gravity law of universal gravitation weight friction static friction elastic force tension force compression force normal force sliding friction rolling friction fluid friction

2.2 Types of Forces
• Many types of forces act on objects.
• Gravity is an attractive force between all objects that have mass.

(cont.)
2.2 Types of Forces
• The ball does not travel in a straight line because of the unbalanced force of gravity acting on it.

2.2 Types of Forces
• The Law of Universal Gravitation states that all objects with mass are attracted to each other.
• The magnitude of attraction depends on the mass of each object and the distance between them.

2.2 Types of Forces
(cont.)
• The gravitational force becomes stronger as either or both objects increase in mass or move closer together.
• The gravitational force becomes weaker as either or both objects decrease in mass or move farther apart.

2.2 Types of Forces
• Mass is the amount of matter in an object and does not change with location.
• Weight is the gravitational force on a object and changes with location.
• Weight is a force and a vector.
• Weight changes with height above Earth.

2.2 Types of Forces
• Friction is a force that opposes the movement between two surfaces in contact.
• A book pushed across a table slows down because of friction.
• Friction is an unbalanced force acting on the book to slow it down.

2.2 Types of Forces
• Static friction is the force between two surfaces in contact that keeps them from sliding when a force is applied.
• A force is applied to a heavy box, but the box doesn’t move.
• The forces are balanced, the force pushing the box equals the force of static friction pushing in the opposite direction.

2.2 Types of Forces
• When the force pushing on the box is larger than the maximum static friction force, the box begins to slide.
• The frictional force that acts on the sliding box is called sliding friction.

2.2 Types of Forces
(cont.)
• The book pushed across the table slows down because of sliding friction.
• Without sliding friction, the book would continue moving without a force being applied.

2.2 Types of Forces
(cont.)
• Usually friction is present and an unbalancing force must be applied to keep an object moving.
• When friction is greatly reduced, objects move with nearly constant velocity without an applied force.
– Rolling friction
– Smoother surfaces
– Lubricating surfaces

2.2 Types of Forces
• An elastic force occurs when a material is stretched or compressed.
• A diving board exerts an upward elastic force on the diver when it is bent downward.

2.2 Types of Forces
• A tension force is a pulling force exerted by an object when it is stretched, such as a rubber band.

2.2 Types of Forces
• A compression force is a pushing force exerted by a material when it is squeezed or compressed.
• The size of the compression force exerted by a material is equal to the size of the force that compresses the material.

2.2 Types of Forces
• A normal force is the force exerted by an object that is perpendicular to the surface of the object.

2.2 Types of Forces
(cont.)
• The cup is exerting a downward force on the table, caused by gravity.
• The table is exerting an upward normal force on the cup, caused by compression.

(cont.)
2.2 Types of Forces

(cont.)
2.2 Types of Forces

2.2 Types of Forces
• Friction balances forces applied in a horizontal direction.
• Friction equals the horizontal force on an object that is not changing motion.

2.2 Types of Forces
• Upward normal force balances the downward force of gravity on an object that is not moving vertically.

2.2 Types of Forces
(cont.)

2.2 Types of Forces
A(n) ____ force is the force exerted by an object that is perpendicular to the surface of the object.
A compression
B elastic
C normal
D tension
Lesson 2 Review

2.2 Types of Forces
Which force causes a rolling ball to slow down?
A sliding friction
B static friction
C normal force
D gravity
Lesson 2 Review

2.2 Types of Forces
The gravitational force between two objects ____ as ____ increases.
A increases; distance
B decreases; mass
C increases; velocity
D decreases; distance
Lesson 2 Review