thus, a steep incline means a large parallel component and a small horizontal one. Conversely, a gradual incline means a large horizontal component and a small vertical one.

m

mg

mg

cos

mg sin

Extreme cases: When = 0, the ramp is flat; red = mg; blue = 0.When = 90, the ramp is vertical; red = 0; blue = mg.

Inclined Plane - Pythagorean Theorem

m

mg

mg cos

mg sin

(mg sin )2 + (mg cos )2 = (mg)2 (sin2 + cos2 ) = (mg)2 (1) = (mg)2

Let’s show that the Pythagorean theorem holds for components on the inclined plane:

Inclined Plane: Normal Force

m

mgmg cos

mg sin

N = mg cos

Recall normal force is perpen-dicular to the contact surface. As long as the ramp itself isn’t accelerating and no other forces are lifting the box off the ramp or pushing it into the ramp, N matches the perpendicular component of the weight. This must be the case, otherwise the box would be accelerating in the direction of red or green. N > mg cos would mean the box is jumping off the ramp. N < mg cos would mean that the ramp is being crushed.

Net Force on a Frictionless Inclined Plane

m

mgmg cos

mg sin

N = mg cos

With no friction, Fnet = mg + N = mg cos + mg sin + N = mg sin.

(mg cos + N = 0 since their magnitudes are equal but they’re in directions opposite. That is, the perpendicular component of the weight and the normal cancel out.)

Therefore, the net force is the parallel force in this case.

Acceleration on a Frictionless Ramp

m

mg

mg cos

mg sin

Here Fnet = mg sin = m a. So, a = g sin. Since sin has

no units, a has the same units as g, as they should. Both the net force and the acceleration are down the ramp.

Incline with friction at equilibrium

m

mg

mg cos

mg sin

fs = mg sin

N = mg cos

At equilibrium Fnet = 0, so all forces must cancel out. Here, the normal force cancels the perpendicular component of the weight, and the static frictional force cancels the parallel component of the weight.


Incline with friction at equilibrium (cont.)

m

mg

mg cos

mg sin

fs = mg sin

N = mg cos

fs s N = s mg cos. Also,fs = mg sin (only because we have equilibrium). So, mg sin s mg cos .Since the mg’s cancel and tan = sin / cos, we have s tan.


Incline with friction at equilibrium (cont.)

mg

mg cos

mg sin

fs = mg sin

N = mg cosSuppose we slowly crank up the angle, gradually making the ramp steeper and steeper, until the box is just about to budge. At this angle,

fs = fs, max = s N = s mg cos .

So now we have

mg sin = s mg cos,

and s = tan.

An adjustable ramp is a convenient way to find the coefficient of static friction between two materials.

(Neither of these quantities have units.)

Acceleration on a ramp with friction

mg

mg cos

mg sin

fk = k mg cos

N = mg cos

In order for the box to budge, mg sin must be greater than fs, max which means tan must be greater than s. If this is the case, forget about s and use k.

fk = kN = k mg cos.

Fnet = mg sin - fk = ma.

So, mg sin - k mg cos = ma.

The m’s cancel, which means a is independent of the size of the box. Solving for a we get:

a = g sin - k g cos. Once

again, the units work out right.

Parallel applied force on ramp

mg mg cos

mg sin

fk

N

FA

In this case FA and mg sin are

working together against friction. Assuming FA + mg sin > fs, max the box budges and the 2nd Law tells us FA + mg sin - fk = ma.Mass does not cancel out this time.

If FA were directed up the ramp, we’d have acceleration up or down the ramp depending on the size of FA compared to mg

sin. If FA were bigger, friction

acts down the ramp and a is up the ramp.

Non-parallel applied force on ramp

mgmg cos

mg sin

fk

N

Suppose the applied force acts on the box, at an angle above the horizontal, rather than parallel to the ramp. We must resolve FA into parallel and perpendicular components (orange and gray) using the angle + .

FA serves to increase acceleration directly and indirectly: directly by orange pulling the box down the ramp, and indirectly by gray lightening the contact force with the ramp (thereby reducing friction).

