Gravity and Friction Ch. 2 Section 1

Force A push or pull of an object Some forces can only be applied when touched Other forces are applied even when objects do not touch!

TOUCHING! Force Contact Force Contact Force- a push or pull on one object by another that is touching it TOUCHING! Force

NOT TOUCHING! Noncontact Force Noncontact Force- a force that one object can apply to another object without touching it NOT TOUCHING!

Forces Forces have both magnitude and direction What is this like? Arrows can be used to represent this (just as we did for velocity and acceleration)

Newtons The SI unit for force is a newton (N) You apply a force of about 1N when lifting a stick of butter You apply a force of 20N when lifting a 2L bottle of water

An attractive force that exists between all objects that have mass Gravity An attractive force that exists between all objects that have mass Objects fall to the ground because Earth exerts an attractive force on them You also exert an attractive force on objects!

The amount of matter in an object Mass The amount of matter in an object Often measured in kilograms (kg) The mass of an object -the amount of matter the object has- stays the same no matter where the object is.

The Law of Universal Gravitation All objects are attracted to each other by gravitational forces The strength of the force depends on the mass of each object and the distance between them Sir Isaac Newton in the late 1600s

Gravitational Force and Mass Notice the force arrows are the same size between the objects!

Gravitational Force and Distance Notice the force arrows are the same size between the objects!

Effect of Distance on Gravity The gravitational force between objects decreases as the distance between the objects increase

The gravitational force exerted on an object Weight The gravitational force exerted on an object Earth is more massive than anything on it, so everything is pulled toward it

Weight Near Earth’s surface an object’s weight is the gravitational force exerted on the object by Earth Because weight is a force it is measured in Newtons

Does an astronaut weight the same on Earth as on the moon? NO!

Does an astronaut have the same mass on the Earth as on the moon? YES!

A force that resists the motion of two surfaces that are touching Friction A force that resists the motion of two surfaces that are touching

Surfaces are Uneven! Atoms cling together at many points of contact when slid against each other creating friction

Friction The direction of the friction force is always in a direction opposing motion

Friction To push a heavy box, you must apply enough force to overcome friction So you have to be winning the net force battle

Static Friction vs. Sliding Friction The initial force to get a stationary object to move Example: If you push on a crate, it takes more force to get it going than it takes to keep it sliding. The force of friction of a sliding object Example: The force of friction against the tires of a sliding car

Static Friction vs. Sliding Friction

Fluid Friction Friction between a surface and a fluid (water, air, etc.) Fluid friction in air is air resistance SURFACE AREA MATTERS!

Reducing Friction Soap on your hands creates a lubricant How can you reduce air resistance (friction) on a falling object?