F = maF = ma

When Making a sudden When Making a sudden stop in your car, what stop in your car, what

pushes you into the seat pushes you into the seat belts?belts?

A.A. Nothing, your own motion Nothing, your own motion does itdoes it

B.B. Your seatYour seat

C.C. Nothing, your seat belts Nothing, your seat belts just tightenjust tighten

D.D. The ForceThe Force

““The Force”The Force”

““An energy field created by all living An energy field created by all living things. It surrounds us, penetrates things. It surrounds us, penetrates us, and binds the galaxy together.”us, and binds the galaxy together.” The Force has two components:The Force has two components:

Light sideLight side Dark sideDark side

The Real ForceThe Real Force

Something that causes an object’s Something that causes an object’s motion to change (causes motion to change (causes acceleration)acceleration)..

A “push” or a “pull.”A “push” or a “pull.” Common Examples of forces: Gravity Common Examples of forces: Gravity

fields, pushing on something, fields, pushing on something, compressing a spring, a magnetic field, compressing a spring, a magnetic field, tension, friction, and the “normal” force.tension, friction, and the “normal” force.

Units are Newtons (N)Units are Newtons (N)

[Enter question here][Enter question here]

A.A. Your weightYour weight

B.B. A magnet pulling on another magnetA magnet pulling on another magnet

C.C. A bully pushing you down the stairsA bully pushing you down the stairs

D.D. A rope pulling a cartful of turnipsA rope pulling a cartful of turnips

E.E. All of the aboveAll of the above

F.F. None of the aboveNone of the above

Which of the following is an Which of the following is an example of a force?example of a force?

Types ofTypes of ForcesForces

•Field•Balloon bending Balloon bending stream of waterstream of water•Projectile Projectile MotionMotion•Magnetic fieldsMagnetic fields

ContactContact Me slapping you upside Me slapping you upside

the headthe head Baseball bat on a Baseball bat on a

baseballbaseball

Easily forgotten Forces!!Easily forgotten Forces!!

Important contact force: Important contact force: FRICTION FRICTION Acts in the opposite direction of motionActs in the opposite direction of motion Produces Produces HEATHEAT AIR RESISTANCE is a form of FRICTION!!AIR RESISTANCE is a form of FRICTION!! SLOWS MOTIONSLOWS MOTION Can be desired (tires on road) or Can be desired (tires on road) or

undesired (skates on ice)undesired (skates on ice)

Easily forgotten forces!!Easily forgotten forces!!

Important Important field forcesfield forces: GRAVITY and : GRAVITY and Magnetic FieldsMagnetic Fields Opperate at a distance (don’t need to “touch”)Opperate at a distance (don’t need to “touch”) Decreases with the distance between objects.Decreases with the distance between objects.

WEIGHT is a FORCE caused by gravityWEIGHT is a FORCE caused by gravity FFweightweight = mass x gravity = mass x gravity

As gravity changes, WEIGHT changes, NOT As gravity changes, WEIGHT changes, NOT MASSMASS

[Enter question here][Enter question here]

A.A. Gravitational Gravitational ForceForce

B.B. Electric ForceElectric Force

C.C. Frictional ForceFrictional Force

D.D. Magnetic ForceMagnetic Force

E.E. THE ForceTHE Force

So I’m cruising around Belton in my So I’m cruising around Belton in my awesome car when I see a dog in the awesome car when I see a dog in the middle of the road! What is the main middle of the road! What is the main force that lets me stop my car so the force that lets me stop my car so the

doggy can live?doggy can live?

Force is a vectorForce is a vector

Has Has magnitudemagnitude and and directiondirection AmountAmount of force makes a difference of force makes a difference

Little push vs. big shoveLittle push vs. big shove Earth gravity vs. moon gravityEarth gravity vs. moon gravity

Direction makes a differenceDirection makes a difference Push vs. PullPush vs. Pull Thrust vs. DragThrust vs. Drag

Forces “add” to give a net force.Forces “add” to give a net force. NetNet force determines what an object will do. force determines what an object will do.

The law of inertia: An object at rest will stay at rest unless acted upon by an outside net force.

An object in motion will stay in motion unless acted upon by an outside net force.