FA

FA cos( + )

FA sin( + )


Non-parallel applied force on ramp (cont.)

mgm

g cos

mg sin

fk

N

FA

FA cos( + )

FA sin ( + )

Because of the perp. comp. of FA, N < mg cos. Assuming FA sin( + ) is not big enough to lift the box off the ramp, there is no acceleration in the perpendicular direction. So, FA sin( + ) + N = mg cos. Remember, N is what a scale would read if placed under the box, and a scale reads less if a force lifts up on the box. So, N = mg cos - FA sin( + ), which means fk = k N = k [mg cos - FA sin( + )].continued on next slide

Non-parallel applied force on ramp (cont.)

mg mg cos

mg sin

fk

N

FA

FA cos( + )

FA sin( + )

Assuming the combined force of orange and blue is enough to budge the box, we have

Fnet = orange + blue - brown = ma.

Substituting, we have

FA cos( + ) + mg sin

- k [mg cos - FA sin( + )] = ma.

Support Beam

mg

1

T1

2

T2


Hanging Sign Problem

mg

1

T1

2

T2Since the sign is not accelerating in any direction, it’s in equilibrium. Since it’s not moving either, we call it Static Equilibrium.

Thus, red + green + black = 0.


Hanging sign f.b.d.Free Body Diagram

mg

T1

T2

Vector Equation:

T1 + T2 + mg = 0


As long as Fnet = 0, this is true no matter many forces

are involved.

Hanging sign force triangleFnet = 0 means a closed vector polygon !

T2

mg

T1

1 2

T1 cos1 T2 cos2

T1 sin1

T2 sin2

T1 cos1 = T2 cos2

Horizontal:

Vertical:T1 sin1 + T2 sin2 = mg

We use Newton’s 2nd Law twice, once in each dimension:

Hanging sign equationsComponents & Scalar Equations

Support Beam

35

T1

62

T2

75 kg

Answers:

Accurately draw all vectors and find T1 & T2.

T1 = 347.65 N

T2 = 606.60 N

Hanging sign sample

Vector Force Lab SimulationGo to the link below. This is not exactly the same as the hanging sign problem, but it is static equilibrium with three forces. Equilibrium link

1. Change the strengths of the three forces (left, right, and below) to any values you choose. (The program won’t allow a change that is physically impossible.)

2. Record the angles that are displayed below the forces. They are measured from the vertical.

3. Using the angles given and the blue and red tensions, do the math to prove that the computer program really is displaying a system in equilibrium.

4. Now click on the Parallelogram of Forces box and write a clear explanation of what is being displayed and why.

3 - Way Tug-o-WarBugs Bunny, Yosemite Sam, and the Tweety Bird are fighting over a giant 450 g Acme super ball. If their forces remain constant, how far, and in what direction, will the ball move in 3 s, assuming the super ball is initially at rest ?

Bugs: 95 N

Tweety: 64 N

Sam: 111 N

To answer this question, we must find a, so we can do kinematics. But in

order to find a, we must first find Fnet.

38° 43°


3 - Way Tug-o-War (continued)

Sam: 111 N

Bugs: 95 N

Tweety: 64 N

38° 43°

87.4692 N

68.3384 N

46.8066 N

43.6479 N

First, all vectors are split into horiz. & vert. comps. Sam’s are purple, Tweety’s orange. Bugs is already done since he’s purely vertical. The vector sum of all components is the same as the sum of the original three vectors. Avoid much rounding until the end. continued on next slide

95 N

87.4692 N

68.3384 N

46.8066 N

43.6479 N



16.9863 N

40.6626 N

Next we combine all parallel vectors by adding or subtracting: 68.3384 + 43.6479 - 95 = 16.9863, and

87.4692 - 46.8066 = 40.6626. A new picture shows the net vertical and horizontal forces on the super ball. Interpretation: Sam & Tweety together slightly overpower Bugs vertically by about 17 N. But Sam & Tweety oppose each other horizontally, where Sam overpowers Tweety by about 41 N.


