Chapter 13The forces of Nature
8th grade Science
I. What is Force?
A. Isaac Newton- is considered the
founder of modern physics and is “one of
the greatest names in the history of human
thought.”
 B. Force- is any push or pull.

E.g.- jet engine, magnet, door closing.
 A force gives energy to an object, sometimes
causes it to start moving, stop moving, or
change directions.

II. Combining Forces
When two forces are acting in the same
direction, they are added together.
A.
•
•
B.
E.g.- You and a friend try to push a wagon
full of rocks
When the total force of an object is in one
direction, the force is called unbalanced.
An unbalanced force changes the motion
of an object.
C. When two forces act in opposite
directions, the combine by subtraction. If
one force is greater than the other, the
object would move in the direction of the
greater force. (e.g. tug-of-war)
D. If the two forces are equal the object
would remain stationary. There would be
no force acting on the object.
E. Forces that are in opposite directions and
are equal in size are called Balanced
Forces. When forces are balanced, there
is no change in motion.
III. Friction
When two surfaces are touching, a force
called Friction exists.
A.
•
•
B.
Friction will cause a moving object to slow
down and finally stop.
Friction is a force that acts in the opposite
direction to the motion of a moving object.
Friction occurs from the fact that objects
and surfaces are not perfectly smooth.
C. Friction is dependent upon how hard the
surfaces are forced together and on the
materials of which the surfaces are
made.
D. The force you apply to an object must be
greater than that force in order to move
the object.
E. Sliding Friction- when solid objects slide
over each other.
F. Rolling Friction- is produced by objects such as
wheels or ball bearings.


This type of friction tends to be less than sliding
friction.
E.g.- Imagine cars without wheels, rolling vs. sliding.
G. Fluid Friction- the force exerted by a fluid. Fluid
friction is less than sliding friction.


Lubricants are used to change friction to fluid
friction, reducing friction.
Lubricant is a slippery substance such as grease.
H. Friction can be helpful in certain activities. E.g.walking and driving.
IV. Newton’s Laws of Motion
A.
B.
In 1665 and 1666, Isaac Newton
developed three laws that describe
motion, -rest, -constant motion, accelerated motion.
Newton’s First Law-