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Chapter 7
Motion & Forces
7.1
Motion

Speed & Velocity
 An object is moving if it changes position
against a background that stays the same.
 The stationary background is the frame of
reference.
 Speed: describes how fast an object moves.
 How do we determine speed?
Speed measurements involve time &
distance

Speed: distance traveled per unit of time.
 The SI unit for speed is meters per second or
m/sec or sometimes km/h (kilometers per
hour) or in United States mph (miles per hour)
Constant speed is the simplest type of
motion

When an object travels equal distances in
equal amounts of time, it has constant speed.
 Ex.: Cruise control in a car.
 96 m/s means that you travel 96 meters every
second of time, just like 60 mph means that
you travel 60 miles each hour of time.
Distance–Time Graph

Distance plotted on vertical axis (y-axis).
 Time plotted on horizontal (x-axis).
 Constant speed results in a straight line on
distance-time graph. The slope is the speed.
 The steeper the slope – the faster the speed. See
graph p. 219

An object at rest has a slope of zero, which is
graphed as a horizontal line.
Speed = distance/time

Speed = d / t [distance divided by time]
 Most objects do not have constant speed.
 Use the above formula to calculate average
speed.
 Average speed is the total distance / total time.
 Avg. speed does NOT tell you the speed at any
given time.
Distance (m)
faster
fast
Not moving
(i.e. no change in distance)
Slope = 0
Time (s)
Velocity denotes both speed &
direction

4.5 m/s north or 4.5 m/s toward the highway
 Velocity changes if either speed or direction changes.
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See Math Skills p. 221
Law of Conservation of Momentum: the total amount
of momentum in a system is conserved.
 The total momentum of 2 cars that crash is the same
after the crash as it was before the crash.
 The car with greater momentum will determine which
way the cars will move after the collision.
Momentum

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Momentum: a quantity defined as the product of mass
& velocity. [Mass times Velocity]
A larger object moving the same speed as a smaller
object has more momentum.
Momentum = mass x velocity
P=mv
Momentum includes direction (velocity). Its momentum
is in the same direction as its velocity.
No movement = no momentum. See Math Skills p. 223
7.2
Acceleration & Force

Acceleration: change in velocity divided by the
time interval in which the change occurred.
 Any change in velocity is acceleration…i.e.
any change in speed or direction.
 Acceleration occurs if you speed up, slow
down or change direction.
 Acceleration is ALWAYS in the direction of
the acting force.
Acceleration
Acceleration = final velocity – initial velocity
time interval

The SI unit for acceleration is meters per second per
second or m/s/s or m/s2

If acceleration is a small number, velocity is changing
gradually. If acceleration is a large number, the
velocity is changing more rapidly.

A decrease in speed is negative acceleration and will
result in your answer being a negative number.
See Math Skills p. 226
Acceleration can be determined from a
velocity-time graph

On a velocity-time graph, velocity goes on the
vertical axis and the time is on the horizontal axis.
 A slanted straight line indicates constant acceleration.
i.e. the velocity changes the same amount each time.
(slope of line = acceleration)
 Acceleration is zero if velocity is constant (because there is
no change in speed or direction) See graphs p. 227

Movement w/constant speed in a straight line is constant
velocity.
Velocity (m/s)
Acceleration
Speeding up
quickly
Acceleration
Speeding up
slowly
No acceleration i.e. (constant velocity)
Slope = 0
Time (s)
Force:
Push
or
Pull
Force

Force: the cause of acceleration, or change in an
objects velocity.
 When you throw or catch a ball you exert a force to
change the ball’s velocity.
 Balanced Forces: forces that act on an object that
combine to produce a net force of zero.
 Balanced forces are equal and opposite i.e. A tug of
war (<<< >>>) or an offensive lineman pushing a
defensive lineman, but there is no acceleration.
(>>> <<<)
Unbalanced forces

Unbalanced Forces result in a net force!
 Someone “wins” in the tug of war & football
linemen examples.
 Balanced forces never change velocity
(NO ACCELERATION)
 Unbalanced forces always change velocity
(ACCELERATION)
Friction & Air resistance

