Newton’s Laws of Motion
Newton’s 1st Law
Newton’s 2nd Law and
Friction
Newton’s 3rd Law and Law
of Universal Gravitation
Newton’s 1st Law
Law of Inertia
Everything in motion stays in motion,
and everything at rest stays at rest
unless an outside force acts upon it.
Everything in motion and at rest has
inertia.
Mass
Mass determines the amount
of inertia possessed by an
object.
Mass is the amount of matter
in an object.
Mass is NOT Volume
Volume is the amount of space
an object takes up.
EXAMPLE:
A pillow will have a volume larger
than a car battery but less mass.
Mass is NOT weight
Weight is the force of gravity
acting on a body
Objects in space will still have
mass but will not have weight.
Metric Units: mass - kilogram
weight - Newton
On the surface of the earth,
mass and weight are
proportional.
1 kg = 9.8 Newtons
Calculating the weight of an
object
Since weight depends on the
mass of an object AND the
gravitational pull of the earth,
we can say that
Weight = mass x gravity
Or
Weight = mg
The weight of an object is
measured in Newtons.
Because weight depends on
gravity, it is also a force.
What is the weight of a 500
gram rock?
1. Convert to standard units
500 grams = .5 kg
2. Remember the constant for
gravity is 9.8 m/s2.
3. Plug into the equation and
solve.
Weight = mg
Weight = (.5 kg)(9.8 m/s2)
Weight = 4.9 N (Newtons)
Questions
1. Your empty hand is not harmed if it bangs against the
2.
3.
4.
5.
wall, but it is harmed if you are carrying a heavy load.
Why?
Does a person diet to lose mass or to lose weight?
Can the force of gravity on a 1 kg mass ever be greater
than on a 2 kg mass? Explain how.
A car at a junk yard is compressed until its volume is less
than 1 cubic meter. Has its mass changed? Has its weight
changed? Has its volume changed? Explain.
If you jump up in a bus that is moving at a constant
velocity, will you land farther back in the bus? Explain.
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Answers
1. Your empty hand has little
inertia therefore it cannot
exert a very large force on
the wall (F=mg).
When you’re holding a
heavy weight your hand’s
inertia is increased by the
mass of the weight.
2. You lose weight as a result
of losing mass. You lose fat
which has mass and if in a
gravity field also has weight.
3. The pull of gravity or weight
depends on “g”. If g is small
enough (say on the moon), a
1 kg mass on Earth (9.8N) will
weigh more than a 2 kg mass
on the moon (3.2N).
3. Continued
F=mg for the moon would be
(2kg)(1.6m/s2) which = 3.2N
4. Mass is the same, same
amount of stuff-mass is a
measure of the amount of
stuff or inertia an object has.
Weight is the same since
same amount of mass and
same force of gravity.
4. Continued
Volume is different since the
car is taking up less space.
5. NO. You and the bus are
moving together at a constant
speed. You will appear to move
straight up and down to an
observer in the bus, but will
appear to move in a curved
projectile path to an observer
on the ground.
Newtons 2nd Law of Motion
Law of Acceleration
What causes an object to
accelerate?
-or change its state of
motion?
NET FORCE
What is a Net Force?
A net force is the combination of all forces
acting on an object
Applied Forces
Net Force
15 N
5N
10 N
5N
5N
10 N
Equilibrium
Applied Forces
Net Force
0N
5N
5N
If the net force is zero, the motion
of the object will not change.
This is a condition called
EQUILIBRIUM.
If an object is in equilibrium, we
have to go back to the Law of
Inertia.
Acceleration
Acceleration is directly related to
the net force.
Acceleration ~ Net Force
The larger the net force is acting on an
object, the greater the acceleration of
the object.
Acceleration
Acceleration is inversely proportional
to the mass of an object
Acceleration ~ 1
mass
Try to push a 250 pound crate. Now try to
push a 25 pound crate. Which could you
move faster?
Newton’s 2nd Law of Motion
The acceleration of an object is
directly proportional to the net force
acting on it, is in the same direction
as the net force, and inversely
proportional to the mass of the
object.
F
a =m
OR
F = ma
F
m
a
Friction
Friction is a force that affects
motion by slowing an object
down.
It is always opposite to the
direction of motion and affects
how much an object can
accelerate.
Friction is present because of
irregularities in the surfaces of
objects that are in contact with
one another.
Fluids like water and air also exert
a friction force on a moving
object.
Friction
Direction of motion
FRICTION BETWEEN TIRES AND ROAD
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Newton’s 3rd Law of Motion
Action / Reaction
Law of Interaction
Newton realized that force is not
a thing in itself, but part of an
interaction between one thing
and another.
Forces always occur in pairs
One force is called the action
force and the other the reaction
force.
Action: Object A exerts a force
on Object B.
Reaction: Object B exerts an
equal and
opposite
force on Object A.
Newton’s 3rd Law
For each action force, there is an
equal and opposite reaction
force.
Whenever one object exerts a
force on a second object, the
second object exerts an
equal and opposite force on
the first object.
Action / Reaction Forces
Action: Tire pushes road
Reaction: Road pushes tire
Action: Rocket pushes gas
Reaction: Gas pushes rocket
Action: Earth pulls ball
Reaction: Ball pulls Earth
Action / Reaction Forces
Consider the apple at
rest on the table. If
we call the
gravitational force
exerted on the apple
action, what is the
reaction force? Are
there any other
action/reaction forces
present?
Action / Reaction Forces
If a cement truck and a car have a head-on collision, which
vehicle will experience the greater impact force?
A. The cement truck
B. The car
C. Both the same
D. ….it depends on other factors
Newton’s Law of Universal
Gravitation
Every object in the universe is
physically attracted to every
other object.
Examples: Earth and Sun
Earth and Moon
You and the earth
Newton’s Law of Universal
Gravitation
The amount of gravitational pull
between two objects depends upon
two things:
• The amount of mass each body
has
• How far apart the two bodies
are from
each other
Newton’s Law of Universal
Gravitation
F=
Gm1m2
d2
Where:
F = Force of gravitational pull
between the two objects
m1 and m2 are the masses of
the two bodies
d is the distance between their
centers of mass
G is the universal gravitational
constant (6.67 x 10-11 Nm2/kg2)
Can you attract another person
gravitationally?
A 50-kg person and a 75-kg person are sitting on a
bench .5 m apart. Calculate the gravitational
force each exerts on the other.
m1m2
11 (50kg)( 75kg)
F  G 2  (6.67 x10 )
2
d
.5
6
F  1.0 x10 N
Which is so small, it is unnoticed except with very
delicate instruments.
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