Galileo’s Unique IdeaGalileo’s Unique Idea

Objects don’t need a force to keep Objects don’t need a force to keep moving!moving! Every object naturally wants to Every object naturally wants to

maintain its statemaintain its state of motion or rest of motion or rest INERTIA! INERTIA! (resistance to change in motion)(resistance to change in motion)

Refined by Newton in 1800’s:Refined by Newton in 1800’s:

Basic Info: InertiaBasic Info: Inertia

Inertia depends on:Inertia depends on: MassMass Shape/Mass Distribution of object- rotational Shape/Mass Distribution of object- rotational

inertiainertia Solid Cylinder (like a wheel of cheese,)Solid Cylinder (like a wheel of cheese,) Hoop (like a bicycle tire)Hoop (like a bicycle tire)

Inertia does NOT depend on:Inertia does NOT depend on: Velocity/Speed of objectVelocity/Speed of object

It takes the same amount of force to speed a bus It takes the same amount of force to speed a bus up as to slow it down!up as to slow it down!

[Enter question here][Enter question here]

A.A. A 0.5 mg cockroach.A 0.5 mg cockroach.

B.B. A 2.0 g lump of cheese.A 2.0 g lump of cheese.

C.C. A 35 g ball of goo.A 35 g ball of goo.

D.D. A 15 kg baby goatA 15 kg baby goat

E.E. A 300 kg circus freak on roller A 300 kg circus freak on roller skates.skates.

Which of these objects has the Which of these objects has the greatest amount of inertia?greatest amount of inertia?

[Enter question here][Enter question here]

A.A. Slow to a stopSlow to a stopB.B. Travel in a parabolaTravel in a parabolaC.C. Continue in a straight line.Continue in a straight line.D.D. Maintain constant speed.Maintain constant speed.E.E. Both A and BBoth A and BF.F. Both C and DBoth C and DG.G. All of the aboveAll of the above

You pay a large amount of money to You pay a large amount of money to be sent to outer space. You finally be sent to outer space. You finally

reach a point where all the reach a point where all the gravitational forces acting on you are gravitational forces acting on you are close to zero. If you are traveling at close to zero. If you are traveling at

the speed of light and then run out of the speed of light and then run out of fuel you will…..fuel you will…..

Net ForceNet Force

Net Income: How much money did Net Income: How much money did you make after you counted up all you make after you counted up all profit and expense?profit and expense?

Net Force: How much force is there Net Force: How much force is there after all individual forces are added after all individual forces are added up?up?

[Enter question here][Enter question here]

A.A. The object will slow to a stop.The object will slow to a stop.

B.B. The object will begin to fall.The object will begin to fall.

C.C. The object’s motion will not change.The object’s motion will not change.

D.D. The universe will open up and The universe will open up and swallow the object.swallow the object.

E.E. Time will stop for us but the object Time will stop for us but the object will grow old and smelly.will grow old and smelly.

What if…What if…The The netnet force acting on an force acting on an

object is zero?object is zero?

I have a mass attached I have a mass attached to the ceiling by a to the ceiling by a

thread. There is also a thread. There is also a thread attached at the thread attached at the bottom. If you pull the bottom. If you pull the bottom string which bottom string which

will break first?will break first?A.A. Both at the Both at the same timesame time

B.B. The top oneThe top one

C.C. The bottom oneThe bottom one

D.D. Depends on Depends on how hard you how hard you pull itpull it

The The amountamount of gravitational force exerted on of gravitational force exerted on an objectan object

1 lb = 4.448 N1 lb = 4.448 N 1 N = 0.225 1 N = 0.225 lb lb

MASSalways the same

(kg)

WEIGHTdepends on gravity

(N)

FW = mgFW (also W or Fg): weight

(N)m: mass (kg)g: acceleration due to

gravity (m/s2)

Weight and mass are two completely different

things!!!

Would you weigh more on Earth Would you weigh more on Earth or Jupiter?or Jupiter?

greater gravitygreater gravity

greater weightgreater weight

greater massgreater mass

Jupiter because...Jupiter because...

WeightWeight Find your weight on different places!Find your weight on different places!

