MOTION

Investigate the relationship

between the amount of force
applied and
the mass of the object to the
amount of change in the
object’s motion.
(S8FE-1a-15)
And, specifically you are to:
1. differentiate balanced and unbalanced
forces;
2. investigate the effects of force and mass
to the motion of the object;
3. relate first law of Newton to the second
law; and
4. cite situations in life where Newton’s
laws are applied.
WHAT IS
FORCE?
A push or pull that acts on an
object
Can cause a resting object to
move
Can accelerate a moving
object by changing its speed or
direction
a) Will this object move by itself?
b) How can we make this object move?
c) While it is moving, how can we make the object speed
up or slow down?
d) How can make it stop?
e) How can we make it change its direction?
A VECTOR QUANTIY IT
HAS BOTH MAGNITUDE
AND DIRECTION.
How is force measured?
Unit of Force
 Newton (N)
 1 kg to accelerate 1 m/s2

kg  m
1N 
s 2
 Combining Forces
Forces in the same direction are
added together
Force in the opposite direction
are subtracted
Net Force
Overall force acting on an object
Balanced vs. Unbalanced Forces

 Balanced
 Combine to produce a net force of zero
 No change in the object’s motion
 Unbalanced
 Net force equals the size of the larger
force minus the size of the smaller force
 Net force does not equal zero
 Causes an object to accelerate
 Friction

 Force that opposes the motion of

objects that touch as they move past
each other
 Acts at the surface where objects
are in contact
 Gravity
 Force that acts
between two masses
 Attractive force
 Pulls objects together
 Earth’s gravity
 Acts downwards
towards the center of
the earth
EVALUATION
1. A book is at rest on top of a table. Which of
the following is correct?

A. There is no force acting on the book.

B. The book has no inertia.
C. There is no force acting on the table.
D. The book is in equilibrium.
2. Which of the following situations involves
friction?

A. A bicycle rolling down a hill

B. A baseball player sliding into 2nd base
C. A diver falling through the air to a pool
D. All of the above experience some friction.
3. What is gravity?

A. Newton’s first law

B. The force that objects exert on each
other because of their masses
C. The downward pull on the Earth
D. The friction that an object has put on it
4. Which is the best example of gravity?

A. A car hits a tree, and its motion stops

B. A breeze blows, and a sailboat moves
C. A book is pushed, and it moves across the
table
D. A person drops a ball, and it falls to the
ground
5. How does Earth ‘s gravity affect
objects near Earth?
A. It pushes them away.
B. It pulls them in
C. It makes them larger.
D. It makes them move faster.
 Newton’s
Laws of Motion
I. Law of Inertia
II. F=ma
III. Action-Reaction
 Newton’s Laws of Motion
 1st Law – An object at rest will stay at rest,
and an object in motion will stay in motion at
constant velocity, unless acted upon by an
unbalanced force.
 2nd Law – Force equals mass times
acceleration.
 3rd Law – For every action there is an equal
and opposite reaction.
 1st Law of Motion
(Law of Inertia)

An object at rest will stay at

rest, and an object in motion
will stay in motion at constant
velocity, unless acted upon by
an unbalanced force.
 1 Law
st

Inertia is the
tendency of an
object to resist
changes in its
velocity:
whether in
motion or These pumpkins will not move
motionless. unless acted on by an unbalanced
force.
 1 Law
st

 Once airborne,
unless acted on
by an
unbalanced
force (gravity
and air – fluid
friction), it
would never
stop!
1 Law
st

Unless acted
upon by an
unbalanced
force, this golf
ball would sit on
the tee forever.
 Why then, do we observe every
day objects in motion slowing
down and becoming motionless
seemingly without an outside
force?
It’s a force we sometimes cannot see –
friction.
 What is this unbalanced force that acts on
an object in motion?

There are four main types of friction:

Sliding friction: ice skating
Rolling friction: bowling
Fluid friction (air or liquid): air or water
resistance
Static friction: initial friction when moving
an object
Newtons’s 1st Law and You

Don’t let this be you. Wear seat belts.

Because of inertia, objects (including you) resist changes
in their motion. When the car going 80 km/hour is stopped
by the brick wall, your body keeps moving at 80 m/hour.
2 Law
nd
 2 Law
nd

The net force of an

object is equal to the
product of its mass and
acceleration, or F=ma.
 2 Law
nd

When mass is in kilograms and

acceleration is in m/s/s, the unit of
force is in newton (N).
One newton is equal to the force
required to accelerate one kilogram
of mass at one
meter/second/second.
 2nd Law (F = m x a)
 How much force is needed to accelerate a
1400 kilogram car 2 meters per second/per
second?
 Write the formula
F=mxa
 Fill in given numbers and units
 F = 1400 kg x 2 meters per second/second
 Solve for the unknown
 2800 kg-meters/second/second or 2800 N
 If mass remains constant,
doubling the acceleration,
doubles the force. If force
remains constant, doubling
the mass, halves the
acceleration.
Newton’s 2nd Law proves that different masses
accelerate to the earth at the same rate, but with
different forces.
• We know that objects
with different masses
accelerate to the
ground at the same
rate.
• However, because of
the 2nd Law we know
that they don’t hit the
ground with the same
force.
F = ma F = ma
98 N = 10 kg x 9.8 m/s/s 9.8 N = 1 kg x 9.8
m/s/s
 3 Law
rd

For every action, there is an

equal and opposite reaction.
3 Law
rd

According to Newton,
whenever objects A and
B interact with each
other, they exert forces
upon each other. When
you sit in your chair,
your body exerts a
downward force on the
chair and the chair
exerts an upward force
on your body.
 3 Law
rd

There are two

forces resulting
from this
interaction - a force
on the chair and a
force on your body.
These two forces
are called action
and reaction forces.
 Newton’s 3rd Law in Nature

 Consider the propulsion of a

fish through the water. A fish
uses its fins to push water
backwards. In turn, the water
reacts by pushing the fish
forwards, propelling the fish
through the water.
 The size of the force on the
water equals the size of the
force on the fish; the direction
of the force on the water
(backwards) is opposite the
direction of the force on the
fish (forwards).
3rd Law

Flying gracefully through

the air, birds depend on
Newton’s third law of
motion. As the birds
push down on the air
with their wings, the air
pushes their wings up
and gives them lift.
Consider the flying motion of birds. A
bird flies by use of its wings. The wings
of a bird push air downwards. In turn,
the air reacts by pushing the bird
upwards.
The size of the force on the air equals
the size of the force on the bird; the
direction of the force on the air
(downwards) is opposite the direction
of the force on the bird (upwards).
Action-reaction force pairs make it
possible for birds to fly.
Other examples of Newton’s Third Law

The baseball
forces the bat to
the left (an
action); the bat
forces the ball
to the right (the
reaction).