16.9863 N

40.6626 N

Fnet = 44.0679 N

Find Fnet using the Pythagorean theorem. Find

using trig: tan = 16.9863 N / 40.6626 N. The

newtons cancel out, so = tan-1(16.9863 / 40.6626) = 22.6689. (tan-1 is the same as arctan.) Therefore, the superball experiences a net force of about 44 N in the direction of about 23 north of west. This is the combined effect of all three cartoon characters.


3 - Way Tug-o-War (final)

a = Fnet / m = 44.0679 N / 0.45 kg = 97.9287 m/s2. Note the conversion from grams to kilograms, which is necessary since 1 m/s2 = 1 N / kg. As always, a is in the same

direction as Fnet.. a is constant for the full 3 s, since the forces are constant.

22.6689

97.9287 m/s2

Now it’s kinematics time: Using the fact

x = v0 t + 0.5 a t 2

= 0 + 0.5 (97.9287)(3)2

= 440.6792 m 441 m,rounding at the end.

So the super ball will move about 441 m at about 23 N of W. To find out how far north or west, use trig and find the components of the displacement vector.

3 - Way Tug-o-War Practice Problem

The 3 Stooges are fighting over a 10 000 g (10 thousand gram) Snickers Bar. The fight lasts 9.6 s, and their forces are constant. The floor on which they’re standing has a huge coordinate system painted on it, and the candy bar is at the origin. What are its final coordinates?

78

Curly: 1000 N

Moe: 500 N

93Larry: 150 N

Hint: Find this angle first.

Answer:

( -203.66 , 2246.22 ) in meters

How to budge a stubborn mule

Big Force

Little Force

It would be pretty tough to budge this mule by pulling directly on his collar. But it would be relatively easy to budge him using this set-up. (explanation on next slide)

How to budge a stubborn mule (cont.)

overhead viewtree mule

little force

Just before the mule budges, we have static equilibrium. This means the tension forces in the rope segments must cancel out the little applied force. But because of the small angle, the tension is huge, enough to budge the mule!

tree mule

little force

T T

(more explanation on next slide)

How to budge a stubborn mule (final)

tree mule

little force

T T

Because is so small, the tensions must be large to have vertical components (orange) big enough to team up and cancel the little force. Since the tension is the same throughout the rope, the big tension forces shown acting at the middle are the same as the forces acting on the tree and mule. So the mule is pulled in the direction of the rope with a force equal to the tension. This set-up magnifies your force greatly.

Relative Velocities in 1 DSchmedrick and his dog, Rover, are goofing around on a train. Schmed can throw a fast ball at 23 m/s. Rover can run at 9 m/s. The train goes 15 m/s.


Question 1: If Rover is sitting beside the tracks with a radar gun as the train goes by, and Schmedrick is on the train throwing a fastball in the direction of the train, how fast does Rover clock the ball?

vBT = velocity of the ball with respect to the train = 23 m/s

vTG = velocity of the train with respect to the ground = 15 m/s

vBG = velocity of the ball with respect to ground = 38 m/s

This is a simple example, but in general, to get the answer we

add vectors: vBG = vBT + vTG (In this case we can simply add magnitudes since the vectors are parallel.)

Relative Velocities in 1 D (cont.)

• Velocities are not absolute; they depend on the motion of the person who is doing the measuring.

• Write a vector sum so that the inner subscripts match.

• The outer subscripts give the subscripts for the resultant.

• This trick works even when vectors don’t line up.

• Vector diagrams help (especially when we move to 2-D).

vBG = vBT + vTG

vBT = 23 m/s vTG = 15 m/s

vBG = 38 m/scontinued on next slide

Question 2: Let’s choose the positive direction to be to the right. If Schmedrick is standing still on the ground and Rover is running to the right, then the velocity of Rover with respect

to Schmedrick = vRS = +9 m/s.

From Rover’s perspective, though, he is the one who is still and Schmedrick (and the rest of the landscape) is moving to the left at 9 m/s. This means the velocity of Schmedrick with

respect to Rover = vSR = -9 m/s.

