Chapter 2
Motion
3 Properties of Motion:
• Speed: Change in distance per unit of
time. distance/time or v=d/t. The units can
be mi/h, km/h, m/s, etc..
• Velocity: Same as speed but direction is
specified as well.
• Acceleration: a = v/t = d/t/t = d/t2. Change
in speed per unit of time.
Calculating Speed
• A car travels 875 miles in 9 hours
The speed is:
d = 875 mi
t = 9 hr
v=d/t
v = ? mi/hr
v = 875 mi / 9 h = 97 mi/h
• A car travels at 75 mi/h for 42 hours. What is the
distance traveled?
• The distance is:
v=75 mi/h
v = d/t
t=42 h
vxt=d
d=?
d=vxt
d = 75 mi/h x 42 h = 3150 mi
Velocity
• Describes the speed and direction of an
object. e.g. 60 mi/h to the west.
60 km/h east
60 km/h northwest
Acceleration
• Rate at which motion is changed. Change
in velocity per unit of time.
• Acceleration = change in velocity
time elapsed
a = v2-v1
t
Calculation Acceleration
• A car is moving at 60 km/h. The driver
accelerates to 80 km/h. If it takes 4 seconds to
increase the velocity from 60 km/h to 80 km/h,
then what is the acceleration?:
v2=80 km/h
a=v2-v1
v1=60 km/h
t
t=4 s
a=80 km/h-60 km/h
4s
a= 20 km/h = 5 km/h
4s
s
Calculation of Acceleration
• What is the acceleration of a bicycle which goes
from rest to 12 mi/h in 1.3 minutes?
a=?
a = v 2 – v1 / t
v2 = 12 mi/h
a = 12 mi/h – 0 mi/h
1.3 min
v1 = 0 mi/h
a = 12 mi/h
t = 1.3 min
1.3 min
a = 9.2 mi/h
min
Acceleration
• Negative acceleration occurs when you
decelerate or apply brakes.
• A change in direction is a change of
acceleration, since it is a change in
velocity.
Forces
• A push or a pull that is capable of changing the
state of motion of an object.
• Net force is a sum of all the forces acting on an
object.
• Parallel forces are added. The net force is the
sum.
• Opposite parallel forces are subtracted. The net
force is the difference of the greater force minus
the smaller force.
• Two net forces which aren’t together or opposite
give rise to a new net force. You will have a new
direction and a new strength.
Forces
• Force strength and direction can be
represented by arrows. The arrowhead is
in the direction of the force exerted. The
length of the arrow is proportional to the
strength of the force.
Horizontal Motion on Land
• Forces applied to an object, such as a ball,
must counteract resistance forces for the
ball to continue to move forward at a given
speed.
• Resistance forces are Ffloor and Fair.
• The net force is the Force applied minus
the resistance forces:
Fnet=Fapplied-Fresistance
A. The ball is rolling to the left with not forces in the direction of motion. The ball slows
to a stop because the only forces are in the opposite direction.
B. A force is applied to a moving ball by a hand that moves along with the ball.
The forces applied equal the sum of the opposing
forces, so the ball continues to move with a constant velocity.
Inertia
• The behavior of matter that causes it to
persist in its state of motion.
• The tendency of an object to remain in
unchanging motion or at rest in the
absence of an unbalanced force; e.g.
friction, gravity, etc.
• Satellites continue to move through space
without additional forces being applied due
to lack of resistance.
Falling Objects
• Galileo threw a solid iron
ball and a solid wooden
ball from the top of the
tower of Pisa. Both balls
hit the ground at nearly
the same time. Therefore,
the velocity is the same.
• The velocity of an object
does not depend on its
weight. The differences
are explained by air
resistance.
Falling Objects
• Objects fall to the floor
when dropped due to the
force of gravity.
• In a free fall (no
resistance forces) the
object should cover a
distance proportional to
the square of time:
d α t 2.
• An object should fall 4
times as far in 2 s as in 1
s. (22=4), 9 times as far in
3 s (32=9)
Falling objects
• The velocity of falling
objects increases at a
constant rate. The
change in velocity in a
period of time is
acceleration, so an
object accelerates
towards the surface of
the earth due to the
force of gravity.
