Forces

• Force


  A push or a pull
• What a force can do

  • Change an object's speed
  • Change an object's direction of movement
  • Change an object's shape
• Force metre


  Used to measure forces
• Contact forces


  • Friction
  • Air resistance
  • Tension
  • Normal contact force
• Non-contact forces


  • Gravity
  • Magnetism
  • Forces due to static electricity
• How force metres work

  Spring connected to a metal hook, stretches when force is applied, bigger the force the longer the spring stretches
• Newton


  Unit of force, greater the force the bigger the number
• Balanced forces


  Equal in size but act in opposite directions
• If forces on an object are balanced

  • Stationary object stays still
  • Moving object continues to move at same speed and direction
• An object can be moving even if there are no forces acting on it
• Force diagram


  • Arrows represent size and direction of forces
  • Labelled with force name and size in Newtons
• Situations with balanced forces


  • Standing on the ground
  • Hanging objects
  • Floating in water
• Newton's first law


  An object remains in the same state of motion unless a resultant force acts on it
• Objects with uniform motion


  • Car travelling at constant velocity
  • Runner at top speed
  • Falling object
• Objects with non-uniform motion


  • Accelerating car
  • Sprinting runner
  • Falling object
• Newton's second law


  Acceleration of an object is directly proportional to the resultant force exerted on it and inversely proportional to its mass
• Moment


  Turning effect of a force around a pivot
• Acceleration


  How quickly your velocity changes
• Newton's Second Law


  Acceleration of an object is directly proportional to the resultant force exerted on it and inversely proportional to its mass
• The harder you push something the faster it will speed up
• Resultant force


  Measured in newtons (N)
• Mass


  Measured in kilograms (kg)
• Acceleration


  Measured in metres per second per second (m/s^2)
• Moment


  A turning effect of a force
• A see-saw will balance if the moments on each side of the pivot are equal
• If a nut is difficult to undo with a short spanner, a longer spanner will help because there will be a bigger moment on the nut
• Using the same principle, you can increase the moment applied by a lever or a crowbar, and this can help you move heavy objects more easily
• Deformation


  Change of shape
• Elastic deformation


  Reversed when the force is removed
• Inelastic deformation


  Not fully reversed when the force is removed - there is a permanent change in shape
• A rubber band undergoes elastic deformation when stretched a little. A metal drinks can undergoes inelastic deformation when it is squashed
• Contact forces


  • Frictional force
  • Normal force
  • Applied force
  • Tension force
• Unit that measures forces

  Newton (N)
• Instrument that measures forces


  Force gauge or dynamometer
• If the forces acting on an object are balanced, it will continue to move at a constant velocity or remain at rest
• Two things needed to create a moment


  A force and a distance from the pivot point (lever arm)
• As the distance to the pivot decreases


  The moment of an applied force also decreases, assuming the force remains constant
• Spring constant


  A measure of the stiffness of a spring
• Extension


  Increase in length
• Compression


  Decrease in length