Therefore, vRS = - vSR

The moral of the story is that you get the opposite of a vector if you reverse the subscripts.

Relative Velocities in 1 D (cont.)


vSR

vRS

Relative Velocities in 1 D (cont.)Question 3: If Rover is chasing the train as Schmed goes by throwing a fastball, at what speed does Rover clock the ball now?

vBT = 23 m/s vTG = 15 m/s

vBG = 29 m/s

Note, because Rover is chasing the train, he will measure a slower speed. (In fact, if Rover could run at 38 m/s he’d say the fastball is at rest.) This time we need the velocity of the ball with respect to Rover:

vBR = vBT + vTG + vGR = vBT + vTG - vRG = 23 + 15 - 9

= 29 m/s.

Note how the inner subscripts match up again and the outer most give the subscripts of the resultant. Also, we make use of the fact that

vGR = - vRG.

vRG = 9 m/s

River Crossing

Current 0.3 m/s

campsite

boat

You’re directly across a 20 m wide river from your buddies’ campsite. Your only means of crossing is your trusty rowboat, which you can row at 0.5 m/s in still water. If you “aim” your boat directly at the camp, you’ll end up to the right of it because of the current. At what angle should you row in order to trying to land right at the campsite, and how long will it take you to get there?

river


River Crossing (cont.)

Current 0.3 m/s

campsite

boat

river

0.3 m/s

0.5 m/s

Because of the current, your boat points in the direction of red but moves in the direction of green. The Pythagorean theorem tells us that green’s magnitude is 0.4 m/s. This is the speed you’re moving with respect to the campsite. Thus, t = d / v = (20 m) / (0.4 m/s) = 50 s. = tan-1(0.3 / 0.4) 36.9.

0.4 m/s


River Crossing: Relative Velocities

Current 0.3 m/s

campsite

river

0.3 m/s

0.5 m/s 0.4 m/s

The red vector is the velocity of the boat with respect to the water, vBW, which is what your speedometer would read.Blue is the velocity of the water w/ resp. to the camp, vWC. Green is the velocity of the boat with respect to the camp, vBC.

The only thing that could vary in our problem was . It had to be determined so that red + blue gave a vector pointing directly across the river, which is the way you wanted to go.


River Crossing: Relative Velocities (cont.)

vWC

vBW vBC

vBW = vel. of boat w/ respect to water

vWC = vel. of water w/ respect to camp

vBC = vel. of boat w/ respect to camp

vBW + vWC = vBC

Look how they add up:

The inner subscripts match; the out ones give subscripts of the resultant. This technique works in 1, 2, or 3 dimensions w/ any number or vectors.

Law of SinesThe river problem involved a right triangle. If it hadn’t we would have had to use either component techniques or the two laws you’ll also do in trig class: Law of Sines & Law of Cosines.

Law of Sines: sin A sin B sin C a b c

= =

Side a is opposite angle A, b is opposite B, and c is opposite C.

A B

C

c

b a

Law of Cosines

Law of Cosines: a

2 = b

2 + c

2 - 2 b c cosA

This side is always opposite this angle.

These two sides are repeated.

It doesn’t matter which side is called a, b, and c, so long as the two rules above are followed. This law is like the Pythagorean theorem with a built in correction term of -2 b c cos A. This term allows us to work with non-right triangles. Note if A = 90, this term drops out (cos 90 = 0), and we have the normal Pythagorean theorem.

A B

C

c

b a

vWA = vel. of Wonder Woman w/ resp. to the air

vAG = vel. of the air w/ resp. to the ground (and Aqua Man)

vWG = vel. of Wonder Woman w/ resp. to the ground (Aqua Man)

Wonder Woman Jet ProblemSuppose Wonder Woman is flying her invisible jet. Her onboard controls display a velocity of 304 mph 10 E of N. A wind blows at 195 mph in the direction of 32 N of E. What is her velocity with respect to Aqua Man, who is resting poolside down on the ground?

