Motion and ForceChapter 10-11

Motion

What are some examples of Motion?

Large scale Small scale Fast vs. slow Frame of reference?

What is motion?

Objects change in position relative to a reference point.› Reference point should be stationary

Motio

n

Dista

nce

D

ispla

cem

ent

Total length which an object moves

Can be a straight line, but doesn’t have to be

Doesn’t have a particular direction

Change in position of an object Straight line from the starting point to the

final point MUST BE A STRAIGHT LINE!!! Shorter or equal to distance, never more Must have a particular direction

(toward/away)

Example 2A+B= DISTANCEC = DISPLACEMENT

John’s house

Jane’s house

School

A

C

B

How do we determine the rate of movement?

Speed – the distance traveled in a given amount of time

To calculate speed, must know both distance and time› Speed = distance/time speed = d/t› (SI Units)

Distance – meters (m) Time – seconds (s) m/s

CHECKER AUTO PARTS 500

SPEED = DISTANCE

TIME

DISTANCE = 315 laps

TIME = 3 hoursSPEED = ?

Track length = 1 mile

V

s

t

Constant speed – object covers equal distances in equal amounts of time (doesn’t change)› Example: earth’s rotation

Most objects don’t have constant speed

Distance – Time Graph

X-axis – independent variable Y-axis – dependent variable Time is usually independent b/c it will pass

whether distance is traveled or not Constant speed is a straight line The slope (rise/run) give the speed of the object Steeper the slope, the faster the speed

Average speed = distance / time Instantaneous speed – speed at that

very instant› Example - speedometer

Constant speed – doesn’t change› Example – Earth’s rotation

Velocity

Speed in a certain direction Used for navigation, weather Direction can be N, S, E, or W of a fixed

point Negative or positive along the line of

motion Velocity = displacement (m) / time (s) SI units

› m/s

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

Can add 2 velocities that are going same direction to get resultant velocities

If moving in opposite directions, you subtract to get the resultant velocity

V

d

t

Acceleration

Any change in velocity

Change in direction causes acceleration› Examples – moon,

race track

Positive acceleration – object is speeding up› Examples: a car goes from 0mi/hr to 60

mi/hr in 3 seconds. Negative acceleration – object is

slowing down› Example: skier stopping from 20m/s in .5s

Acceleration can be determined by the change in velocity over a change in time

ACCELERATION = FINAL VELOCITY – INITIAL VELOCITY

TIME

a = (vf - vi)/(tf - ti) SI units:

› (m/s)-(m/s) / s = m/s2

ZERO ACCELLERATION Identify two conditions

that must be met for these joggers to be at zero acceleration.

1. No change of direction.2. No change

(increase/decrease) in speed.

Acceleration can be determined by a velocity-time graph› The slope of the line gives you the value of

acceleration› Positive slope – object is speeding up (positive

acceleration)

› Negative slope – object is slowing down (negative acceleration)

› Horizontal line – velocity is not changing, thus the acceleration is 0m/s2

Force

Force – an action which changes an objects state of rest or motion› Has magnitude and direction

Examples of FORCES:› Gravity› Friction › Engines

Net force – combination of all the forces acting on an object› If net force =0, the object is balanced

Doesn’t move› If net force is unbalanced, object

accelerates in the direction which force is greater

Friction

Friction – force that acts against a motion in progress› Constant force has to be applied to an

object to keep it moving Example: car will eventually stop, if gas is

not applied› Friction also affects stationary object

Example: truck parked on hill – friction of brakes provides a force against gravity

Sta

tic Friction

Kin

etic Frictio

n

Friction b/w 2 stationary surfaces Greater than kinetic

Friction b/w 2 moving surfaces Less than static b/c it takes more force to make

an object start moving than to keep it moving Sliding friction – 2 objects slide past each other Rolling friction – round object rolls over a flat

surface› Usually less than sliding

Air resistance opposes motion› Example: as a car moves, it must

push the air out of the way› The easier the air is pushed out of

the way, the faster it will go› Designing the shape of the car so

that less air must be displaced is called streamlining

Fric

tion

can

be h

arm

ful

or h

elp

ful

Harm

ful

Help

ful

Road rash Carpet burn Wear and tear on car tires Racing Air hockey

B/w road and tires makes driving possible

Sand on icy roads Stopping a car Racing

Laws of Motion

Sir Isaac Newton described the relationship b/w motion and force in 3 laws

1st Law of Motion - Inertia An object at rest remains

at rest and an object in motion maintains its velocity unless it experiences an unbalanced force.› Example: as car stops

suddenly, body keeps moving

Inertia – tendency of an object to resist a change in velocity (speed & direction) until acted upon

Inertia is related to mass of an object› Object with small mass has

less inertia than object with large mass Example: softball & bowling

ball Other examples of inertia:

› Lean toward side around curves

› Seatbelts keep you from continuing to move

› Car seat has a more equal distribution of weight keeping baby safe

2nd Law of Motion

The unbalanced force acting on an object equals the objects mass times its acceleration

Force=mass x accelerationF=ma

SI unit› Newtons (N) = Kg x m/s2

Which is harder to push?

Example: empty grocery cart vs. full cart

Law of Universal Gravitation – all objects in the universe attract each other through gravitational force

Force = G (m1 x m2/d2)

G=6.673 x 10-11 N x m2/kg2

Gravitational force increases as 1 or both masses increase

Gravitational force decreases as distance b/w masses increase

Acceleration depends on the mass of the object and the unbalanced force applied› More mass – harder to accelerate› More force – faster acceleration

When gravity is the only force acting on an object it is in free fall› If there were no air resistance, all objects would

fall to the Earth at the same speed: 9.8m/s2 (acceleration due to gravity)

Weight is the force of gravity on an objectWeight = mass x gravity w = mg

SI unit is Newton (N) › Kg x m/s2

The mass of an object is always the same, but weight changes as gravity changes.› Example: an astronaut has a mass of 66kg.

What is his weight on Earth and on the moon? On the moon gravity is 1.6 m/s2

The force of gravity is constant:› Air resistance

increases as you fall› It eventually equals

force of gravity› Equal force of gravity

& air = 0 acceleration

› No acceleration means constant velocity (max) called terminal velocity

Orbiting objects are in free fall:› Move forward then free

fall toward other object Projectile motion –

curved path that an object follows when thrown, launched, or projected› Combination of forward

horizontal motion and downward horizontal motion

Newton’s 3rd Law – action/reaction For every action force, there is an

equal and opposite reaction force. Can occur when there is no motion:

› Example: sitting in chair Or can occur in motion:

› Example: rocket

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

Momentum Momentum- the product of the mass &

velocity of an objectmomentum = mass x velocity

p = mv Si Units

Kg x m/s Has direction because velocity has

direction When force changes motion, momentum

changes as well

Example: calculate the momentum of a 6 kg bowling ball moving at 10m/s down the alley toward the pins.

Law of Conservation of Momentum:› The total amount of momentum is a

system is conserved› Used to predict motion of cars after a

collision