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Forces


  • A Force is a push or a pull on an object. Forces can:
  1. Speed up an object
  2. Slow down or stop an object
  3. Change the direction of an object
  4. Change the shape of an object
  • Forces are measured in units called Newtons (N).
  • Forces may be balanced (if no movement occurs) or unbalanced (if movement occurs).
  • An example of unbalanced forces is the Action and Reaction of a bullet fired from a gun (action), and the following recoil of the gun (reaction).
  • Forces may be contact (if objects touch e.g. friction, cohesion and adhesion) or non-contact (if objects do not touch e.g. gravity, static electricity and magnetism).

FRICTION

  • Friction is a force opposing the movement of one surface over another.
  • The Magnitude of the Force of Friction depends on the following factors:
  1. The roughness of the surfaces (e.g. stepping on banana peel compared with carpet)
  2. The force pushing the surfaces together (e.g. A heavy truck's tyres compared with a bicycle's tyres on the road)
  3. Whether the surfaces are moving or stopped
  • 3 Types of Friction
  1. Static Friction - acting between 2 stationary bodies (e.g. holding a person on a chair)
  2. Sliding Friction - acting between surfaces where one is moving (e.g. sliding furniture across the floor, writing with a pencil on paper)
  3. Rolling Friction - acting between surfaces of objects where one has a rounded shape (e.g. car tyres on the road, ball bearings); less than sliding friction
  • 4 Ways to Reduce Friction
  1. Reducing the force pushing both surfaces together
  2. Using a lubricant such as oil or water between the surfaces
  3. Using ball bearings or rollers between both surfaces
  4. Polishing both surfaces

THE FORCE RULE

  • Force is affected in 2 ways:
  1. An object of greater mass has a greater force (e.g. An adult baseballer will hit the ball further than a child)
  2. An object with greater acceleration has a greater force (e.g. A faster karate expert can strike with greater force than a slower person)
  • Force Rule

Forces

Force (Newtons) =Mass (kilograms) X Acceleration (m/s2)
F = m X a

  • Force Rule Example

Question: Daring Darius, the human cannonball whose mass is 100 kg, is accelerated from a cannon at 5 m/s2 . What force was used?

Answer: F = m X a
= 100 X 5
= 500 Newtons

FORCE OF GRAVITY

  • Mass is the amount of matter in an object. It is measured in kilograms. The mass of an object remains the same anywhere in the universe.
  • Weight is the force of gravity on an object. It is measured in Newtons. The weight of an object differs depending on its position in the universe (e.g. A person's weight on the moon with less gravity will be less than that on earth).
  • Rule for Weight

Forces

Weight (N)= Mass(kg) X Gravitational Acceleration (9.8m/s2)
W = m X g