Motion

Speed

MotionMotion: A change in position

Depends on reference point

Is the mom moving relative to the dad? Is the mom moving if you were on the side of

the road?

Distance vs. Displacement

What is distance?How is displacement different from distance?

What is speed?What is velocity?

DistanceDistance (d) – how far an object

travels.Does not depend on direction.

Imagine an ant crawling along a ruler.

What distance did the ant travel?d = 3 cm

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Distance Distance is scalar which means

magnitude or quantity.

Now what distance did the ant travel?d = 3 cm

Does his direction change the answer?

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Distance Distance does not depend on direction. Let’s follow the ant again.

What distance did the ant walk this time?

d = 7 cm

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Displacement Displacement (x) – difference between an

object’s final position and its starting position. Does depend on direction.

Displacement = final position – initial position x = xfinal – xinitial

In order to define displacement, we need directions.

Examples of directions: + and – N, S, E, W Angles

Displacement vs. Distance

Example of distance:The ant walked 3 cm.

Example of displacement:The ant walked 3 cm EAST.

An object’s distance traveled and its displacement aren’t always the same!

DisplacementFind the distance and

displacement of the ant.

Distance: 7 cmDisplacement: +3 cm

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+-

Displacement vs. Distance

An athlete runs around a track that is 100 meters long three times, then stops. What is the athlete’s distance and displacement?

Distance = 300 m

Displacement = 0 m

Why?

SpeedThe distance an object moves in a certain amount of time

Formula:

Unit for SpeedSince the formula is

Then the unit for speed must be a unit of distance divided by a unit of time.

For example: km/h m/s

SpeedLook familiar?Knowing any of the 2 variables, you can solve for the 3rd.

Calculating SpeedA car drives 100 meters in 5

seconds.

What is the car’s average speed?s = d/ts = (100 m) / (5 s) = 20 m/s

Graphing Speed

Distance (km)

25 50 50 50 100

Time (min)

20 30 40 50 60

Distance (km)

25 50 75 100

125

Time (min)

20 40 60 80 100

Distance (km)

25 40 55 65 75

Time (min)

5 15 30 45 70

Distance (km)

25 50 75 100

125

Time (min)

25 45 60 70 75

Interpreting GraphsThe shape, or slope, of the

line tells the story!

Interpreting a GraphNow you try! What does each graph tell you?

Tell the Story!What story does the graph tell you as you ride your bike to your friend’s house?

Interpret This Graph

Scalar Quantity – has magnitude but not direction.Distance and speed are scalar quantities.

Vector Quantity – has magnitude and direction.Displacement and velocity are vector quantities.

Scalar vs. Vector