Friction is a force between 2 objects in contact
with each other that opposes their motion.
 Ex.: a shopping cart rolling along a sidewalk
will eventually come to a stop. It stops because
of friction.
 Friction also acts on objects that are not
moving. A car parked on hill with its parking
brake applied or someone sitting in a chair.
Frictional force varies w/the surfaces
that are in contact

Frictional forces are great when the surfaces
are rough.
 Smooth surfaces produce less friction.
Air Resistance is a form of friction

Air resistance is the result of interaction between the
surface of a moving object & air molecules.
 The amount of air resistance depends on size & shape
of the object as well as the speed of the object.
 Larger objects create more air resistance as does
greater speed.
 Design of cars, planes etc. take into account air
resistance.
Gravity
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Gravity: the force of attraction between 2 particles of
matter due to their mass.
Newton’s Law of Universal Gravitation states that all
objects in the universe are attracted to all other
objects in the universe.
The amount of the force depends on 2 factors: 1. the
mass of the objects &
2. the distance between the 2 objects.
The greater the mass… the greater gravity is.
Gravity rapidly decreases as distance is increased.
7.3
Newton’s Laws of Motion
Newton’s First Law of Motion
An object at rest will remain at rest and an object in
motion will remain in motion at a constant velocity
unless acted upon by an unbalanced force.
 Ex.: sitting in a car that is stopping. You are pushed
backward by your seatbelt. Also, when the car turns,
you really aren’t pushed against the car. You are
really trying to continue going straight.

Inertia
Inertia really is another name for Newton’s 1st
Law of Motion.
 Inertia: the tendency of an object to remain at
rest or, if in motion, to remain in motion at a
constant velocity.
 A small mass object has low inertia – it takes
only a small force to be accelerated.
 A large mass object (such as a car) has high
inertia – it takes a large force to be accelerated.

Newton’s Second Law of Motion

The unbalanced Force acting on an object equals the
mass times the acceleration. F = m a
F=ma
 Consider pushing a full grocery compared to pushing an
empty grocery cart. You must push the full cart with
greater force for it to accelerate at the same rate.
 If you & a friend both push empty carts but you push
w/greater force, your cart will have greater acceleration.
See Math Skills p. 236.

Force is measured in Newtons. 1N = 1kg x 1 m/s2
Free fall & Weight
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Free fall: the motion of a body when only the force of
gravity is acting on it.
Free fall is due to gravity, g. g = 9/8 m/s2
If there is no air resistance all objects fall at the same
rate – 9.8 m/s2
A penny will hit the ground at the same time as a
cannon ball if dropped at same time from the same
height.
A heavier object has a greater gravitational force, but
because it has more mass it needs a greater force to
accelerate. On Earth, the ratio of weight divided by
mass of ANY object is always 9.8 m/s2.
Weight = mass x free-fall acceleration

w=mg
 The force on a body due to gravity is called its
weight. Weight is measured in Newtons
 If I have a mass of 82 kg & gravity is 9.8 m/s2.
Then I weigh 82 kg x 9.8 m/s2 = 804 N
 Mass & weight are different
 Mass = amount of matter in an object.
 Weight = force of gravity acting on your mass.
 Force & weight are the same
Velocity is constant when air
resistance balances weight

When air resistance equals the force of gravity,
objects stop accelerating. (i.e. balanced force)
 The objects continue to fall at a constant
velocity (i.e. no acceleration). This is called
terminal velocity.
 Terminal velocity is approx. 200 mi/h or
or about 320 km/h.
Newton’s Third law of Motion

For every action, there is an equal & opposite
reaction.
 Ex.: 1. kick a soccer ball – the ball experiences a
change in motion and the ball is exerting an equal &
opposite force on your foot.
2. while swimming in a pool you push off the side of
the pool. How far you go into the pool is determined
by how hard you pushed off the pool.
3. Jet planes & the space shuttles utilize Newtons 3rd
law to propel themselves. See fig. 7-17 on p. 240