Chapter 10 Motion. Measuring Motion Motion—when an object changes its position relative to a reference point Motion—when an object changes its position.

Chapter 10 Chapter 10 MotionMotion

Measuring MotionMeasuring Motion

MotionMotion—when an object changes —when an object changes its position relative to a reference its position relative to a reference pointpoint– DistanceDistance—how far an object has —how far an object has

movedmoved– DisplacementDisplacement—distance and —distance and

direction of an object’s change of direction of an object’s change of position from a starting pointposition from a starting point

MotionMotion

Problem:Problem:– Is your desk moving?Is your desk moving?

We need a We need a reference pointreference point......– nonmoving point from which motion nonmoving point from which motion

is measuredis measured

MotionMotion

MotionMotion– Change in position in relation to a Change in position in relation to a

reference point.reference point.

Reference point

Motion

MotionMotion

Problem:Problem: You are a passenger in a car You are a passenger in a car

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

You have mistakenly set You have mistakenly set yourselfyourself as the reference point.as the reference point.


SpeedSpeed—distance an object travels per —distance an object travels per unit of timeunit of time– RateRate—any change over time—any change over time– Calculation for speed: Calculation for speed: speed = speed =

distance/distance/timetime– Speed that doesn’t change over time—Speed that doesn’t change over time—

constantconstant speed speed– Speed is usually not constant; usually an Speed is usually not constant; usually an

object has object has changingchanging speed. speed.– AverageAverage speed—speed of motion when speed—speed of motion when

speed is changing: speed is changing: speed = total speed = total distance/total travel timedistance/total travel time

– InstantaneousInstantaneous speed—speed at any given speed—speed at any given point in timepoint in time

SpeedSpeed

SpeedSpeed– rate of motion rate of motion – distance traveled per unit timedistance traveled per unit time

time

distancespeed

vd

t

Speed & VelocitySpeed & Velocity

Instantaneous SpeedInstantaneous Speed– speed at a given instantspeed at a given instant

Average SpeedAverage Speed

time total

distance totalspeed avg.


A distance-time A distance-time graphgraph displays displays motion of an object over time.motion of an object over time.– Plot distance on a(n) Plot distance on a(n) verticalvertical axis. axis.– Plot time on a(n) Plot time on a(n) horizontalhorizontal axis. axis.

VelocityVelocity—speed and direction of —speed and direction of an object’s motionan object’s motion

Motion of Earth’s crust—so Motion of Earth’s crust—so slowslow we don’t noticewe don’t notice


Problem:Problem:– A storm is 10 km away and is A storm is 10 km away and is

moving at a speed of 60 km/h. moving at a speed of 60 km/h. Should you be worried?Should you be worried?– It depends on It depends on

the storm’s the storm’s direction!direction!


VelocityVelocity– speed in a given directionspeed in a given direction– can change even when the speed is can change even when the speed is

constant!constant!

AccelerationAcceleration

AccelerationAcceleration—change in velocity’s —change in velocity’s raterate– PositivePositive acceleration—speed is acceleration—speed is

increasing.increasing.– NegativeNegative acceleration—speed is acceleration—speed is

decreasing.decreasing.– When an object changes speed or When an object changes speed or

directiondirection, it is accelerating., it is accelerating.


Calculating accelerationCalculating acceleration– Acceleration = Acceleration = change inchange in velocityvelocity/time/time– Change in velocity = Change in velocity = final velocityfinal velocity – initial – initial

velocityvelocity– Unit for acceleration—meters per second Unit for acceleration—meters per second

squaredsquared– PositivePositive acceleration—positive number with a acceleration—positive number with a

positive slope on a velocity-time graphpositive slope on a velocity-time graph– NegativeNegative acceleration—negative number with acceleration—negative number with

a negative slope on a velocity-time grapha negative slope on a velocity-time graph


Amusement park acceleration—Amusement park acceleration—Roller coastersRoller coasters– Changes in Changes in speedspeed cause cause

acceleration.acceleration.– Changes in Changes in directiondirection cause cause

acceleration.acceleration.


AccelerationAcceleration– the rate of change of velocitythe rate of change of velocity– change in speed change in speed oror direction direction

t

vva if

a: acceleration

vf: final velocity

vi: initial velocity

t: time

a

vf - vi

t

Motion and ForceMotion and Force

ForceForce—a push or pull that one body —a push or pull that one body applies to anotherapplies to another– A force can cause an object’s A force can cause an object’s motionmotion to to

change.change.– When two or more forces combine at the When two or more forces combine at the

same time, they create a same time, they create a net forcenet force..– Balanced forces are equal in Balanced forces are equal in sizesize and and

opposite in opposite in directiondirection..– Unbalanced forcesUnbalanced forces are unequal in size and / are unequal in size and /

or are not in the same direction.or are not in the same direction.

