Chapter 4. Motion An objects change in position relative to a reference point When an object changes position with respect to a frame of reference,

Chapter 4

Motion An object’s change in position relative to a reference point

When an object changes position with respect to a frame of reference, the object is in motion. north, south, east, west, up, or down

A person sits in the passenger seat of a car that is traveling down a street. Describe the person as seen by each of the following observers: A person sitting in the backseat of the car

A person standing on the sidewalk as the car passes

The driver of a second car moving in the same direction and passing the first car

A person stands near a bus stop. Describe the standing person’s motion as seen by the following observers: A person sitting in an approaching bus

A person riding in a car moving away from the bus stop

Another person standing at the bus stop

Do any observers say that the person sitting in the passenger seat of the car in the first slide was moving? Explain

Do any observers say that the person sitting in the passenger seat of the car in the first slide was not moving? Explain

Do any observers say that the person standing near the bus stop in the second slide was moving? Explain

Do any observers say that the person standing near the bus stop in the second slide was not moving? Explain

Based on these answers, explain what it means when someone says an object is “moving.”

Motion requires an object moving over a distance or how far it has moved.

When an object moves displacement takes place.

What is the difference between distance and displacement?

Which should you use when calculating how many gallons of gas you will need for a road trip?

Which does the saying “as the crow flies” refer to?

Speed tells you how fast something traveled over specific time.

Speed is calculated as distance d divided by time t.

s=d/t Speed can also be measure in small spurts called instantaneous speed.

▪ Ex. Speedometer in a car

I have dinner reservations at 8 pm at the Spaghetti Warehouse in Marietta off of exit 259. I just got on I-75 South from Barrett Parkway at exit 269. Assuming the distance from here to there is 10 miles, and it is now 7:50 pm, how fast must I drive to make my reservation?

I know that Speed equals distance divided by time

Distance is 10 miles Time is 10 minutes

Speed = distance = 10 miles = 1 mile / minute!

time 10 minutes

Average Speed = total distance covered travel time

Average Speed

Total Time

Total Distance Covered

D

TS

Velocity not only tells you the speed of an object but it also tells the direction.

When velocity is calculated you use the same equation as speed, but you must specify direction.

It takes you 60 mins to travel 2km, what is your average speed?

A graph can be used to study motion

Distance – Time graph x-axis= time y-axis= distance

The slope of the line is the speed

Make a line graph of the data of a runner. Determine the during which time interval the runner is moving faster, moving slower, and standing still. Explain the relationship between the slope of the line and the runner’s speed.

Time (s)

Position (m)

0 0

10 20

20 30

30 30

0

5

10

15

20

25

30

35

0 10 20 30

Time

Dis

tan

ce

Motion Diagram

Distance01 m2 m3 m4 m5 m6 mTime0.5 s1 s1.5 s2 s2.5 s3 s

Position

(meters)

Time - (seconds)

1 2 3

Position versus Time

1

3

5

7

Position

Time

Position versus Time

Which line shows the object covering the most ground in a certain amount of time?

We call the distance an object travels in a certain amount of time “speed”

Speed = Distance covered Time

Two Types of Instantaneous Average

Distance

(meters)

Time - (seconds)

1 2 3

Distance versus Time

1

3

5

7

Distance divided by time on this kind of graph is the:

___________ of the object

___________ of the line

Position

Time

Position versus Time

Which line shows the object covering the most ground in a certain amount of time?

Position

Time

Position versus Time

Which object has the highest rate of speed or rather, which object is moving the fastest?

Position

Time

Position versus Time

Rank the five colors in order from slowest to fastest.

Velocity is Speed + Direction

How can we change our speed?

How can we change our velocity?

Whenever your speed or direction changes you accelerate.

Calculate acceleration

▪ a= vf-vi/t

Think about your daily life. List 5 examples of acceleration. Classify each example of acceleration as an increase in speed, a decrease in speed, a change in direction, or a combination.

Your friend Lynn is getting frustrated that y’all are driving in circles [literally] trying to find the store. Lynn looks at the speedometer, sees its not moving from 25 mph and says: “Well, at least we are not accelerating while driving around the around the mall.” What do you say to Lynn?

Spacing between dots represent what?

Arrows represent what two things?

If the object started to move faster towards the left, what would the diagram look like now?

Construct a motion diagram for the ball in this video!

Video

Which diagram below resembles your diagram the most?

A B C D

In what direction is your CHANGE in velocity for that video/motion diagram?A B C

Change in velocity is zero

In what direction is your CHANGE in velocity for that video/motion diagram?A B C

Change in velocity is zero

Combine the two and you get:

v

Devise a story for an object whose motion is consistent with the motion diagram.

v v v = 0

Devise a story for an object whose motion is consistent with the motion diagram to the right.

v

v

v = 0

We just did a motion diagram for a ball on its way down.

Imagine, if I throw the ball up in the air? What does a motion diagram look like for the ball while it is moving upwards?

While its at the top?

I am standing on a cliff and I drop a rock from rest. After two seconds, the rock is moving at a speed of 20 m/s. What is the rate of acceleration for this rock? I know that acceleration equals change in velocity per unit time

Change in Velocity is 20 m/s Time is 2 seconds

Acceleration = velocity = 20 m/s = 10 m/s/s or

time 2 s 10 m/s2

If I can accelerate at a rate of 15 mph per second, how long will it take me to accelerate from 30 to 90 miles per hour?

If I can accelerate at a rate of 15 mph per second, how long will it take me to accelerate from 30 to 90 miles per hour?

a) 2 seconds

b) 4 seconds

c) 8 seconds

d) 900 seconds

You are in your car going 75 mph. If you can decelerate your car at a rate of 25 mph per second, then how long does it take you to stop your car?

You are in your car going 75 mph. If you can decelerate your car at a rate of 25 mph per second, then how long does it take you to stop your car?

a) 100 seconds

b) 1/3 second

c) 3 seconds

d) 50 seconds

What is the direction of the objects acceleration compared to the velocity of the car?

A flowerpot falls off a second-story windowsill. The flowerpot starts from rest and hits the sidewalk 1.5s later with a velocity of 14.7m/s. Find the average acceleration of the flowerpot.

v = d / t = velocity equals change in displacement over change in timea = v / t = acceleration equals change in velocity over change in time

Free Fall Only Starting at Rest!!!!!!!a = g = 10 m/s2 [actually 9.8 m/s2]v = g t = velocity is g multiplied by timed = ½ g t2 = distance is one half multiplied by g multiplied by times squared.