# Forces

• Created by: bob
• Created on: 12-05-13 11:32

## Newton's 1st Law of Motion

• Every object remains in a state of constant velocity or remains stationary unless a resultant force acts upon it.
• A resultant force is needed to produce an accleration.
• If the resultant force is zero/no resultant force, the object will continue to move at a constant velocity/remain stationary.
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## Newton's 2nd Law of Motion

• Resultant force = mass x acceleration
• Acceleration and force are vectors
• Resultant force is proportional to acceleration (and moves in the same direction)
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## Newton's 3rd Law of Motion

• Every action has an equal and opposite reaction
• If body A exerts a force on body B, then body B exerts an equal and opposite in direction reaction force of the same type on body A.
• They are equal in magnitude, force type, and the time and line of action is the same.
• They act in opposite directions and on different bodies.
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## Gravity

• GRAVITY IS NOT A FORCE - it is acceleration
• Weight = downwards force
• Weight = mass x gravity
• Magnitude of acceleration in freefall = gravitational field strength
• Centre of gravity = the point where all the weight of an object is concentrated
(it is usually in the centre of the object)
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## Diagrams

• A free body diagram shows all the forces acting on one object (at the centre of gravity)
• Weight always acts directly down
• Normal contact force (reaction force) acts perpendicular to the surface (ground/slope)
• Friction/drag forces always oppose motion.
• TENSION IS A FORCE
• Vector diagram = closed polygon if the forces are in equilibrium (resultant force = 0)
• Forces in equilibrium = constant velocity/stationary state (N1)
• Draw vector arrows from tip to tail (resultant vector = tail of first --> tip of last)
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