ForceForce

F = maF: force (N)m: mass (kg)a: accel (m/s2)

1 N = 1 kg ·m/s2

am

F

a

Fm

ForceForce

What forces are being What forces are being exerted on the football?exerted on the football?

Fkick

Fgrav

ForceForce

Balanced ForcesBalanced Forces– forces acting on an forces acting on an

object that are object that are opposite in direction opposite in direction and equal in sizeand equal in size

– no change in no change in velocityvelocity

ForceForce

Net ForceNet Force– unbalanced forces that are not opposite and unbalanced forces that are not opposite and

equalequal– velocity changes (object accelerates)velocity changes (object accelerates)

Ffriction

W

Fpull

Fnet

NN


Inertia and MassInertia and Mass– InertiaInertia—an object’s resistance to any —an object’s resistance to any

change in motionchange in motion– Objects with greater Objects with greater massmass have greater have greater

inertia.inertia.– Newton’s Newton’s first law of motionfirst law of motion —an object —an object

moving at a constant velocity keeps moving at a constant velocity keeps moving at that velocity unless a net force moving at that velocity unless a net force acts on it; an object at rest will stay at acts on it; an object at rest will stay at rest unless a net force acts on it.rest unless a net force acts on it.


Auto crashes—the law of Auto crashes—the law of inertiainertia at workat work– A passenger not wearing a seat belt A passenger not wearing a seat belt

keeps moving keeps moving forwardforward at the car’s at the car’s speed even after the car stops.speed even after the car stops.

– A passenger wearing a seat belt A passenger wearing a seat belt slows downslows down as the car slows down as the car slows down and stops.and stops.

CalculationsCalculations

What force would be required to What force would be required to accelerate a 40 kg mass by 4 m/saccelerate a 40 kg mass by 4 m/s22??

GIVEN:

F = ?

m = 40 kg

a = 4 m/s2

WORK:

F = ma

F = (40 kg)(4 m/s2)

F = 160 N

m

F

a


A 4.0 kg shotput is thrown with 30 N of A 4.0 kg shotput is thrown with 30 N of force. What is its acceleration?force. What is its acceleration?

GIVEN:

m = 4.0 kg

F = 30 N

a = ?

WORK:

a = F ÷ m

a = (30 N) ÷ (4.0 kg)

a = 7.5 m/s2

m

F

a


Mrs. J. weighs 557 N. What is her mass? Mrs. J. weighs 557 N. What is her mass?

GIVEN:

F(W) = 557 N

m = ?

a(g) = 9.8 m/s2

WORK:

m = F ÷ a

m = (557 N) ÷ (9.8 m/s2)

m = 56.8 kg

m

F

a


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

CalculationsCalculations A roller coaster starts down a hill at 10 m/s. A roller coaster starts down a hill at 10 m/s.

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

CalculationsCalculations Sound travels 330 m/s. If a lightning bolt Sound travels 330 m/s. If a lightning bolt

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


How long will it take a car traveling 30 m/s How long will it take a car traveling 30 m/s to come to a stop if its acceleration is to come to a stop if its acceleration is -3 m/s-3 m/s22??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 MotionGraphing Motion

slope =slope =

steeper slope =steeper slope =

straight line =straight line =

flat line =flat line =

Distance-Time Graph

A

B

faster speed

constant speed

no motion

speed


Who started out faster?Who started out faster?– A (steeper slope)A (steeper slope)

Who had a constant speed?Who had a constant speed?– AA

Describe B from 10-20 min.Describe B from 10-20 min.– B stopped movingB stopped moving

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

A = 80 m/minA = 80 m/min

– B = (1200m) ÷ (30min) B = (1200m) ÷ (30min) B = 40 m/minB = 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


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

Changing slope = Changing slope = changing velocitychanging velocity


0

1

2

3

0 2 4 6 8 10

Time (s)

Sp

ee

d (

m/s

)

Speed-Time Graph

slope =slope =

straight line =straight line =

flat line =flat line =

accelerationacceleration– +ve = speeds up+ve = speeds up– -ve = slows down-ve = slows down

constant accel.constant accel.

no accel. no accel. (constant velocity)(constant velocity)


0

1

2

3

0 2 4 6 8 10

Time (s)

Sp

ee

d (

m/s

)

Speed-Time GraphSpecify the time period when Specify the time period when

the object was...the object was... slowing downslowing down

– 5 to 10 seconds5 to 10 seconds speeding upspeeding up

– 0 to 3 seconds0 to 3 seconds

moving at a constant speedmoving at a constant speed– 3 to 5 seconds3 to 5 seconds

not movingnot moving– 0 & 10 seconds0 & 10 seconds

Chapter 10 Motion. Measuring Motion Motion—when an object changes its position relative to a reference point Motion—when an object changes its position.

Documents

object changes speed

speed velocityvelocityspeed

speed cause acceleration

sizeno change

doesnt change

object travels

storms direction

direction cause acceleration