MOTIONCHAPTER 9

Describing Motion•An object is in motion if its distance from

another object changes.

•In order to determine that, you must have a

REFERENCE POINT!

The reference point is the place or object used for comparison to determine if something is moving.

Frames of ReferenceIs the girl in red moving?

It depends!!

What is your

reference point?

Speed & Velocity•Speed is the distance an object travels in

a unit of time. It is a type of RATE.

•Calculating SPEED:

Speed = distancetime

Calculating Speed • Speed units are DERIVED units!! They come

from the formula for speed!

• Distance/time

• Which of the following could NOT be speed units?

Km/h, m/s, g/L, min/m, mph, ft/min, cm/ms, h/mi, N/m, in/s, m/min, mi/day

Average Speed vs. Instantaneous Speed

•Average Speed = TOTAL distance over TOTAL timeThink: “The Tortoise & the Hare”

•Instantaneous speed = The rate at which an object is moving at a given instant in time.

Who had the greatest AVERAGE speed?

Who had the greatest INSTANTANEOUS speed?

VELOCITY•Velocity is both the speed of the object

and the DIRECTION of the object!•Speed with direction is the VELOCITY!!

•A plane moves at 125 mph = SPEED

•A plane moves at 125 mph, EAST = VELOCITY

•This is critically important to various people – like air traffic controllers or metereologists!

Motion Graphs•Graphing the motion of an object allows

us to “see” the patterns of motion.The most important factor is the SLOPE!!

What is happening?

Moving at a constant

velocity/speed

NO slope = no motion! Staying at

25 m

On distance – time graphs:

•Which means, whatever the slope is doing, the speed is doing the same!

AS THE SLOPE GOES,SO GOES THE SPEED!

SLOPE = SPEED!!•Since slope is RISE/RUN, and distance is on the “rise” or y-axis, and time is on the “run” or the x-axis.

Distance/Time = SPEED

KNOW SLOPES!!•Straight, horizontal line – What is the

slope?•Straight line – Is the slope changing?•Steep line – What is the slope, LOW or

HIGH? •Curved line – What is the slope doing? Is

it staying the same or changing?

ACCELERATION•The rate at which velocity changes.•(Speeding up or slowing down)

•Remember that velocity involves both SPEED and DIRECTION so a change in velocity could involve a change in speed or a change in direction!

•Therefore, acceleration refers to increasing speed, decreasing speed OR changing direction.

Acceleration!

Slowing Down = Acceleration!

Constantly changing direction = Acceleration!!

Formula for acceleration

•Acceleration = Final velocity– initial velocity

Time

a = Vf – Vi

t

Decide if the following situations involve acceleration•A car approaching a red light.•A skateboard rolling down a steep hill.•An egg being dropped from the top of a

building•A car going a constant speed on a circular

track.•Pushing a box across the floor.•Hitting a baseball with the baseball bat•Cruising down the straight highway on

cruise control.

What does acceleration look like graphed on a distance – time graph??

The green line shows a changing

slope, so the speed is

changing!

ACCELERATION!