Physics ch 3 9 14 2010

Chapter 3: LINEAR
• straight-line path—
linear motion
• http://highered.mcgraw
Chelcie Liu asks his students to check their neighbors and
predict which ball will reach the end of the equal-length tracks
The rules of motion involve three concepts:
Become familiar with them and be able to distinguish between them.
Here we'll consider only the simplest form of motion—that along a
straight-line path—linear motion.
Motion Is Relative
• Everything moves.
• Even things that appear at rest move. They move relative to the
sun and stars.
• You're moving at about 107,000 km/hr relative to the sun. And
even faster relative to the center of our galaxy.
• When we discuss the motion of something, we describe motion
relative to something else.
Motion Is Relative
• When walking down the aisle of a moving bus, your speed is
relative to the floor of the bus – which is likely quite different from
your speed relative to the road.
• a racing car with a speed of 300 km/hr is relative to the track.
• Unless stated otherwise, when we discuss the speeds of things in
our environment we mean relative to the surface of the Earth.
Scalars are quantities which are
fully described by a magnitude
Examples of scalar quantities are
distance, speed, mass, volume,
temperature, density and energy.
• Linear Motion
Linear motion is the movement of
an object along a straight line.
Distance vs Displacement
The total length that is
• the shortest distance of
traveled by that object.
the object from point O
Unit: metre (m)
in a specific direction.
Type of Quantity: Scalar
Unit: metre (m)
Type of Quantity: Vector
• Distance traveled =
• 200 m
• Distance is a scalar
• Displacement = 120 m, in
the direction of Northeast
• Displacement is a vector
Vectors are quantities which are
fully described by both a magnitude
and a direction.
Examples of vector quantities are
displacement, velocity,
acceleration, force,
Speed is the rate of change
in distance.
• Type of quantity:
Scalar quantity
• Speed is a measure of how fast
• for motor vehicles (or long
something moves, measured by
distances) the units kilometers
a unit of distance divided by a
per hour (km/h) or miles per
unit of time
hour (mi/h or mph) are
• Any combination of distance and
time units is legitimate for
measuring speed;
commonly used.
• shorter distances, meters per
second (m/s) are often useful
Approximate Speeds in Different Units
Velocity is the rate of change
in displacement
• Vector
Instantaneous Speed
• Cars vary in speed on a
• You can tell the speed of
the car at any instant by
looking at its
• The speed at any instant
is the instantaneous
Average Speed
• Average speed is defined as:
• the whole distance covered divided by the
total time of travel
• it doesn't indicate the different speeds and
variations that may have taken place during
shorter time intervals.
Check Yourself
1. What is the average speed of a cheetah that sprints 100 m in 4 s? How
about if it sprints 50 m in 2 s?
2. If a car moves with an average speed of 60 km/h for an hour, it will travel
a distance of 60 km.
(a) How far would it travel if it moved at this rate for 4 h? (b) For 10 h?
3. In addition to the speedometer on the dashboard of every car is an
odometer, which records the distance traveled. If the initial reading is set at
zero at the beginning of a trip and the reading is 40 km one-half hour later,
what has been your average speed?
4. Would it be possible to attain this average speed and never go faster than
80 km/h?
• Speed = distance /
• Velocity =
distance/time +
• The car on the circular
track may have a
constant speed, but its
velocity is changing
every instant. Why?
• We distinguish
• Velocity alone is
between average
assumed to mean
velocity and
velocity as we do for
• If something moves
at an unchanging or
constant velocity,
then, its average
and instantaneous
velocities will have
the same value.
• The same is true for
• Constant velocity
• A car that rounds a
means constant
curve at a constant
speed with no change
speed does not have
in direction. .
a constant velocity—
its velocity changes
as its direction
• Constant velocity
means constant
speed with no change
• Constant velocity
and constant speed,
however, can be very
in direction.
• A car that rounds a
curve at a constant
speed does not have
a constant velocity—
its velocity changes
as its direction
• Constant velocity
• A car that rounds a
means constant speed
curve at a constant
with no change in
speed does not have a
constant velocity—its
velocity changes as its
direction changes.
• The best way to imagine a situation with
several physical quantities is by drawing a
• To picture the behavior of the speed of an
object, we plot the distance on the vertical
axis and the time on the horizontal axis.