First, find your mass in kilograms: take First, find your mass in kilograms: take your weight and divide it by 2.2 your weight and divide it by 2.2 (because 1 kg weighs 2.2 lbs)(because 1 kg weighs 2.2 lbs)

Now, to find how many Newtons you Now, to find how many Newtons you weigh, take your mass (m) and multiply weigh, take your mass (m) and multiply it by the gravitational acceleration (g). it by the gravitational acceleration (g). Remember: F=ma, or Remember: F=ma, or FFWW = mg = mg

ggearthearth: 9.8 m/s: 9.8 m/s22

ggmoonmoon: 1.67 m/s: 1.67 m/s22

ggmarsmars: 3.7 m/s: 3.7 m/s22

ggsunsun: 274.13 m/s: 274.13 m/s22

ggplutopluto: 0.42m/s: 0.42m/s22

To convert Newtons to Pounds, To convert Newtons to Pounds, remember: remember: 1 lb = 4.448 N1 lb = 4.448 N

[Enter question here][Enter question here]

A.A. 5.8 N5.8 N

B.B. 58 N58 N

C.C. 588 N588 N

D.D. 5880 N5880 N

E.E. 60 kg60 kg

F.F. None of the aboveNone of the above

What is the weight of a 60 What is the weight of a 60 kg person on earth?kg person on earth?

[Enter question here][Enter question here]

A.A. 1 N1 NB.B. 10 N10 NC.C. 100 N100 ND.D. 1000 N1000 NE.E. 1.67 N1.67 N

What is the weight of a 60 kg What is the weight of a 60 kg person on the moon? person on the moon?

(g(gmm= 1.67 m/s= 1.67 m/s22))

Constant acceleration Constant acceleration from gravityfrom gravity

Galileo found that balls Galileo found that balls of different masses fell of different masses fell at the same rate.at the same rate.

The The forceforce of gravity is of gravity is stronger for more stronger for more massive objects.massive objects.

But it requires more But it requires more force to accelerate a force to accelerate a more massive object more massive object (inertia).(inertia).

These two effects cancel These two effects cancel each other out.each other out.

Force of NormalForce of Normal The force that a surface exerts on an The force that a surface exerts on an

object.object. Force is always in a direction Force is always in a direction

perpendicular to the surface the object or perpendicular to the surface the object or system is in contact withsystem is in contact with

Usually a balancing force when an object Usually a balancing force when an object is in contact with another objectis in contact with another object

Designated FDesignated FNN or N or N

Force of Normal - Force of Normal - ExampleExample

Let’s say the box in this diagram has Let’s say the box in this diagram has a mass of 25 kg. a mass of 25 kg.

That means it has a weight of 245 N.That means it has a weight of 245 N. Since the ground is level, the Since the ground is level, the

Normal Force is going to be equal to Normal Force is going to be equal to the box’s weight, but the force is the box’s weight, but the force is exerted in a different direction (up).exerted in a different direction (up).

Force of Normal - Force of Normal - Example 2Example 2

That last one was easy! Let’s say this box That last one was easy! Let’s say this box is seated on an incline and still has a mass is seated on an incline and still has a mass of 25 kg.of 25 kg.

The weight of the box (mg) is still directed The weight of the box (mg) is still directed straight down, but the Normal Force is at straight down, but the Normal Force is at an angle, which makes it a little harder to an angle, which makes it a little harder to find…find…

FFNN = wcos = wcosθθ

Force of FrictionForce of Friction Static Friction is the force Static Friction is the force

which keeps an object which keeps an object stationarystationary

Kinetic Friction slows an Kinetic Friction slows an object in motionobject in motion

Force is dependent on Force is dependent on surface type and pressure surface type and pressure pushing the two objects pushing the two objects togethertogether

FFff = = FFNN

is the coefficient of is the coefficient of friction, and describes the friction, and describes the “roughness” of a surface.“roughness” of a surface.

Force is directed Force is directed oppositeopposite motion or expected motionmotion or expected motion

Find the Force of Friction…Find the Force of Friction…

For a 5 kg box that For a 5 kg box that is being pushed is being pushed across a table which across a table which has a µ of 0.655. :) has a µ of 0.655. :)

Find the Force of Friction…Find the Force of Friction…

For the shoes of a For the shoes of a 60 kg student as 60 kg student as they walk across a they walk across a floor with a µ of floor with a µ of 0.654.0.654.

