Motion and ForcesDescribing Motion

•Motion•Speed & Velocity

An object at rest will remain at rest, and an object in motion will continue in motion at a constant velocity unless acted upon by a net force.

Newton’s First Law

Motion

Problem:− Is your desk moving?

We need a reference point...− nonmoving point from which motion is measured

Motion

Motion− Change in position in relation to a reference point.

Reference point

Motion

Relative Motion

If you are sitting in a chair reading this sentence, you are moving. You are not moving relative to your desk or your school building, but you are moving relative to the other planets in the solar system and the Sun.

Motion

• Distance• Distance describes how far an object has

moved. • The SI unit of length or distance is the

meter (m). Longer distances are measured in kilometers (km).

• Shorter distances are measured in centimeters (cm).

Motion

• Displacement is the distance and direction of an object's change in position from a reference point.

• Suppose a runner jogs to the 50-m mark and then turns around and runs back to the 20-m mark.

• The runner travels 50 m in the original direction (north) plus 30 m in the opposite direction (south), so the total distance she ran is 80 m.

Motion

Problem:• You are a passenger in a car stopped at a stop

sign. Out of the corner of your eye, you notice a tree on the side of the road begin to move forward.

• You have mistakenly set yourself as the reference point.

Speed & Velocity

Speed− rate of motion − distance traveled per unit time

timedistance

speed

sdt

Calculating Speed

The SI unit for distance is the meter and the SI unit of time is the second (s), so in SI, units of speedSometimes it is more convenient to express speed in other units, such as kilometers per hour (km/h).

Speed & Velocity

Instantaneous Speed− speed at a given instant− Speed odometer reading

Average Speed− the total distance traveled divided by the total time of travel

time totaldistance total

speed avg.

Changing Instantaneous Speed

When something is speeding up or slowing down, its instantaneous speed is changing.If an object is moving with constant speed, the instantaneous speed doesn't change.To determine direction you need to know the velocity

Speed & Velocity

Problem:− A storm is 10 km away and is moving at a speed of 60

km/h. Should you be worried?

It depends on the It depends on the storm’s direction!storm’s direction!

Speed & Velocity

Velocity− speed in a given direction− can change even when the speed is constant!

Speed an Acceleration Video

Calculations Your neighbor skates at a speed of 4 m/s. You can skate Your neighbor skates at a speed of 4 m/s. You can skate

100 m in 20 s. Who skates faster?100 m in 20 s. Who skates faster?

GIVEN:

d = 100 mt = 20 ss = ?

WORK:s = d ÷ t

s = (100 m) ÷ (20 s)

s = 5 m/s

You skate faster!sdt

Calculations Sound travels 330 m/s. If a lightning bolt strikes the Sound travels 330 m/s. If a lightning bolt strikes the

ground 1 km away from you, how long will it take for ground 1 km away from you, how long will it take for you to hear it?you to hear it?

GIVEN:

s = 330 m/sd = 1km = 1000mt = ?

WORK:t = d ÷ s

t = (1000 m) ÷ (330 m/s)

t = 3.03 s

sdt

Graphing Motion

slope =

steeper slope =

Straight line =

flat line =

Single point = instantaneous instantaneous speedspeed

Distance-Time Graph

A

B

faster speedfaster speed

constant speedconstant speed

no motionno motion

speedspeed

Graphing MotionWho started out faster?− A (steeper slope)

Who had a constant speed?− A

Describe B from 10-20 min.− B stopped moving

Find their average speeds.− A = (2400m) ÷ (30min) A =

80 m/min− B = (1200m) ÷ (30min) B =

40 m/min

Distance-Time Graph

A

B

0

100

200

300

400

0 5 10 15 20

Time (s)

Dis

tan

ce (

m)

Distance-Time Graph

Graphing Motion

Acceleration is indicated by a curve on a Distance-Time graph.

Changing slope = changing velocity

Graphing Motion

0

1

2

3

0 2 4 6 8 10

Time (s)

Sp

ee

d (

m/s

)

Speed-Time Graph

Specify the time period when the object was...slowing down− 5 to 10 seconds

speeding up− 0 to 3 seconds

moving at a constant speed− 3 to 5 seconds

not moving− 0 & 10 seconds

Speed and Velocity

Check on Learning

A

B

C

D

m/s

s

kg

hr

1 _____is a unit of speed.

A

B

C

D

speed, velocity

rate, speed

rate, velocity

speed, acceleration

2_____ describes how fast something is going, whereas, _____describes how fast something is going and in a certain direction.

A

B

C

D

In/hour

m/s

cm/min

none of the answers

3 The SI unit for speed is _____.

A

B

C

D

24 m

6 m

4 m

none of the answers

4 If a person walked at 2 m/s for 12 s, he/she would travel a distance of___

A

B

C

D

slope

Y-intercept

origin

None of the answers

5 The _____ of a distance versus time graph is speed.

A

B

C

D

36.0 m

2160 m

40.0 m

36.0 miles

6How far would you travel moving at 12 m/s for 3.00 minutes?

A

B

C

D

average speed

velocity

given speed

Instantaneous speed

7 The speed at any instant of time is known as_____

A

B

C

D

3974 mi/hr

3 mi/hr

30 mi/hr

30 m/s

8If a total distance of 750 m is covered in a time interval of 25 s, the average speed is _____?

A

B

C

D

3 hours

1 hour

5 hours

50 hours

9How long would it take to travel 50 km traveling at a speed of 10 km/hr?

A

B

C

D

30 s

30 South

30 m/s

30 m/s, South

10 Which of the following is a measure of velocity?

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