# Friction and Gravity by fjzhangxiaoquan

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```									Friction and Gravity
Outline Notes
Friction
• Friction: force that two surfaces exert on each other when
they rub against each other
The Causes of Friction
• The strength of the force of friction depends on two factors:
how hard the surfaces push together and the types of surfaces
involved.

• Friction acts in a direction opposite to the direction of the
object’s motion.
Static Friction
• Static friction: friction that acts on objects that are not moving

• Example: pushing on a heavy desk that does not move
Sliding Friction
• Sliding friction: occurs when two solid surfaces slide over each
other

• Example: using the hand brake on a bicycle
Rolling Friction
• Rolling friction: when an object rolls across a surface

• Example: skateboarding
Fluid Friction
• Fluid friction: occurs when a solid object moves through a fluid

• Example: riding a bike, fluid friction occurs between you and
the air
Gravity
• Gravity: force that pulls objects toward each other
Universal Gravitation
• The force of gravity acts between all objects in the universe

• Any two objects in the universe will be attracted to one
another. This is not noticed because Earth’s attraction is much
greater.
Factors Affecting Gravity
• Two factors affect the gravitational attraction between
objects: mass and distance.

• Mass: measure of the amount of matter in an object

• The more mass, the greater the gravitational force.

• The more distance, the lesser the gravitational force.
Weight and Mass
• Weight: force of gravity on an object at the surface of a planet
Gravity and Motion
Free Fall
• Free fall: when the only force acting on an object is gravity

• In free fall the force of gravity is an unbalanced force, which
causes an object to accelerate.

• Acceleration due to gravity= 9.8m/s2
Air Resistance
• Air resistance: objects falling through the air experience fluid
friction

• Falling objects with a greater surface area experience more air
resistance.

• Air resistance increases with velocity.

• Terminal velocity: greatest velocity a falling object reaches,
reached when the force of air resistance = the weight of the
object
Projectile Motion
• Projectile: object that is thrown

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