Gravity & Friction

Force

Contact Forces

• friction
• normal
• tension
• spring

Noncontact Force

• Gravity
• Magnetic
• Electrical

Forces have STRENGTH & DIRECTION

SI Unit of Force

Law of Universal Gravitation

• all objects are attracted to each other by gravitational force
• Strength depends on mass & distance

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• Gravitational force increases if mass increases
• Gravitational force decreases if distances increases.

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• Mass - amount of matter; measure w/balance; SI unit is kg; location doesn't effect mass
• Weight - pull of gravity; measure w/spring scale; SI unit is N; about 10 times its mass; location can effect weight

Friction

• 1. Static Friction: prevents surfaces from sliding past each other; increases up to a limit when force is applied to object

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• 2. Sliding Friction:
• slows the motion of objects sliding past each others; does not increase during motion

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• 3. Fluid Friction
• fluid - gas or liquid
• air resistance
• decreases surface area decreases air resistance

Source of Friction

• microscopic roughness
• electrical charges

Reducing Friction

• add lubricants
• reduce surface area

Balanced/Unbalanced Forces

Newton's Laws of Motion

Newton's First Law

• Objects at rest remain at rest; objects in motion remain in motion; unless acted on by an outside force.

Also called Law of Inertia
tendency of object to resist change in motion

Newton's Second Law

• force = mass times acceleration
• more force needed to change acceleration of a more massive object

Newton's Third Law

• For every action, there is an equal but opposite reaction