CatalystCatalyst1. What is the difference between speed and

velocity?2. Put these objects in order from slowest to fastest:●Peregrine falcon●Cheetah●Human●Car (at the highway speed limit)●Mako shark●Ocean wave●Maglev train

Motion & ForcesMotion & Forces

II. Describing Motion Motion Speed & Velocity Acceleration

A. MotionA. Motion

Problem: Is your desk moving?

We need a reference point...nonmoving point from which

motion is measured

A. MotionA. Motion

MotionChange in position in relation to

a reference point.

Reference point

Motion

A. MotionA. 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.

B. Speed & VelocityB. Speed & Velocity

Speed rate of motion distance traveled per unit time

time

distancespeed

sd

t

B. Speed & VelocityB. Speed & Velocity

Velocityspeed in a given directioncan change even when the speed

is constant!

vd

tUnits:D=metert-=secondsV=m/s

B. Speed & VelocityB. Speed & Velocity

Instantaneous Speedspeed at a given instant

Average Speed

time total

distance totalspeed avg.

B. Speed & VelocityB. 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 storm’s direction!

More PracticeMore Practice

Try problems 1-5 on the Speed Practice Problems sheet on my website on a separate sheet of paper.

Use the guess method we practiced when we worked on density.

How do distance and How do distance and displacement differ?displacement differ?

Distance is the total distance of the path travelled.

Displacement-measured from start point to ending point in one line. (if you start & stope in the same spot, displacement =0.

Graphing MotionGraphing Motion

slope =

steeper slope =

straight line =

flat line =

Distance-Time Graph

A

B

faster speed

constant speed

no motion

speed

Graphing MotionGraphing Motion 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 See pg 334 in text See pg 335

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 MotionGraphing Motion

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

Changing slope = changing velocity

Solve- Solve- not in your notesnot in your notesThe diagram below shows the position of a cross-country skier at various times. At each of the indicated times, the skier turns around and reverses the direction of travel. In other words, the skier moves from A to B to C to D. Use the diagram to determine distance traveled by the skier during these three minutes. Can you determine the displacement?

The skier covers a distance of (180 m + 140 m + 100 m) = 420 m and has a displacement of 140 m, rightward

A football coach paces back and forth along the sidelines. The diagram below shows several of coach's positions at various times. At each marked position, the coach makes a "U-turn" and moves in the opposite direction. In other words, the coach moves from position A to B to C to D. What is the coach's resulting displacement and distance of travel?

The coach covers a distance of(35 yds + 20 yds + 40 yds) = 95 yards and has a displacement of 55 yards, left.

not in your notes

CalculationsCalculationsYour 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

v = ?

WORK:

v = d ÷ t

v = (100 m) ÷ (20 s)

v = 5 m/s

You skate faster!vd

t

CalculationsCalculationsSound 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

C. AccelerationC. Acceleration

Acceleration the rate of change of velocitychange in speed or direction

t

vva if

A : acceleration(m/s2)

Vf : final velocity (m/s)

Vi : initial velocity (m/s)

T : time (sec)

a

vf - vi

t

3 ways an object can 3 ways an object can accelerateaccelerate

1. Positive acceleration=“speeding up”

2. Negative acceleration (aka. Deceleration= “slowing down”

3. Changing direction- traveling around a curve with constant speed.

Free FallObjects that fall in

air towards Earth, due to gravity. Objects fall at the rate of 9.8m/s2

(acceleration due to gravity= 9.8m/s2)

T=1sV=9.8m/s

T=2sV=19.6 m/s

T=3sV=29.4m/s

Terminal VelocityTerminal Velocity

the highest velocity that will be reached by a falling object.

Graphing MotionGraphing Motion

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 +ve = speeds up -ve = slows down

constant accel.

no accel. (constant velocity)

0

1

2

3

0 2 4 6 8 10

Time (s)

Sp

ee

d (

m/s

)

Graphing MotionGraphing Motion

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

CalculationsCalculationsA 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/s2

a

vf - vi

t

CalculationsCalculationsHow 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

B. MomentumB. Momentum

p = mv

p: momentum (kg ·m/s)m: mass (kg)v: velocity (m/s)

m

p

v

All objects have mass; so if an object is moving, then it has momentum

Law of Conservation of Law of Conservation of MomentumMomentum

Momentum is not created nor destroyed during a crash.

if no net force acts on a system, then the total momentum of the system does not change.

In a closed system, the loss of momentum of one object equals the gain in momentum of another object-momentum is conserved.