We know the first two vectors; we need to find the third. First we’ll find it using the laws of sines & cosines, then we’ll check the result using components. Either way, we need to make a vector diagram. continued on next slide

The 80 angle at the lower right is the complement of the 10 angle. The two 80 angles are alternate interior. The 100 angle is the supplement of the 80 angle. Now we know the angle between red and blue is 132.

Wonder Woman Jet Problem (cont.)


10

32 v

WA

v AG

vWG

vWA + vAG = vWG 80

195 mph

304

mph

vWG

8032

100

Wonder Woman Jet Problem (cont.)

195 mph

304

mph

v

132

By the law of cosines v 2 = (304)2 + (195)2 - 2 (304) (195) cos 132.

So, v = 458 mph. Note that the last term above appears negative, but it’s actually positive, since cos 132 < 0. The law of sines says:

sin 132 sin v 195

=

So, sin = 195 sin 132 / 458, and 18.45

80

This mean the angle between green and the horizontal is 80 - 18.45 61.6

Therefore, from Aqua Man’s perspective, Wonder Woman is flying at 458 mph at 61.6 N of E.

Wonder Woman Problem: Component Method

32

vW

A =

304

mph

v AG = 195 mph

10

This time we’ll add vectors via components as we’ve done before. Note that because of the angles given here, we use cosine for the vertical comp. of red but sine for the vertical comp. of blue. All units are mph.

304

195103.3343

165.3694

52.789

299.3816


Wonder Woman: Component Method (cont.)

304

195

103.

3343

165.3694

52.789

299.

3816

103.3343

52.789 165.3694

299.

3816

402.

7159

mph

218.1584 mph

458.

0100

mph

Combine vertical & horiz. comps. separately and use Pythag. theorem. = tan-1(218.1584 / 402.7159) = 28.4452. is measured from the vertical, which is why it’s 10 more than .

Comparison of MethodsWe ended up with same result for Wonder Woman doing it in two different ways. Each way requires some work. You will only want to use the laws of sines & cosines if:

• the vectors form a triangle.

• you’re dealing with exactly 3 vectors. (If you’re adding 3 vectors, the resultant makes a total of 4, and this method would require using 2 separate triangles.)

Regardless of the method, draw a vector diagram! To determine which two vectors add to the third, use the subscript trick.

floor

Free body diagrams #1

mF1

F2

Two applied forces; F2 < mg; coef. of kinetic friction = k

For the next several slides, draw a free body diagram for each mass in the set-up and find a (or write a system of 2nd Law equations from which you could find a.)

v

F1

F2

fk

mg

ma = F1 - fk = F1 - kN = F1 - k (mg - F2) (to the right). There is not enough info to determine whether or not N is bigger than F2.

N

answer:

Free body diagrams #2Bodies start at rest; m3 > m1 + m2; frictionless

pulley with negligible mass. answer :

T1

m3 g

T1

m1 g T2

T2

m2 g

Let’s choose clockwise as the + direction.

m1: T1 - m1 g -T2 = m1a

m2: T2 - m2 g = m2a

m3: m3 g - T1 = m3a

system: m3 g - m1 g - m2 g = (m1 + m2 + m3)a (Tensions are internal and cancel out.)

So, a = (m3 - m1 - m2) g / (m1 + m2 + m3)

If masses are given, find a first with last equation and substitute to find the T ’s.

m1

m3

m2


m2

m1m3

v

k

m1 > m3

m1 g

T1

m3 g

T2

Note: T1 must be > T2 otherwise m2 couldn’t accelerate.

T2 - m3 g = m3 a T1 - T2 - k m2 g = m2 a m1 g - T1 = m1 a

system: m1 g - km2 g - m3 g = (m1 + m2 + m3) a

T1 T2

fk

m2 g

N

answer:


mv

answer:

Rock falling down in a pool of water

mg - D = ma. So, a = (mg - D) / m. Note: the longer the rock falls, the faster it goes and the greater D becomes, which is proportional to v. Eventually, D = mg and a becomes zero, as our equation shows, and the rock reaches terminal velocity.