Here, the total distance travelled ( y) divided by the time
taken ( x) is the gradient of the slope. This is also equal to
the average speed of the object - remembering that
In this case, the speed is
constant as the slope of
the distance-time graph
is constant.
By re-arranging the equation we can plot slopes of either distance, or
time, on a graph to find their values. For example, we can see how to
find the distance from a speed-time graph by rearranging to get:
• We then plot a speed-time graph as
shown below:
The blue rectangle has an
area equal to the speed
multiplied by the time.
We can see from the
equation above, that this
is equal to the distance
• The speedometer of a car moving to the east
reads 100 km/h. It passes another car that
moves to the west at 100 km/h. Do both cars
have the same speed? Do they have the same
• During a certain period of time, the
speedometer of a car reads a constant 60
km/h. Does this indicate a constant speed? A
constant velocity?
• We can change the
velocity of something
by changing its speed,
by changing its
direction, or by
changing both its speed
and its direction.
Acceleration on Galileo's Inclined Planes
• Galileo lacked suitable timing devices to fime
falling objects
• he used inclined planes to slow down
accelerated motion and investigate it more
Galileo found that a ball rolling down an
inclined plane will pick up the same amount of
speed in successive seconds; that is, the ball
will roll with unchanging acceleration.
Acceleration on Galileo's Inclined Planes
• a ball rolling down a plane inclined at a certain
angle might be found to pick up a speed of 2
meters per second for each second it rolls. This
gain per second is its acceleration. Its
instantaneous velocity at 1-second intervals, at
this acceleration, is then 0, 2, 4, 6, 8, 10, and so
forth meters per second
Galileo found greater accelerations for
steeper inclines.
• The ball attains its maximum
acceleration when the incline is
tipped vertically. Then the
acceleration is the same as that of a
falling object
Free Fall
Table 3.2 shows the instantaneous speed of a
freely falling object at 1-second intervals
Free Fall
• During each second of fall, the object gains a
speed of 10 meters per second.
• Free-fall acceleration is approximately equal
to 10 m/s2
freely falling objects use g because the
acceleration is due to gravity
• g varies slightly in different locations,
dependent on mass
• Where accuracy is important, the value of 9.8
m/s2 should be used.
Free Fall
• When a falling object is free of all restraints—
no friction, air or otherwise, and falls under
the influence of gravity alone, the object is in
a state of free fall.
• The instantaneous velocity of an
object falling from rest can be
expressed in shorthand notation
as V = gt
• the instantaneous velocity or
speed in meters per second is
simply the acceleration g = 10
m/s2 multiplied by the time t in
• a falling rock is equipped with a
• In each succeeding second of fall,
you'd find the rock's speed increasing
by the same amount: 10 m/s.
How about an object
thrown straight upward?
• Once released, it continues to move upward
for a while and then comes back down.
• At the highest point, when it is changing its
direction of motion from upward to
downward, its instantaneous speed is zero.
Then it starts downward just as if it had been
dropped from rest at that height.
How about an object
thrown straight upward?
• the object slows as it rises. at the rate of 10 meters per
second each second—the same acceleration it experiences
on the way down.
the instantaneous speed at points of equal elevation in the
path is the same whether the object is moving upward or
The velocities are opposite, because they are in opposite
the downward velocities have a negative sign, indicating
the downward direction
How about an object
thrown straight upward?
• Whether moving upward or
downward, the acceleration
is 10 m/s2 the whole time.
• up positive, and down negative.
Time of Fall (seconds)
Distance Fallen
½ 10 t 2
Air Resistance
• is responsible for different
• a feather and a coin in the presence
of air fall with different
• But in a vacuum, the feather and
coin fall with the same acceleration
• it is a rate of a rate
• Acceleration is not
velocity, nor is it even a
change in velocity.
Acceleration is the rate
at which velocity itself
vertical motion
• The relationship between time up or down
and vertical height is given by
• We're talking here of vertical motion.
• How about running jumps? Hang time
depends only on the jumper's vertical speed
at launch. While airborne, the jumper's
horizontal speed remains constant while the
vertical speed undergoes acceleration.
Interesting physics!
Summary of Terms
Speed How fast something moves. The distance
traveled per unit of time.
Velocity The speed of an object and specification
of its direction of motion.
Acceleration The rate at which velocity changes
with time; the change in velocity may be in
magnitude or direction or both.