TensionTension

Force applied by a Force applied by a string or cable string or cable when the object or when the object or system is hangingsystem is hanging

Direction can be Direction can be variablevariable

A force of tension A force of tension is described as Fis described as FTT or T or T

What is the What is the mass of a 294 mass of a 294

N boxN box

A.A. 30 kg30 kg

B.B. 2880 kg2880 kg

C.C. 294 N294 N

D.D. 66 lbs66 lbs

Basic Info: Basic Info: Force Force DiagramsDiagrams

Definition: A Diagram that shows all Definition: A Diagram that shows all the forces acting the forces acting onon a body a body

Does Does NOTNOT include forces exerted include forces exerted byby the body!the body!

Forces are drawn as vectors.Forces are drawn as vectors.

Free Body DiagramFree Body Diagram Simple drawing of all Simple drawing of all

forces working on an forces working on an object or systemobject or system

Use a box to Use a box to represent the object represent the object or systemor system

All forces move away All forces move away from the box.from the box.

Remember: gravity Remember: gravity will always affect an will always affect an object and so Fg will object and so Fg will always be in a F.B.D!always be in a F.B.D!

Diagram the Diagram the forces acting on a forces acting on a car stuck in a car stuck in a traffic jam.traffic jam.

Diagram the Diagram the forces acting on a forces acting on a car driving on IH-car driving on IH-35.35.

Basic Info: Unbalanced Basic Info: Unbalanced ForcesForces

Objects that are NOT in equilibriumObjects that are NOT in equilibrium Will accelerate!Will accelerate!

Objects in equilibrium will not accelerate Objects in equilibrium will not accelerate (inertia)(inertia) Net force required to speed up, slow down, Net force required to speed up, slow down,

turn, etc.turn, etc.

Which of these situations shows balanced forces?

Which will accelerate?

A B

C

D: all of them

E: none of them

1.1. As a 1150 kg car is driving on the highway the As a 1150 kg car is driving on the highway the engine is supplying a force of 900 N. The total engine is supplying a force of 900 N. The total force of friction (including air resistance and force of friction (including air resistance and the internal friction of the engine) is 460 N. the internal friction of the engine) is 460 N. What is the net force on the car in the What is the net force on the car in the direction of motion? direction of motion?

2.2. A 5 kg box slides across a table that has a A 5 kg box slides across a table that has a coefficient of friction of 0.236. If the force coefficient of friction of 0.236. If the force applied to the box is 25 N, what is the net applied to the box is 25 N, what is the net force acting on the box in the direction of force acting on the box in the direction of motion?motion?

3.3. A force of 150 N is applied to a 15 kg wood A force of 150 N is applied to a 15 kg wood block to make it slide across a piece of block to make it slide across a piece of sandpaper.sandpaper.

a)a) What is the force of normal acting on the wood What is the force of normal acting on the wood block?block?

b)b) What is the force of friction acting on the wood What is the force of friction acting on the wood block if the block if the μμ of the sandpaper is 0.995? of the sandpaper is 0.995?

c)c) What is the net force acting on the wood block?What is the net force acting on the wood block?

4.4. A 8 kg block of cheese is sitting on a ramp A 8 kg block of cheese is sitting on a ramp with a 30with a 30oo incline and a µ of 0.745. Draw a incline and a µ of 0.745. Draw a force diagram showing all the forces force diagram showing all the forces acting on the cheese.acting on the cheese.

The mass of a refrigerator that slides across a kitchen floor

that has a µ of 0.245 and an applied frictional

force of 300 N.A.A. 125 kg125 kg

B.B. 1220 kg1220 kg

C.C. 73.5 kg73.5 kg

D.D. 7.50 kg7.50 kg

Basic Info: balanced ForcesBasic Info: balanced Forces

Objects are balanced only if their net Objects are balanced only if their net force is zero in both the vertical and force is zero in both the vertical and horizontal directionshorizontal directions Meaning all the forces in the x direction add Meaning all the forces in the x direction add

up to zero AND all the forces in the y direction up to zero AND all the forces in the y direction add up to zeroadd up to zero

All forces that act in a angle needs to be All forces that act in a angle needs to be broken into components using trig.broken into components using trig. Meaning using a right triangle with x and y Meaning using a right triangle with x and y

components.components.

Vector ComponentsVector Components

In order to find In order to find the components the components of a vector (like of a vector (like force) you will force) you will need to use those need to use those timeless timeless Trigonometric Trigonometric FunctionsFunctions..

Vector ComponentsVector Components So we have a So we have a

person pulling a person pulling a sled 30sled 30oo with with respect to the respect to the horizontal at a horizontal at a force of 50 N.force of 50 N.

