Types of Forces & Newton’s Third Law of Motion

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

Gravity The ball does not travel in a straight line because of the unbalanced force of gravity acting on it.

The Law of Universal Gravitation 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.

The Law of Universal Gravitation The gravitational force becomes stronger as either or both objects increase in mass. The gravitational force becomes weaker as the distance between the objects increases.

Friction 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.

Friction Without friction, you could not walk, drive, skate, etc. Sometimes we want to INCREASE it. (cleats, surfboard wax)

Friction Friction is the force between two surfaces in contact that keeps them from sliding. When a force is applied to a heavy box, and the box doesn’t move… The forces are balanced, the force pushing the box equals the force of friction pushing in the opposite direction.

Sliding Friction When the force pushing on the box is larger than the friction force, the box begins to slide. The frictional force that acts on the sliding box is called sliding friction. motion

Sliding Friction 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.

Sliding Friction 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 & Fluid Friction Rolling Friction – when an object rolls over a surface. Fluid Friction – when an object moves through a gas OR a liquid (a fluid).

Elastic 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 types: tension and compression.

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

Compression Forces A compression force is a pushing force exerted by a material when it is squeezed or compressed.

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

Normal Forces 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.

Newton’s Third Law of Motion Newton’s Third Law of Motion states that when one object exerts a force on a second object, the second object exerts an equal force in the opposite direction on the first object.

Newton’s Third Law of Motion The forces two objects exert on each other are called force pairs. The forces in a force pair are equal in size, and opposite in direction.

Newton’s Third Law of Motion One of the forces in a force pair is called the action force and the other force is called the reaction force.

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

The gravitational force between two objects ____ as ____ increases. 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

For every action force, there is a reaction force that is ____. A in the same direction B equal to the two objects C equal and in the opposite direction D unequal and in the direction of the object with less mass