Transcript Chapter 2: MOTION AND SPEED

Chapter 2:
MOTION AND SPEED
Section 1—DESCRIBING MOTION
Motion occurs when an object
changes its position.
 To know whether the
position of something
has changed, you need
a reference point.
 A reference point helps
you determine how far
an object has moved.
Sometimes you may want to know not only
your distance, but also your direction from
a reference point.
 Displacement is the
distance and
direction of an
object’s change in
position from a
reference point.
DISTANCE VS. DISPLACEMENT
What is speed?
 Speed is the distance
an object travels per
unit of time.
 Any change over time is
called a rate.
 Speed is the rate at
which distance is
traveled.
CALCULATING SPEED
 Speed = distance
time
 If s = speed, d =
distance, and t = time,
this relationship can
be written as:
s = d
t
 Suppose you ran 2 km
in 10 minutes. Your
speed or rate of
change of position,
would be:
 s = d = 2 km
t
10 min
 0.2 km/min
=
CONSTANT SPEED
 If an object is in motion and neither slows down
nor speeds up, the object is traveling at a constant
speed. (Ex. Car traveling on a freeway—CRUISE
CONTROL)
CHANGING SPEED
 Much of the time, the
speeds you experience
are not constant. (Ex.
Riding a bicycle for 5
km)
CHANGING SPEED
AVERAGE SPEED
 Describes speed of
 For the bicycle trip, the
motion when speed is
changing.
 AVERAGE SPEED is the
total distance traveled
divided by the total time
of travel.
total distance traveled
was 5 km and the total
time was 15 min. or .25 h.
The AVERAGE SPEED
was: s = d = 5 km =
t 0.25 h
 20 km/h
INSTANTANEOUS SPEED
 INSTANTANEOUS
SPEED is the speed
at a given point in
time. (Ex. CAR’S
SPEEDOMETER)
VELOCITY
 VELOCITY includes
the speed of an object
and the direction of its
motion.
 Ex. HURRICANE—
traveling at a speed of
60 km/h; located 100
km east of your
location
Velocity
 VELOCITY IS SPEED WITH DIRECTION!
VELOCITY
 SPEED
same
 DIRECTION
different
(VELOCITY =
DIFFERENT)
VELOCITY
 SPEED
constant
 DIRECTION
changing
(VELOCITY = CHANGING)
VELOCITY
 SPEED
constant
 DIRECTION
changing
(VELOCITY =
CHANGING)
SPEED UNITS
REMEMBER…
 VELOCITY includes the speed and direction of
an object;
 Therefore, a change in velocity can be either a
change in how fast something is moving or a
change in the direction it is moving.
CHAPTER 2: MOTION
AND SPEED
Section 2: ACCELERATION
ACCELERATION is a change in
velocity.
 Acceleration occurs
when an object changes
its speed, its direction,
or both.
When you think of acceleration, you
probably think of something speeding up
(positive acceleration);
Calculating ACCELERATION
 Remember…
Acceleration is the rate of change in velocity.
 The change in velocity or speed is divided by
the length of the time interval over which the
change occurred.
 Acceleration = change in velocity
time
How is the change in velocity
calculated?
 Always subtract the initial velocity—(the
velocity at the beginning of the time
interval)—from the final velocity—(the
velocity at the end of the time interval).
Change in velocity = final vel. – initial vel.
 Change in velocity = vf – vi
 a = (vf – vi) =
t
(units) m/s
s
UNITS
 The SI unit for velocity is meters/second (m/s), and
the SI unit for time is seconds (s).
 So, the unit for acceleration is
meters/second/second. This unit is written as m/s2
and is read “meters per second squared.”
CALCULATING POSITIVE ACCELERATION
 Suppose a jet airliner starts at rest at the end of a
runway and reaches a speed of 80 m/s in 20 s.
Because it started from rest, its initial speed was
zero. Its acceleration can be calculated as follows:
 a = (vf – vi) = (80m/s-0m/s)= 4 m/s2
t
20s
CALCULATING NEGATIVE
ACCELERATION
 Now imagine a skateboarder is moving at a speed
of 3 m/s and comes to a stop in 2 s. The final
speed is zero and the initial speed was 3 m/s. The
skateboarder’s acceleration is calculated as
follows:
 a = (vf – vi) = (0m/s-3m/s)= -1.5 m/s2
t
2s
ACCELERATION…
 Will always be positive if an object is speeding up
 Will always be negative if an object slowing down
Chapter 2: MOTION
AND SPEED
Section 3—MOTION AND FORCES
What is a force?
 A force is a push or a pull that one body
exerts on another.
 A force can cause the motion of an object to
change.
OBVIOUS VS. NOT SO OBVIOUS
 Some forces are
obvious…the force
applied to a soccer ball
as it is kicked into the
goal
 Some forces are not
so obvious…the force
of the floor being
exerted on your feet
OR
gravity pulling down on
your body
BALANCED FORCES
 When two or more forces act on an object at
the same time, the forces combine to form
the net force.
 What is the net force acting on this box?
The net force on the box is zero,
because the two forces cancel each
other.
 Forces on an object that are equal in size and
opposite in direction are called balanced
forces.
UNBALANCED FORCES
 When two students
are pushing with
unequal forces in
opposite directions.
 A net force occurs in
the direction of the
larger force.
UNBALANCED FORCES
 The students are
pushing on the box
in the same
direction.
 The net force is
formed by adding
the two forces
together.
IT IS IMPORTANT TO REMEMBER…
 Students often assume that NO MOTION =
NO FORCE (not true), but an object’s lack of
motion is because the forces acting on it are
balanced.
 NO MOTION = BALANCED FORCES
 MOTION = UNBALANCED FORCES
What is inertia?
 Inertia is the tendency of an object to resist any
change in motion. (NEWTON’S 1st LAW—The
Law of Inertia)
 QUESTION: Would a bowling ball or a table
tennis ball have a greater inertia? Why?
Remember—Mass is the amount of matter in
an object, and a bowling ball has more mass
than a table-tennis ball.
 The INERTIA of an
object is related to its
MASS.
 The greater the mass of
an object, the greater its
inertia.
 ↑ MASS = ↑ INERTIA
British Scientist Sir Isaac Newton (1642-1727)
was able to describe the effects of forces on
the motion of objects.
 These rules are known as Newton’s Laws of
Motion.
 According to 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 (Part I—Car-CC). Also, if an object is at
rest, it stays at rest, unless a net force acts on
it (Part II—Soccer ball).
SHORT VERSION—Newton’s 1st
Law
• An object will
resist any
change in
motion.
What happens in a car crash?
 This can be explained
by the law of inertia…
 When a car traveling
about 50 km/h collides
head-on with
something solid, the
car crumples, slows
down, and stops
within appproximately
0.1s.
A passenger without a seatbelt…
 Will continue to move forward at the same
speed that the car was traveling
 Within 0.02 s after the car stops, unbelted
passengers slam into the steering wheel,
dashboard, etc.
 They are traveling at the car’s original speed of
50 km/h