Physics

• Extension of a spring

  Directly proportional to the force extending it
• Momentum


  Mass × velocity
• Moment


  Force × perpendicular distance from the pivot
• If 2 forces are equal and opposite, they are balanced. The resultant force is zero.
• Forces acting on an object

  • 2 N North
  • 2 N South
  • 4 N East
  • 3 N West
• 2 N North and 2 N South are balanced


  Resultant force is 1 N East
• Limit of proportionality


  The point beyond which it no longer obeys Hooke's Law
• Elastic limit


  The point beyond which the rubber will not return to original shape
• Permanent deformation


  When the force is removed, bonds have been broken and the rubber has a permanent deformation
• Force


  Mass × gravity
• Scalar


  A magnitude, but no direction, e.g. speed
• Total length of spring


  Original length + extension
• Weight


  Mass × gravitational field strength
• Gravitational force


  The force by which the earth attracts other objects by mass. It is a conservative force.
• Electrostatic force


  The force of an object due to charge. It is also a conservative force.
• Both gravitational and electrostatic forces act along the imaginary line joining the objects.
• Both gravitational and electrostatic forces are inversely proportional to the square of the distance between the objects, this is known as the inverse-square law.
• The magnitude of both gravitational and electrostatic forces increases with the magnitude of the charges or mass.
• Force is a vector quantity because it has both magnitude and direction.
• Resultant force


  Subtract the magnitude of the smaller force from the magnitude of the larger force. The direction is in the same direction as the larger force.
• Air resistance acting in the other direction

  Balances until the object reaches a constant speed
• Extension


  Length - unloaded length
• Hooke's law


  Extension is directly proportional to force applied
• Objects that obey Hooke's law will return to their original length after being stretched.
• If an object continues to be stretched, it can be taken past the limit of proportionality.
• Force of collision


  Equal to the rate of change of momentum
• Car safety features


  Seatbelts, airbags and crumple zones all work to change the shape of the car, which increases the time taken for the collision
• Crumple zones


  Areas of a car that are designed to deform or crumple on impact
• Force

  Change in momentum/time taken
• Principle of Moments


  When a body is balanced, the total clockwise moment about a point equals the total anticlockwise moment about the same point