Transcript Chapter 10 Motion Measuring Motion

Chapter 10 Motion
Measuring Motion

Motion—when an object changes its
position relative to a reference point
– Distance—how far an object has moved
– Displacement—distance and direction of
an object’s change of position from a
starting point
Motion

Problem:
– Is your desk moving?

We need a reference point...
– nonmoving point from which motion is
measured
Motion

Motion
– Change in position in relation to a
reference point.
Reference point
Motion
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.
Measuring Motion

Speed—distance an object travels per unit
of time
– Rate—any change over time
– Calculation for speed: speed = distance/time
– Speed that doesn’t change over time—constant
speed
– Speed is usually not constant; usually an object
has changing speed.
– Average speed—speed of motion when speed is
changing: speed = total distance/total travel
time
– Instantaneous speed—speed at any given point
in time
Speed
d

Speed
– rate of motion
– distance traveled per unit time
v t
distance
speed 
time
Speed & Velocity

Instantaneous Speed
– speed at a given instant

Average Speed
total distance
avg. speed 
total time
Measuring Motion

A distance-time graph displays motion
of an object over time.
– Plot distance on a(n) vertical axis.
– Plot time on a(n) horizontal axis.


Velocity—speed and direction of an
object’s motion
Motion of Earth’s crust—so slow we
don’t notice
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!
Speed & Velocity

Velocity
– speed in a given direction
– can change even when the speed is
constant!
Acceleration

Acceleration—change in velocity’s rate
– Positive acceleration—speed is increasing.
– Negative acceleration—speed is
decreasing.
– When an object changes speed or
direction, it is accelerating.
Acceleration

Calculating acceleration
– Acceleration = change in velocity/time
– Change in velocity = final velocity – initial
velocity
– Unit for acceleration—meters per second
squared
– Positive acceleration—positive number with a
positive slope on a velocity-time graph
– Negative acceleration—negative number with a
negative slope on a velocity-time graph
Acceleration

Amusement park acceleration—Roller
coasters
– Changes in speed cause acceleration.
– Changes in direction cause acceleration.
Acceleration
vf - vi
a t

Acceleration
– the rate of change of velocity
– change in speed or direction
a
v f  vi
t
a:
vf:
vi:
t:
acceleration
final velocity
initial velocity
time
Motion and Force

Force—a push or pull that one body
applies to another
– A force can cause an object’s motion to
change.
– When two or more forces combine at the
same time, they create a net force.
– Balanced forces are equal in size and
opposite in direction.
– Unbalanced forces are unequal in size
and / or are not in the same direction.
Force
F
a
m
F = ma
F
m a
F: force (N)
m: mass (kg)
a: accel (m/s2)
1 N = 1 kg ·m/s2
Force
What forces are being
exerted on the football?
Fkick
Fgrav
Force

Balanced Forces
– forces acting on an
object that are opposite
in direction and equal
in size
– no change in velocity
Force

Net Force
– unbalanced forces that are not opposite and equal
– velocity changes (object accelerates)
Fnet
Ffriction
Fpull
N
N
W
Motion and Force

Inertia and Mass
– Inertia—an object’s resistance to any
change in motion
– Objects with greater mass have greater
inertia.
– Newton’s first law of motion —an object
moving at a constant velocity keeps
moving at that velocity unless a net force
acts on it; an object at rest will stay at
rest unless a net force acts on it.
Motion and Force

Auto crashes—the law of inertia at
work
– A passenger not wearing a seat belt
keeps moving forward at the car’s speed
even after the car stops.
– A passenger wearing a seat belt slows
down as the car slows down and stops.
Calculations

What force would be required to accelerate a
40 kg mass by 4 m/s2?
GIVEN:
WORK:
F=?
m = 40 kg
a = 4 m/s2
F = ma
F
m a
F = (40 kg)(4 m/s2)
F = 160 N
Calculations

A 4.0 kg shotput is thrown with 30 N of force.
What is its acceleration?
GIVEN:
WORK:
m = 4.0 kg
F = 30 N
a=?
a=F÷m
F
m a
a = (30 N) ÷ (4.0 kg)
a = 7.5 m/s2
Calculations

Mrs. J. weighs 557 N. What is her mass?
GIVEN:
WORK:
F(W) = 557 N
m=?
a(g) = 9.8 m/s2
m=F÷a
F
m a
m = (557 N) ÷ (9.8 m/s2)
m = 56.8 kg
Calculations

Your neighbor skates at a speed of 4 m/s. You
can skate 100 m in 20 s. Who skates faster?
GIVEN:
WORK:
d = 100 m
t = 20 s
v=?
v=d÷t
d
v t
v = (100 m) ÷ (20 s)
v = 5 m/s
You skate faster!
Calculations

A 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:
WORK:
vi = 10 m/s
t=3s
vf = 32 m/s
vf - vi
a=?
a = (vf - vi) ÷ t
a t
a = (32m/s - 10m/s) ÷ (3s)
a = 22 m/s ÷ 3 s
a = 7.3 m/s2
Calculations

Sound 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:
WORK:
v = 330 m/s
t=d÷v
d = 1km = 1000m
t = (1000 m) ÷ (330 m/s)
t=?
t
=
3.03
s
d
v t
Calculations

How long will it take a car traveling 30 m/s to
come to a stop if its acceleration is
-3 m/s2?
GIVEN:
WORK:
t=?
vi = 30 m/s
vf = 0 m/s
a = -3 m/s2
t = (vf - vi) ÷ a
t = (0m/s-30m/s)÷(-3m/s2)
vf - vi
a t
t = -30 m/s ÷ -3m/s2
t = 10 s
Graphing Motion
Distance-Time Graph
A

slope =

steeper slope =
speed
faster speed
B

straight line =
constant speed

flat line =
no motion
Graphing Motion
Distance-Time Graph

A
Who started out faster?
– A (steeper slope)

Who had a constant speed?
– A

Describe B from 10-20 min.
– B stopped moving
B

Find their average speeds.
– A = (2400m) ÷ (30min)
A = 80 m/min
– B = (1200m) ÷ (30min)
B = 40 m/min
Graphing Motion
Distance-Time Graph
400

Distance (m)
300
200

100
0
0
5
10
Time (s)
15
20
Acceleration is
indicated by a curve on
a Distance-Time graph.
Changing slope =
changing velocity
Graphing Motion
Speed-Time Graph
3

slope = acceleration
– +ve = speeds up
– -ve = slows down
Speed (m/s)
2

1

straight line =
constant accel.
flat line =
no accel.
(constant velocity)
0
0
2
4
6
Time (s)
8
10
Graphing Motion
Speed-Time Graph
Specify the time period when the
object was...
 slowing down
3
– 5 to 10 seconds

2
speeding up
Speed (m/s)
– 0 to 3 seconds

moving at a constant speed
– 3 to 5 seconds
1

not moving
– 0 & 10 seconds
0
0
2
4
6
Time (s)
8
10