Chapter 12 What is Motion? Describing Motion Frame of Reference Motion Speed & Velocity ...

Chapter 12Chapter 12What is Motion?What is Motion?

Describing Motion Frame of Reference Motion Speed & Velocity Acceleration Momentum

MotionMotion

Problem: Is your desk moving?

We need a frame (point) of reference...a nonmoving point from which

motion is measured

MotionMotion

MotionChange in position in relation to

a point of reference.

Reference point

Motion

MotionMotion

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.

SpeedSpeed

Speed rate at which an object moves distance traveled per unit time

time

distancespeed

sd

t

SpeedSpeed

Instantaneous Speedspeed at a given instant

•ex - the car is going 35 mph right now

Average Speed The average speed of the entire journey

time total

distance totalspeed avg.

VelocityVelocity

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 storm’s direction!

VelocityVelocity

Velocityspeed in a given directionVelocity can change even when

the speed is constant!

Calculations of Calculations of Speed & VelocitySpeed & Velocity

Your neighbor skates at a speed of 4 m/s. You can skate 100 m in 20 s. Who skates faster?

GIVEN:

d = 100 m

t = 20 s

s = ?

WORK:

s = d ÷ t

s = (100 m) ÷ (20 s)

s = 5 m/s

You skate faster!vd

t

Calculations of Calculations of Speed & VelocitySpeed & Velocity

Sound travels 330 m/s. If a lightning bolt strikes the ground 1 km away from you, how long will it take for you to hear it?

GIVEN:

v = 330 m/s

d = 1km = 1000m

t = ?

WORK:

t = d ÷ v

t = (1000 m) ÷ (330 m/s)

t = 3.03 s

vd

t

Graphing Speed & VelocityGraphing Speed & Velocitydistance vs. time graphthe slope is = tospeed steeper slope meansfaster speedstraight line meansconstant speedflat line meansno motion

Distance-Time Graph

A

B

Graphing Speed & VelocityGraphing Speed & Velocity

Who 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 Speed & VelocityGraphing Speed & Velocity

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

Changing slope = changing velocity

AccelerationAcceleration

Acceleration the rate of change of velocitychange in speed or direction

t

vva if

a: acceleration

vf: final velocity

vi: initial velocity

t: time

a

vf - vi

t

AccelerationAcceleration

Positive acceleration “speeding up”

Negative acceleration “slowing down”

Acceleration CalculationsAcceleration CalculationsA roller coaster starts down a hill at 10 m/s.

Three seconds later, its speed is 32 m/s. What is the roller coaster’s acceleration?

GIVEN:

vi = 10 m/s

t = 3 s

vf = 32 m/s

a = ?

WORK:

a = (vf - vi) ÷ t

a = (32m/s - 10m/s) ÷ (3s)

a = 22 m/s ÷ 3 s

a = 7.3 m/s2a

vf - vi

t

Acceleration CalculationsAcceleration CalculationsHow long will it take a car traveling 30 m/s

to come to a stop if its acceleration is - 3 m/s2?

GIVEN:

t = ?

vi = 30 m/s

vf = 0 m/s

a = -3 m/s2

WORK:

t = (vf - vi) ÷ a

t = (0m/s-30m/s)÷(-3m/s2)

t = -30 m/s ÷ -3m/s2

t = 10 sa

vf - vi

t

Graphing AccelerationGraphing Acceleration

0

1

2

3

0 2 4 6 8 10

Time (s)

Sp

ee

d (

m/s

)

Speed-Time Graph

slope =

straight line =

flat line =

acceleration +vel = speeds up -vel = slows down

constant accel.

no accel. (constant velocity)

Graphing AccelerationGraphing Acceleration

0

1

2

3

0 2 4 6 8 10

Time (s)

Sp

ee

d (

m/s

)

Speed-Time GraphSpecify 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

MomentumMomentum

All moving objects have momentum Momentum depends on the mass of the

object and the velocity with which it is traveling

Formula for calculating momentum

momentum = mass x velocity

m

p

v

Conservation of MomentumConservation of Momentum

The total momentum of any group of objects remains the same unless outside forces act on the objects.

Ex. 2 billiard balls hit into one another.

The total momentum of the 2 balls before they collide is the same as the total momentum of the two balls after they collide.

Momentum CalculationsMomentum CalculationsWhat is the momentum of a 0.3 kg bluejay

flying at 17 m/sec?

GIVEN:

p = ?

v = 17 m/s

m = 0.3 kg

WORK:

p = m x v

p = (0.3 kg)x(17 m/s)

p = 5.1 kg x m/sec

mp

v