Chapter 13- Chapter 13- The forces of NatureThe forces of Nature

88thth grade Science grade Science

I. What is Force?I. What is Force?

A. Isaac Newton- is considered the A. Isaac Newton- is considered the founder of modern physics and is “one of founder of modern physics and is “one of the greatest names in the history of human the greatest names in the history of human thought.”thought.”

B. Force- is any push or pull. B. Force- is any push or pull. E.g.- jet engine, magnet, door closing.E.g.- jet engine, magnet, door closing.A force gives energy to an object, sometimes A force gives energy to an object, sometimes

causes it to start moving, stop moving, or causes it to start moving, stop moving, or change directions. change directions.

II. Combining ForcesII. Combining Forces

A.A. When two forces are acting in the same When two forces are acting in the same direction, they are added together. direction, they are added together.

• E.g.- You and a friend try to push a wagon E.g.- You and a friend try to push a wagon full of rocksfull of rocks

• When the total force of an object is in one When the total force of an object is in one direction, the force is called direction, the force is called unbalanced.unbalanced.

B.B. An unbalanced force changes the motion An unbalanced force changes the motion of an object.of an object.

C. When two forces act in opposite C. When two forces act in opposite directions, the combine by subtraction. If directions, the combine by subtraction. If one force is greater than the other, the one force is greater than the other, the object would move in the direction of the object would move in the direction of the greater force. (e.g. tug-of-war) greater force. (e.g. tug-of-war)

D. If the two forces are equal the object D. If the two forces are equal the object would remain stationary. There would be would remain stationary. There would be no force acting on the object. no force acting on the object.

E. Forces that are in opposite directions and E. Forces that are in opposite directions and are equal in size are called are equal in size are called Balanced Balanced Forces. Forces. When forces are balanced, there When forces are balanced, there is no change in motion.is no change in motion.

III. FrictionIII. Friction

A.A. When two surfaces are touching, a force When two surfaces are touching, a force called called FrictionFriction exists. exists.

• Friction will cause a moving object to slow Friction will cause a moving object to slow down and finally stop.down and finally stop.

• Friction is a force that acts in the opposite Friction is a force that acts in the opposite direction to the motion of a moving object. direction to the motion of a moving object.

B.B. Friction occurs from the fact that objects Friction occurs from the fact that objects and surfaces are not perfectly smooth. and surfaces are not perfectly smooth.

C. C. Friction is dependent upon how Friction is dependent upon how hard the hard the surfaces are forced together and on the surfaces are forced together and on the materials of which the surfaces are materials of which the surfaces are made. made.

D. The force you apply to an object must be D. The force you apply to an object must be greater than that force in order to move greater than that force in order to move the object. the object.

E. Sliding Friction- when solid objects slide E. Sliding Friction- when solid objects slide over each other. over each other.

F. Rolling Friction- is produced by objects such as F. Rolling Friction- is produced by objects such as wheels or ball bearings. wheels or ball bearings.

This type of friction tends to be less than sliding This type of friction tends to be less than sliding friction.friction.

E.g.- Imagine cars without wheels, rolling vs. sliding.E.g.- Imagine cars without wheels, rolling vs. sliding.

G. Fluid Friction- the force exerted by a fluid. Fluid G. Fluid Friction- the force exerted by a fluid. Fluid friction is less than sliding friction. friction is less than sliding friction.

Lubricants are used to change friction to fluid Lubricants are used to change friction to fluid friction, reducing friction. friction, reducing friction.

Lubricant is a slippery substance such as grease. Lubricant is a slippery substance such as grease.

H. Friction can be helpful in certain activities. E.g.- H. Friction can be helpful in certain activities. E.g.- walking and driving.walking and driving.

IV. Newton’s Laws of MotionIV. Newton’s Laws of Motion

A.A. In 1665 and 1666, Isaac Newton In 1665 and 1666, Isaac Newton developed three laws that describe developed three laws that describe motion, -rest, -constant motion, -motion, -rest, -constant motion, -accelerated motion.accelerated motion.

B.B. Newton’s First Law-Newton’s First Law-