1. Gravity and Friction

2. Types of Forces The push or pull of an object is a force
Some forces are applied only when objects touch but some forces can be applied when objects do not touch
A contact force is one where the objects are touching while a noncontact force is one where the objects are not touching

3. Forces have both strength and direction and can be represented by an arrow. The length of the arrow represents the strength of the force.
The SI unit of force is the Newton (N)
Gravity is an attractive, noncontact force that exists between all objects that have mass
Mass is the amount of matter in an object

4. The Law of Universal gravitation
Developed by Sir Isaac Newton, the Law of Universal Gravitation states that all objects are attracted to each other by a gravitational force; the strength of the force depends on the mass of each object and the distance between them
When the mass of an object increases, the gravitational force increases
The gravitational force decreases as the distance increases

5. Weight-a gravitational force
Weight is the gravitational force exerted on an object
Because the moon has 1/6 less gravity than Earth, you would weigh 6 times less on the moon than what you would weigh on Earth.
Your mass would stay the same, but your weight would change

6. Friction
Friction is a force that resists the motion of 2 surfaces that are touching
Static friction prevents surfaces from sliding past each other; acts on objects not moving; must use extra force to get a stationary object moving
Sliding friction opposes the motion of surfaces sliding past each other
Fluid friction is between a surface and a fluid; friction between a surface and air is air resistance
What causes friction and how can it be reduced?