MOTION

Do you have to see something move to know that motion has

taken place??All you need to know is that its

position has changed!

Motion occurs when an object changes its

position RELATIVE TO A REFERENCE POINT.

The dog’s position has changed

relative to the tree (reference point) so you

know motion must have occurred.

After you choose your reference point, you can create what is called

a FRAME OF REFERENCE

This is basically just a GRAPH!!

X axis

Y axis

1. DISTANCE• Describes how far something

has moved.

QUICK REVIEW: Distance is measured in what unit

when you use SI??

DESCRIBING MOTION

DISTANCE V/S DISPLACEMENT

Distance is HOW FAR SOMETHING HAS TRAVELED from the reference

point (starting position)Whereas

2. Displacement is DISTANCE and the DIRECTION from the

reference point (starting position) something is.

DESCRIBING MOTION

EXAMPLE

Total Distance Traveled =80 m

Displacement = 20 m North

DESCRIBING MOTION

3. Speed = How far something travels (distance) in a given

amount of timeSpeed (in meters/second) = distance (in meters)

time (in seconds)

(You could also state speed in km/h or km/s or cm/yr)

EX. OF A SPEED PROBLEM

A car traveling at a constant speed covers a distance of 750 m in 25 s.

What is the car’s speed?

Important info: Distance (d) = 750 m

Time (t) = 25 sSpeed = d/t

Speed = 750 m =

25 s

30 m/s

Different Types of Speed

1.Constant Speed = when the speed hardly changes. Like when you use

cruise control on your car.

2.Changing Speed = when the speed changes over a given distance. Like

when you ride a bike or drive through downtown.

3.Average Speed = this describes speed of motion when you have changing speed. It is the total

distance traveled divided by the total time traveled.

Different Types of Speed

4. Instantaneous Speed = The speed at a given point in time. Like going from 0 km/h to 60 km/h. Your speedometer shows you your instantaneous speed.

-Instantaneous Speed changes when you increase or decrease

your speed

-Instantaneous Speed stays the same when your speed is

constant

How would these different types of speed look if your graphed them?

time

Distanc

e

time

Distanc

eCONSTANT SPEED

CHANGING SPEED

GRAPHING MOTIONUse a DISTANCE/TIME GRAPH

-Time = X axis

-Distance = Y axis

A graph of constant speed will represent the motion with a straight

lineTo show two constant speeds which are

different, you use SLOPE = steepness of the line =

speed

Steeper the slope = faster the speed

GRAPHING MOTION

A graph of changing speed will have a zig-zag or curved line

Putting it all together

TIME

DIS

TA

NC

E

FASTEST CONSTANT SPEED

SLOWER CONSTANT SPEED

CHANGING SPEED

What is the slope of this part of the line?

Putting it all together

10 20 300

200

600

1000

What is the average speed (slope) during the 10 min to 20 min period?

Change in distance = 1200-600 = 600

Amount of time = 20-10 = 10 min

Avg. Speed = change in distance (600 m)

Amount of time (10 min)= 60 m/min

Putting it all together

10 20 300

200

600

1000

What is the instantaneous speed at t= 10 min? (it is the slope of the line at that point)

600m – 0m = 600 m

10min – 0min = 10min

600m/10min = 60m/min

VELOCITY

Includes the SPEED of an object and the DIRECTION it is moving

-Velocity of an object can change even though the object’s speed doesn’t –

how??

-The velocity changes if the direction changes even though the

speed is constant

How are Velocity and Speed different??

Velocity includes direction/speed doesn’t!

Example Problem with Velocity

Describe the velocity of an object that travels north 6.9 m in 3 s, then turns and

travels south 2.8 m in 4 s.

Work this just like you did for finding average speed, but

now you include direction!

