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Chapter 13
The Nature of Forces
What is Force?
A force is a push or pull.
• The wind pushes against a sail or a magnet pulls
iron toward it
A force gives energy to an object,
sometimes causing it to start moving, stop
moving or change direction.
• If force is increased then movement increases
You exert a force on an object & the object
exerts a force on you.
Combining Forces
• Force involves direction
1. Same Direction = when 2 forces act in the
same direction they are added together
Total Force on an object is in one direction,
the force is called Unbalanced
2. Opposite Direction = when 2 forces act in
the opposite direction they are subtracted
Forces that are in opposite direction
and are equal in size are Balanced
Combining Forces cont.
+
=
=
+
+
0
=
Friction: A Force Opposing Motion
• Friction is a force that exists between 2
objects that are touching.
• It is a force that acts in the opposite
direction of a moving object.
• Friction will cause a moving object to slow down
and finally stop.
Friction depends on:
1. How hard the surfaces of the touching objects
are forced together

The heavier the objects, the more friction there will be & the
harder it is to move them.
2. The materials that the surfaces of the objects are
made of
 You must exert a force that is larger than the
force of friction in order to move an object.
Types of Friction
1. Sliding Friction – when solid objects slide
over each other
2. Rolling Friction – produced by objects
such as wheels & ball bearings
 Which has less friction? Why?
3. Fluid Friction – force exerted by a fluid
such as water, oil, air
 Air Resistance slows down falling objects
Additional Facts ….
• Lubricants = “slippery” substances that help
reduce friction
Change from sliding to fluid friction
Grease, oil, wax
• Why is Friction a Good Thing?
Sir Isaac Newton
• Born: 4 Jan 1643 in
Lincolnshire, England
• Died: 31 March 1727 in
London, England
• Newton’s Laws describe all
states of motion – at rest,
constant motion & accelerated
motion
Newton’s Laws of Motion
•
First Law = states that an object at rest
will remain at rest or an object in motion
will remain in motion until acted upon by
an unbalanced force
 Inertia – matter tends to resist any change in
motion; the more mass an object has the more
inertia it has
Newton’s Second Law
Force = Mass X Acceleration (F=M*A)
The greater the force, the more the acceleration
The more mass (thus more inertia) an object has
the greater the force required for acceleration of
the object
 Newton – unit of measurement for Force
The force required to accelerate 1 kg of mass 1 m/s/s
Newton’s Third Law
• States that for every action, there is an equal
and opposite reaction
What’s going to happen to the car? In what direction will it go?
GRAVITY
• In the 1500’s, the Italian
scientist Galileo dropped 2
cannonballs of different
masses from the Leaning
Tower of Pisa.
Both cannonballs landed at the
same time.
WHY?
Falling Objects …
Galileo’s discovery was important to Newton
1. All falling objects accelerate at the same
rate
2. The acceleration of a falling object is due to
the force of gravity between the object & the
Earth.
3. Acceleration due to gravity (g) near the Earth’s
surface is 9.8 m/s/s
Newton’s Law of Universal
Gravitation
• States that all objects
in the universe attract
each other by the force
of gravity
•
The size of the force
depends on:
1. The masses of the 2
objects
2. The distance between
the objects
Force of gravity increases as mass of the object increases
Gravitational force decreases as distance between objects
increases
Weight and Mass
• Weight is a measure of the force of gravity
on an object (measured in Newtons)
• Weight = Mass X Acceleration due to Gravity
Wt = M * g
Gravity = 9.8 m/s/s or 9.8 N