Falling Objects
• The acceleration is constant for all objects in
free fall. During each second of fall the object
gains 9.8 m/s in velocity.
• This gain is the acceleration of the falling object,
9.8 m/s2, or 32 ft/s2. The symbol g is used for
this. Thus g= 9.8 m/s2, or 32 ft/s2
• The acceleration of free falling objects varies
slightly from place to place on the earth’s
surface due to the earth’s spin, shape, and
distribution of mass. It is less at the equator,
more at the poles.
Newtons Laws of Motion:
Newton’s First Law of Motion
• The law of inertia: Similar to Galileo’s. Inertia is
the tendency of an object to resist changes in
motion.
• The first law: Every object retains its state of rest
or its state of uniform straight line motion unless
acted upon by an unbalanced force. The net
force must be greater than 0.
• An object moving with uniform straight line
motion will retain that motion unless a net force
causes it to speed up, slow down, or change
directions.
Newton’s Laws of Motion
Second Law of Motion
• A change of motion is evidence of the
action of a net force. This is the conclusion
from Newton’s first law.
• The acceleration of an object is inversely
proportional to its mass.
• a=F/m, so F=ma
• Newton is the metric unit for force:
1 newton (N) = 1 kg x m
s2
Newton’s Laws of Motion
Second Law of Motion
• Newton’s Second Law states that:
The acceleration of an object is directly
proportional to the net force acting on it
and is inversely proportional to the mass
of the object.
Weight and Mass
• Mass=How much an object resists a
change in its motion.
• e.g. Moving a car versus a truck, pushing
them into motion.
• Masses are measured on a balance by
comparing the force of gravity acting on a
standard mass compared to the force of
gravity acting on an unknown mass.
• Masses have units of grams or lbs/ft/s2
Mass and Weight
• Weight=The force of gravity acting on a mass. It
is a force and has units of pounds or newtons.
• F=ma (Force=mass x acceleration)
• Weight is a downward force on a falling object:
downward force = (mass)(acceleration due
to gravity)
g = acceleration due to gravity = 9.8 m/s2
weight = (mass)(g)
Mass and Weight
The pound (lb) in the English system is a unit of force or weight, not mass
1 lb = 4.5 kgxm = 4.5 N
s2
Mass and weight are proportional on a given place on earth.
Newton’s Laws of Motion:
Newton’s Third Law of Motion
• If an object changes its state of motion it is
evidence that an unbalanced force has
been applied. This is the conclusion from
the first and second laws.
• To produce a force a second object is
always pushing or pulling on a first object.
• A single force doesn’t exist by itself. There
is always a matched and opposite force
that occurs at the same time.
Newton’s Laws of Motion:
Newton’s Third Law of Motion
• Newton’s third law of motion states that:
Whenever two objects interact, the force exerted on one
object is equal in size and opposite in direction to the
force exerted on the other object.
• Forces always occur in matched pairs that act in
opposite directions and on two different bodies:
• FA due to B = FB due to A
• For every action there is an equal and opposite reaction.
• When a person walks he or she exerts force on the
ground and the ground exerts force on the person. Since
the earth is so massive the earth will not accelerate but
the person will.
Assignments for Chapter 2
• p. 57 Applying Concepts # 1, 3, 4, 5, 6, 7, 8, 9
• P. 58 Parallel Exercises Group A # 1, 2, 3, 4, 5,
6, 7, 8, 10
• New Book: p. 61-65 # 1, 2, 3, 4, 5, 6, 7, 9, 10,
11, 12, 13, 14, 16, 17, 18, 19, 21, 22, 23, 24, 25,
32, 33, 34, 37, 43
• p. 65 Group A: # 1, 2, 3, 5, 6, 8, 10, 14, 20, 21,
24
Review for Chapter 2
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3 properties of motion
Calculating speed, velocity, acceleration
Negative acceleration
Forces and Resistance
Inertia
Velocity and Acceleration during free fall
Gravitational acceleration
Newton’s 3 laws
Equation for force
Opposing forces