1. Chapter 13- The forces of Nature
8th grade Science

2. 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.

3. II. Combining Forces
When two forces are acting in the same direction, they are added together.
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.

4. 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.

5. + = + = + =

6. III. Friction
When two surfaces are touching, a force called Friction exists.
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.

7. 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.

8. 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.

9. IV. Newton’s Laws of Motion
In 1665 and 1666, Isaac Newton developed three laws that describe motion, -rest, -constant motion, -accelerated motion.
Newton’s First Law-