Free fall Motion under the influence of gravity
Summary of Formulas
Speed = distance/time
Average speed = total distance covered
time interval
Acceleration = change of velocity
time interval
Acceleration (linear) = change in speed
time interval
Freefall velocity from rest v = gt
Distance fallen in freefall from rest
Motion is Relative
1. As you read this, how fast are you moving relative to the
chair you are sitting on? Relative to the sun?
2. What two units of measurement are necessary for
describing speed?
3. What kind of speed is registered by an automobile
speedometer; average speed or instantaneous speed?
4. Distinguish between instantaneous speed and average
5. What is the average speed in kilometers per hour for a
horse that gallops a distance of 15 km in a time of 30 min?
6. How far does a horse travel if it gallops at an average speed
of 25 km/h for 30 min?
Motion is Relative
1. Distinguish between speed and velocity.
2. If a car moves with a constant velocity, does it also move
with a constant speed?
3. If a car is moving at 90 km/h and it rounds a corner, also at
90 km/h, does it maintain a constant speed? A constant
velocity? Defend your answer.
4. Distinguish between velocity and acceleration.
5. What is the acceleration of a car that increases its velocity
from 0, to 100 km/h in 10 s?
6. What is the acceleration of a car that maintains a constant
velocity of 100 km/h for 10 s? (Why do some of your
classmates who correctly answer the previous question get
this question wrong?)
Motion is Relative
1. When are you most aware of motion in a moving vehicle— when it is
moving steadily in a straight line or when it is accelerating? If a car moved
with absolutely constant velocity (no bumps at all), would you be aware of
2. Acceleration is generally defined as the time rate of change of velocity.
When can it be defined as the time rate of change of speed?
3. What did Galileo discover about the amount of speed a ball gained each
second when rolling down an inclined plane? What did this say about the
ball's acceleration?
4. What relationship did Galileo discover for the velocity acquired on an
5. What relationship did Galileo discover about a ball's acceleration and the
steepness of an incline? What acceleration occurs when the plane is
6. What exactly is meant by a “freely falling” object?
1. What is the gain in speed per second for a freely falling object?
2. What is the velocity acquired by a freely falling object 5 s after being
dropped from a rest position? What is it 6 s after?
3. The acceleration of free fall is about 10 m/s2. Why does the seconds unit
appear twice?
4. When an object is thrown upward, how much speed does it lose each
5. What relationship between distance traveled and time did Galileo discover
for accelerating objects?
6. What is the distance fallen for a freely falling object 5 s after being
dropped form a rest position? What is it 6 s after?
7. What is the effect of air resistance on the acceleration of falling objects?
What is the acceleration with no air resistance?
8. Consider these measurements: 10 m, 10 m/s, and 10 m/s2. Which is a
measure of distance, which of speed, and which of acceleration?
• Stand flatfooted next to a wall and make a
mark at the highest point you can reach. Then
jump vertically and mark this highest point.
The distance between the marks is your
vertical jumping distance. Use this to calculate
your hang-time.
1.A person's hang time would be considerably
greater on the moon. Why?
2.Make up two multiple-choice questions that
would check a classmate's understanding of
the distinction between velocity and
• Two balls are released simultaneously from
rest at the left end of equal-length tracks A
and B as shown. Which ball reaches the end of
its track first?
• Refer to the pair of tracks above. (a) On which
track is the average speed greater? (b) Why is
the speed of the ball at the end of the tracks
the same?
Why does a stream of water get
narrower as it falls from a faucet?
3. An artillery shell is shot with an initial velocity of 20 m/s, at an
angle of 60 degrees to the horizontal. At the top of the trajectory,
the shell explodes into two fragments of equal mass. One
fragment, whose speed immediately after the explosion is zero,
falls vertically. How far from the gun does the other fragment
land, assuming the terrain is level and that air drag is negligible?
• 4. A railroad flatcar of weight W can roll
without friction along a straight horizontal track.
Initially a man of weight w is standing on the car,
which is moving to the right with a speed Vo. See
Figure. What is the change in the velocity of the
car if the man runs to the left (in the Figure) so
that his speed relative to the car is Vrel?
• An object, with mass m and speed V relative
to an observer explodes into two pieces, one
three times as massive as the other; the
explosion takes place in deep space. (no
external forces) The less massive piece stops
relative to the observer. How much kinetic
energy is added to the system in the
explosion, as measured in the observer’s
frame of reference?

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