We need to think of We need to think of it like the sled it like the sled being pulled being pulled vertically and vertically and horizontally at the horizontally at the same time, giving same time, giving it both it both components.components.

Θ=30o

F=50NFy

Fx

Vector ComponentsVector Components In order to calculate In order to calculate

the components, we the components, we need to shift Fneed to shift Fyy to to make a right triangle.make a right triangle.

Then we can use trig Then we can use trig functions to solve for functions to solve for FFyy and F and Fxx like they are like they are sides of a right sides of a right triangle.triangle.

To solve for FTo solve for Fxx, we will , we will use cosine because it use cosine because it is adjacent and we is adjacent and we have the hypotenuse.have the hypotenuse.

Θ=30o

F=50NFy

Fx

Vector ComponentsVector Components To solve for vTo solve for vxx, we will use , we will use

cosine because it is the cosine because it is the adjacent side and we have the adjacent side and we have the hypotenuse.hypotenuse.

To solve for vTo solve for vyy, use the same , use the same process but with sine.process but with sine.

Θ=30o

F=50m/s

Fy

Fx

θFFF

Fθ

hyp

oppθ

y

y

sin

sin

sin

cos

cos

cos

FFF

F

hyp

adj

x

x

FFy y equals?equals?

A.A. 43.3 N43.3 N

B.B. 25 N25 N

C.C. 28.9 N28.9 N

D.D. 50 N50 N

Θ=30o

F=50m/s

Fy

θFFF

Fθ

hyp

oppθ

y

y

sin

sin

sin

cos

cos

cos

FFF

F

hyp

adj

x

x

FBD Example 1FBD Example 1

A 50 kg mass is suspended from two wires, as in the diagram below. What is the tension in the wires?

FBD Example 2FBD Example 2

A 25.0 N picture is hanging from two wires. The wires make a 30˚ angle with the top of the picture. Calculate the tensional force on each wire.

A force of 60 N is applied to a rope A force of 60 N is applied to a rope to pull a sled across a horizontal to pull a sled across a horizontal

surface at a constant velocity. The surface at a constant velocity. The rope is at an angle of 30 above the rope is at an angle of 30 above the

horizontal.horizontal.Calculate the magnitude of the Calculate the magnitude of the

component of the 60N force that is component of the 60N force that is parallel to the horizontal surface.parallel to the horizontal surface.

A.A. 60 N60 N

B.B. 30 N30 N

C.C. 52 N52 N

D.D. 35 N35 N

A force of 60 N is applied to a rope A force of 60 N is applied to a rope to pull a sled across a horizontal to pull a sled across a horizontal

surface at a constant velocity. The surface at a constant velocity. The rope is at an angle of 30 above the rope is at an angle of 30 above the

horizontal.horizontal.Determine the magnitude of the Determine the magnitude of the

frictional force.frictional force.

A.A. 60 N60 N

B.B. 30 N30 N

C.C. 52 N52 N

D.D. 18 N18 N

How to draw a free-body How to draw a free-body diagram for an object on a diagram for an object on a

ramp?ramp?

FBD example 3FBD example 3

A 20.0 kg block rests on a frictionless ramp, as in the diagram below. What is the tension in the string holding the block in place?

Formulas!Formulas!

W = mgW = mg FFff = = μμFFNN

FFnetnet = F = FAA – F – Fff

Object on a flat surface:Object on a flat surface: FFNN = Weight = Weight

Object on an incline:Object on an incline: FFNN = Wcos = Wcosθθ

FFAA = Wsin = Wsinθθ

What is the What is the coefficient of friction coefficient of friction of a 30 kg box being of a 30 kg box being

pushed across a table pushed across a table in a constant speed in a constant speed

with an applied force with an applied force of 250 N.of 250 N.A.A. 1.1761.176

B.B. .12.12

C.C. .850.850

D.D. 8.38.3

Even though the force Even though the force of gravity on a less of gravity on a less massive object is massive object is

smaller, it still fall at smaller, it still fall at the same rate as more the same rate as more

massive objects massive objects because:because:

A.A. Air is pushing it down more.Air is pushing it down more.

B.B. Because it has less resistance to Because it has less resistance to changing it motion (inertia). changing it motion (inertia).

C.C. Because it has more resistance to Because it has more resistance to changing it motion (inertia). changing it motion (inertia).

D.D. MagicMagic