D

mg

m


answer:

cotton candy Fe

A large crate of cotton candy and a small iron block of the same mass are falling in air at the same speed, accelerating down. R

mg

R

mg

Since the masses are the same, a = (mg - R) / m for each one, but R is bigger for the cotton candy since it has more surface area and they are moving at the same speed (just for now). So the iron has a greater accelera-tion and will be moving faster than the candy hereafter. The cotton candy will reach terminal vel. sooner and its terminal vel. will be less than the iron’s.

Free body diagrams #6 aThe boxes are not sliding;coefficients of static friction are given.

answer:

m1

m3

m2

1

2

m2

There is no friction acting on m2. It would not be in equilibrium otherwise.

T = m3 g = f1 1 N1 = 1(m1 + m2) g

f1’s reaction pair acting on table is not shown.

m3 g

m3

N1

m1 g

T

m2 g

m2 g

N2

f1

Tm1

2 is extraneous info in this problem, but not in the next slide.

Free body diagrams #6 bBoxes accelerating (clockwise); m1 & m2 are sliding; coef’s of kinetic friction given.

answer:

m1

m3

m2

1

2

v

m2

There is friction acting on m2 now.

It would not be accelerating otherwise. m3 g - T = m3 a; f2 = m2 a; T - f1 - f2 = m1 a,where f1 = 1 N1 = 1(m1 + m2) g

and f2 = 2 N2 = 2 m2 g.

m3 g

N1

m1 g

T

m2 g

m2 g

N2f1

Tm1

f2

f2

Note: f2 appears twice; they’re reaction pairs.


m 1

m2

k

v

Boxes moving clockwise at constant speed.

answer:

m2 g

T

m1 g

T

m1

g sin

fk

m1 g

cos

N

Since a = 0, m2 g = T = m1 g sin + fk = m1 g sin + k m1 g cos m2 = m1 (sin + k cos ). This is the relationship between the masses that must exist for equilibrium.

Constant velocity is the same as no velocity when it comes to the 2nd Law.

Note: sin, cos, and k are all dimensionless quantities, so we have kg as units on both sides of the last equation.

Free body diagrams #8Mr. Stickman is out for a walk. He’s moseying along but picking up speed with each step. The coef. of static friction between the grass and his stick sneakers is s.

v

answer:

mg

N

fs

Here’s a case where friction is a good thing. Without it we couldn’t walk. (It’s difficult to walk on ice since s is so small.) We use fs here since we assume he’s not slipping. Note: friction is in the direction of motion in this case. His pushing force does not appear in the free body diag. since it acts on the ground, not him. The reaction to his push is friction.

Fnet = fs

So, ma = fs fs, max = s m g

Thus, a s g.


ground

m

F

k

v fk

mg

F sin

F c

os

N

Note: is measured with respect to the vertical here.

Box does not get lifted up off the ground as long as F cos mg. If F cos mg, then N = 0.

Box budges if F sin > fs, max = s N = s (mg - F cos ).

While sliding, F sin - k (mg - F cos ) = ma.

answer:

Dot ProductsFirst recall vector addition in component form:

x1, y1, z1 x2, y2, z2 + = x1 + x2, y1 + y2, z1 + z2 It’s just component-wise addition. Note that the sum of two vectors is a vector.

For a dot product we do component-wise multiplication and add up the results:

x1, y1, z1 x2, y2, z2 = x1 x2 + y1 y2 + z1 z2

Note that the dot product of two vectors is a scalar!

Ex: -2, 3, 10 N

1, 6, -5 m = -2 + 18 - 50 = -34 N m

Dot products are used to find the work done by a force applied over a distance, as we’ll see in the future.

Dot Product Properties

• The dot product of two vectors is a scalar.

• It can be proven that a b = a b cos, where is the

angle between a and b.

• The dot product of perpendicular vectors is zero.

• The dot product of parallel vectors is simply the product of

their magnitudes.

