3 Laws of Motion

Essential Question:
How are the concepts of motion
helpful and important to one’s life?

Sir Isaac Newton
(1643-1726)
He was an English
physicist, mathematician,
astronomer, natural
philosopher, alchemist,
and theologian and one
of the most influential
men in human history.

Newton’s Laws of Motion:
LAW OF INERTIA
LAW OF ACCELERATION
LAW OF INTERACTION

Newton’s First Law:
LAW OF INERTIA

Inertia
- the tendency of objects to resist
changes in state of motion
States of motion may be:
-At rest
-Moving with a constant velocity
-Moving with changing velocity
(Accelerating)

Law of Inertia
An object at rest will remain at rest
and an object in motion will remain
in motion moving with constant
velocity unless acted upon by a net
force.

Law of Inertia
An object at rest will remain at rest
and an object in motion will remain
in motion moving with constant
velocity unless acted upon by a NET
FORCE.

net force
- unbalanced force
balanced forces
- forces that are equal in
magnitude and opposite in
direction
equilibrium
- the state in which all forces are
balanced

Law of Inertia
Force, in its simplest sense, is a push
or a pull.
It can be an influence capable of
producing a change in the state of
motion of an object.

Law of Inertia
Types of Forces:
contact forces – forces that require
physical contact between objects
noncontact forces - forces that do
not require physical contact between
objects

Mass and Inertia
 The tendency of objects to resist
changes in state of motion varies
with mass.
 A more massive object has a
greater tendency to resist changes
in its state of motion.

Mass and Inertia
Which has more inertia?

Everyday Applications of the 1st
Law
• The head of a
hammer can be
tightened onto
the wooden
handle by
banging the
bottom of the
handle against a
hard surface.

Law
• To dislodge ketchup
from the bottom of a
ketchup bottle, it is
often turned upside
down and thrusted
downward at high
speeds and then
abruptly halted.

Law
• A brick is painlessly
broken over the
hand of a physics
teacher by slamming
it with a hammer.
(CAUTION: do not
attempt this at
home!)

Law
•Headrests are
placed in cars to
prevent whiplash
injuries during rear-
end collisions.

Law
• While riding a
skateboard (or
wagon or bicycle),
you fly forward off
the board when
hitting a curb or
rock or other object
that abruptly halts
the motion of the
skateboard.

Check Your Understanding
1. Imagine a place in the cosmos far from
all gravitational and frictional influences.
Suppose that you visit that place (just
suppose) and throw a rock. The rock will
a.gradually stop.
b.b. continue in motion in the same
direction at constant speed.

2. Mac and Tosh are arguing in the
cafeteria. Mac says that if he flings the
Jell-O with a greater speed it will have a
greater inertia. Tosh argues that inertia
does not depend upon speed, but rather
upon mass. Who do you agree with?
Explain why.

3. Supposing you were in space in
a weightless environment, would it
require a force to set an object in motion?

4. Fred spends most Sunday afternoons
at rest on the sofa, watching pro football
games and consuming large quantities of
food. What affect (if any) does this
practice have upon his inertia? Explain.

5. Ben Tooclose is being chased through
the woods by a bull moose that he was
attempting to photograph. The
enormous mass of the bull moose is
extremely intimidating. Yet, if Ben makes
a zigzag pattern through the woods, he
will be able to use the large mass of the
moose to his own advantage. Explain this
in terms of inertia and Newton's first law
of motion.

Newton’s Second Law:
LAW OF ACCELERATION

Law of Acceleration
The acceleration of an object as
produced by a net force is directly
proportional to the magnitude of the
net force, in the same direction as
the net force, and inversely
proportional to the mass of the
object.

 Newton’s second law pertains to
the behavior of objects for which
all existing forces are NOT
balanced (there is acceleration).
 The second law states that the
acceleration of an object is
dependent upon two variables -
the net force acting upon the
object and the mass of the object.

Net Force
(N)
Mass
(kg)
Acceleration
(m/s/s)
1. 10 2
2. 20 2
3. 20 4
4. 2 5
5. 10 10
Using the equation of the second law, fill
the table below.

Finding Acceleration: Practice Problems
1. An applied force of 50
N is used to accelerate an
object to the right across a
frictional surface. The
object encounters 10 N of
friction. Use the diagram
to determine the normal
force, the net force, the
mass, and the acceleration
of the object. (Neglect air
resistance.)

2. An applied force of 20 N
is used to accelerate an
object to the right across a
frictional surface. The object
encounters 10 N of friction.
Use the diagram to
determine the normal force,
the net force, the mass, and
the acceleration of the
object. (Neglect air
resistance.)

3. A 5-kg object is
resting on top of a table.
Determine the force of
gravity, the normal
force, the net force, and
the acceleration.
(Neglect air resistance.)

1. How much net force will be required
to move a 1500 kg car with an
acceleration of 4 m/s2
?
2. What is the weight of a 50 kg sack of
rice?

Newton’s Third Law:
LAW OF INTERACTION

Law of Interaction
For every action force, there is
always an equal but opposite
reaction force.

Law of Interaction
• The action and reaction forces are
contact forces (forces acting in
contact).
• Action-reaction forces act on
different bodies.
 a wall-hand system
 a baseball-baseball bat system
• Forces acting are equal in magnitude .

Law of Interaction
1. Bowling ball pushes
pin leftwards
2. Pin pushes bowling
ball rightwards
1. Baseball pushes
glove leftwards
2. Glove pushes the
baseball rightwards

Tell the law present in each situation.
1. The bicycle moves faster as the rider pedals faster
2. A rolling ball stops when blocked by a wall
3. You feel pain when you kick the ball
4. The weight lifter lifting a barbell
5. Birds fly in V- formation
6. A vase on the table
7. Pushing a metal cabinet
8. A swimmer is propelled forward as he swims
through the water

3 Laws of Motion

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