chapter 10 motion. measuring motion motion—when an object changes its position relative to a...
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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.
Measuring MotionMeasuring Motion
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.
Measuring MotionMeasuring Motion
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
Speed & VelocitySpeed & Velocity
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!
Speed & VelocitySpeed & Velocity
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.
AccelerationAcceleration
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
AccelerationAcceleration
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
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
Motion and ForceMotion and Force
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.
Motion and ForceMotion and Force
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
CalculationsCalculations
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
CalculationsCalculations
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
CalculationsCalculations
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
CalculationsCalculations
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
Graphing MotionGraphing Motion
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
Graphing MotionGraphing Motion
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
Graphing MotionGraphing Motion
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)
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 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