6.9 m / 3 s = 2.3m/s NORTH

Then

2.8 m / 4 s = 0.7m/s SOUTH

ACCELERATION= Rate of change of velocity

-When velocity changes = object is accelerating

-So: Acceleration occurs when an object changes speed, changes direction, or changes both speed

and direction

Acceleration due to a Change in Speed

-Increasing Speed = Positive Acceleration

-Decreasing Speed = Negative Acceleration (Deceleration)

Graphing Positive and Negative Acceleration

Time

Speed

Positive Slope = Positive

Acceleration = like going downhill (speeding up)

Time

Speed

Negative Slope = Negative

Acceleration = like climbing a hill

(slowing down)

Graphing Zero Acceleration

Time

Speed

Zero Slope = Zero Acceleration

This means No change in

Velocity = No change in Speed or direction

Putting them all together

Time

Speed

+ A

0 A

- A

Calculating Acceleration

Acceleration (A) m/s2 = Change in Velocity (m/s)

Time (s)Final Velocity = Vf

Initial Velocity = Vi

A (m/s2) = Vf-Vi (m/s)

Time (s)

Calculating Acceleration

If there is no direction change and only speed changes, then a change in velocity = change in speed so you use:

A (m/s2) =Final speed – Initial Speed

Time

Example Problems

1. A plane on a runway reaches 80 m/s in 20 s. What is the planes

acceleration?Important Info: Final Speed = 80m/s

Time = 20 s

A = Vf-Vi = 80m/s – 0m/s = 80 = 4m/s2

t 20s 20

Example Problem

A skateboarder is going 3m/s and stops in 2 seconds.

Important Info: Final Speed = 0m/s

Initial Speed = 3m/s

Time = 2 s

A (m/s2) = Vf-Vi = 0m/s – 3m/s = -1.5 m/s2

t 2s

Working Backwards

The acceleration of a car that comes to a stop in 5 seconds is -4m/s2. What was the car’s speed when it started accelerating

(decelerating)?

A = -4m/s2

t = 5sA = Vf-Vi

t

-4m/s2 = 0 – Vi

5s

X 5s 5s X

-20 m/s = -Vi

20 m/s = Vi

Get your book: Work these problems!

pg. 51 # 6-7

Force

= A push or pull

-Can cause the motion of an object to change (so it can change velocity)

EX: kicking a soccer ball, playing pool, hitting a tennis ball

Does the Atmosphere have force? Do you feel it?

Does gravity have force? Do you feel it?

Net Force

-When two or more forces act on an object at the same time, the forces combine and we call this

NET FORCE

Balanced Forces

-Forces on an object that are equal in size and opposite in direction

-The Forces cancel each other out

-Net Force = 0

Unbalanced Forces-When two or more unequal forces act on

an object

Situation #1

-Forces are unequal and in opposite directions

-Object is moved in the direction of the larger force

-Net force = difference between the two forces

Unbalanced ForcesSituation #2

-Forces are applied in the same direction

=Forces are combined (added together)

-Net force = add two forces together

Inertia= The tendency of an object to resist a

change in its motion

-The velocity (speed and direction) of an object remains constant unless a force

changes it

This said, if an object is at rest, it should

stay at rest unless a force causes it to

move. Time to test this out.

Inertia and Mass

-The greater an object’s mass = The greater its inertia

(the more it resists moving)

DUH! It takes more force to stop a bowling ball by swatting it with a

paddle than it does to stop a tennis ball!

Newton’s First Law of Motion

-An object moving at a constant velocity(constant speed and direction)

keeps moving at that velocity unless an unbalanced net force acts on it.

-AKA: The law of Inertia

Two students push on a box in the same direction, and one pushed in the opposite direction. What is the net force on the box

if each pushes with a force of 50 N.

50N

50N

50N

Add your combined forces (50 + 50 = 100)

Subtract the opposite force from your combined forces

Net Force = 100 N – 50 N = 50 N in the direction the two students are pushing

The downward force of gravity and the upward force of air resistance on a ball

are both 5N. What is the ball’s acceleration? (Remember acceleration = change in velocity divided by time)

5 N

5 NNet force = Difference between the two = 5 N – 5 N = 0

So is there any change in velocity (speed or direction of the object)?

NOPE. Acceleration = 0 m/s2