CHAPTER 8

MOTION AND FORCES

8.1 MOTION

SPEED -

• 65 mi/hr

CONSTANT SPEED -

TIME DISTANCE

0 0

1 15

2 30

3 45

4 ?

AVERAGE SPEED -

SPEED = DISTANCE / TIME

(v = d/t )

TIME DISTANCE AVERAGE

1 5

2 20

3 20

100 25

Graphing Speed

• What does a straight line indicate about the speed?

• What does the slope of a line indicate?

• Draw another line showing speed that is changing.

CHECKER AUTO PARTS 500• http://www.nascar

.com/• SPEED = DISTANCE

TIME

DISTANCE = 315 laps

TIME = 3 hours

SPEED = ? Track length = 1 mile

VELOCITY -

• The speed of these racers may not have changed…Explain why their velocity has changed.

• List some examples of how change of direction may be stated:– North, south, east, west

– 280 degrees (on a compass)

– Up, down…

MOMENTUM -

• Momentum = mass x velocity

( p = mv )

How does mass influence momentum?

How does speed influence momentum?

SUMMARY

D IS T A N C E a n d T IM E

S P E E D

S P E E D a n d D IR E C T IO N

V E L O C ITY

V E L O C IT Y a nd M A S S

M O M E N T U M

M O T IO N

8.2 ACCELERATION AND FORCE

ACCELERATION –

ACCELERATION = FINAL VELOCITY – INITIAL VELOCITYTIME

What evidence of acceleration do you see in these pictures?

(read page 260)

boat speeding up

car slowing down

ACCELERATION (continued)

What evidence of acceleration can be seen in this picture?

(changing direction)

ZERO ACCELLERATION -

• Identify two conditions that must be met for these joggers to be at zero acceleration.

(page 261)

1. No change of direction.

2. No change (increase/decrease) in speed.

FORCE –IS THE CAUSE OF ACCELERATION

• Examples of FORCES:– Gravity– Engines– Rubber band–

• BALANCED FORCES do not change motion.

• UNBALANCED FORCES change motion.

FRICTION -

A. SURFACE FRICTION – depends on roughness of surface.

Ways to INCREASE friction:

1. Sandpaper

2.

Ways to DECREASE friction:

1. Oil

2.

FRICTION (continued )B. AIR FRICTION

(resistance) - depends on the size, shape, or speed of an object.Ways to INCREASE friction:1. Larger surface area2. Ways to DECREASE friction:1. Smaller surface area.2.

GRAVITY -

• Gravity is exerted by all objects on each other.

• Depends on mass of objects.– MORE mass = MORE

gravity

• Depends on distance between objects. – MORE distance = LESS

gravity

Which law of gravitational attraction is illustrated by this cartoon?

                                                                                                                                                                                           

8.3 NEWTON’S LAWS OF MOTION

WEIGHT -

Is the gravitational force an object experiences due to its mass.

Is dependent on ones’ location.

May be described in an equation:

weight = mass x free-fall acceleration( g = 9.8 m/s2 )

The metric unit of measurement is the NEWTON and is measured with a SPRING SCALE.

MASS -Is a measure of the amount of matter in an object.

Is not dependent on location.

The metric unit of measurement is the GRAM and is measured with a BALANCE.

FIRST LAW ( INERTIA) -Objects at rest stay at rest and

objects in motion stay in motion unless an unbalanced force acts on them.

(This law describes what happens when forces on an object are BALANCED and the net force is zero.)

EXAMPLES:

SECOND LAW (ACCELERATION) -

A. This law describes the relationship between force, mass, and acceleration and may be stated as an equation:

force = mass x acceleration ( f = ma )

• (This law describes what happens when forces on an object are UNBALANCED and the net force is not zero.)

B. Acceleration and EQUAL FORCE -

• When the forces are equal, less massive objects accelerate faster than more massive ones.

• EXAMPLES:

C. Acceleration and EQUAL MASS -

• When the masses are equal, a greater force provides greater acceleration.

• EXAMPLES:

FREE FALL -

• When gravity is the accelerating force.

• Acceleration due to gravity is 9.8 m/s/s ( g = 9.8 m/s2 )

• Gravity accelerates all objects at the same rate!

Acceleration due to GRAVITY

TERMINAL VELOCITY -

• Force of air resistanceequals

the gravitational force (weight)

Object stops acceleratingTerminal velocity for a

human body is about 200 mph.

TERMINAL VELOCITY and AMUSEMENT PARKS

• New ride at Wisconsin Dells – Terminal Velocity.

• 100 foot free fall drop• 70 mph• 140 foot tower• $40 first time…$25

second time

THIRD LAW (ACTION = REACTION)• Every action force has

an equal and opposite reaction force

• F x D = f x d– (action = reaction)

• EXAMPLES:

THE THIRD LAW AND ROCKETS

• Unbalanced forces inside the engine compartment allow a rocket to move.

B A L A N C E D F O R C E S

IN E R T IA

F irs t L aw

U N B A L A N C E D F O R C E S

A C C L E R A T IO N

S e co n d L aw

F O R C E S A C T IN P A IR S

A C T IO N = R E A C T IO N

T h ird L aw

N e w to n s L a w s o f M o tion

STUDY FOR THE TEST

• VOCABULARY

• SPEED / VELOCITY / ACCELERATION

• IS IT A BALANCED OR UNBALANCED FORCE

• 1ST, 2ND, OR 3RD LAW EXAMPLES

• THE MOTION LABS