• A dot product is commutative:

• A dot product can be performed on two vectors of the

same dimension, no matter how big the dimension.

a b = b a

Unit Vectors in 2-D

The vector v = -3, 4 indicates 3 units left and 4 units up, which is the sum of its components:

v = -3, 4 = -3, 0 + 0, 4

Any vector can be written as the sum of its components.

Let’s factor out what we can from each vector in the sum:

v = -3, 4 = -3 1, 0 + 4 0, 1 The vectors on the right side are each of magnitude one. For this reason they are called unit vectors.

A shorthand for the unit vector 1, 0 is i.

A shorthand for the unit vector 0, 1 is j.

Thus, v = -3, 4 = -3 i + 4 j

Unit Vectors in 3-D

v = 7, -5, 9 = 7, 0, 0 + 0, -5, 0 + 0, 0, 9

One way to interpret the vector v = 7, -5, 9 is that it indicates 7 units east, 5 units south, and 9 units up. v can be written as the sum components as follows:

= 7 1, 0, 0 - 5 0, 1, 0 + 9 0, 0, 1

= 7 i - 5 j + 9 k

In 3-D we define these unit vectors:

i = 1, 0, 0 , j = 0, 1, 0 , and k = 0, 0, 1

(continued on next slide)

Unit Vectors in 3-D (cont.)

x

y

z

1i

j

1

k 1

The x-, y-, and z-axes are mutually perpendicular, as are i, j, and k. The yellow plane is the x-y

plane. i and j are in this plane. Any point in space

can be reached from the origin using a linear combination of these 3 unit vectors. Ex: P = (-1.8, -1.4, 1.2) so the vector

P

-1.8 i – 1.4 j + 1.2 k will extend from the origin to P.

Determinants

To take a determinant of a 2 2 matrix, multiply diagonals and subtract. The determinant of A is written | A | and it equals 3 (11) - 4 (-2) = 33 + 8 = 41.

114

23A =

In order to do cross products we will need to find determinants of 3 3 matrices. One way to do this is to expand about the 1st row using minors, which are smaller determinants within a determinant. To find the minor of an element, cross out its row and column and keep what remains.

ihg

fed

cba Minor of a:

ih

fe

ig

fd

Minor of b:

hg

ed

Minor of c:

cont. on next slide

Determinants (cont.)

ihg

fed

cba(Minor of a) - b

ih

fe

ig

fd

(Minor of b) + c

hg

ed

(Minor of c )

By definition,

= a

= a - b + c

= a (e i - h f ) - b (d i - g f ) + c (d h - g e)

Determinants can be expanded about any row or column. Besides cross products, determinants have many other purposes, such as solving systems of linear equations.

Cross ProductsLet v1 = x1, y1, z1 and v2 = x2, y2, z2 .By definition, the cross product of these vectors (pronounced “v1 cross v2”) is given by the following determinant.

v1 v2

= x1 y1 z1

x2 y2 z2

i j k

= (y1 z2 - y2 z1) i - (x1 z2 - x2 z1) j + (x1 y2 - x2 y1) k

Note that the cross product of two vectors is another vector!Cross products are used a lot in physics, e.g., torque is a vector defined as the cross product of a displacement vector and a force vector. We’ll learn about torque in another unit.

a b

a b.

Right hand rule

b

a

a b

A quick way to determine the direction of a cross product is

to use the right hand rule. To find a b, place the knife

edge of your right hand (pinky side) along a and curl your

hand toward b, making a fist. Your thumb then points in the direction of

It can be proven that the magnitude of is given by:

a b sin| a b | =where is the angle between

a and b.

Dot Product vs. Cross Product

1. The dot product of two vectors is a scalar; the cross product is another vector (perpendicular to each of the original).

2. A dot product is commutative; a cross product is not. In fact, a b = - b a.

x1, y1, z1 x2, y2, z2 = x1 x2 + y1 y2 + z1 z2

3. Dot product definition:

Cross product definition: v1 v2 = x1 y1 z1

x2 y2 z2

i j k

4. a b = a b cos, and a b sin| a b | =

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Chap 6