Introduction to Gravity and Friction

Noncontact Forces

• A contact force is a push or a pull on one object by an object that is touching it

• Contact forces can be strong or weak

  • weak: pressing buttons on a game controller

  • strong: tectonic plates shifting and causing earthquakes

Contact Forces

• A noncontact force is a force that one object can apply to another object without touching it

• Gravity is a noncontact force

• Magnetic forces and electric forces are also noncontact forces

Forces

• Arrows can be used to show which direction and how strong a force is

• The SI unit for force is the newton (N)

  • To lift a 2-liter bottle of water, you would exert 20 N of force

An attractive force that exists between all objects that have mass

Gravity:

The Law of Universal Gravitation

• The Law of Universal Gravitation was developed in the late 1600s by scientist/mathematician Sir Isaac Newton

• This law states that all objects are attracted to each other by a gravitational force and the strength of the force depends on mass and distance

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When the distance between objects increases, the gravitational force decreases

Gravitational Force and Distance

When the mass of one or both objects increases, so does the gravitational force

Gravitational Force and Mass

Friction:

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

• Prevents surfaces from sliding past each other

• Balances applied force

• Static friction increases as applied force increases

Static Friction

Fluid Friction

Sliding Friction

• Opposes the motion of surfaces sliding past each other

• Sliding force does not change

• Friction between a surface and any fluid that flows

• More surface area=greater resistance