A Force is a push or a pull on an object. Forces can: Speed up an object Slow down or stop an object Change the direction of an object 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: The roughness of the surfaces (e.g. stepping on banana peel compared with carpet) The force pushing the surfaces together (e.g. A heavy truck's tyres compared with a bicycle's tyres on the road) Whether the surfaces are moving or stopped 3 Types of Friction Static Friction - acting between 2 stationary bodies (e.g. holding a person on a chair) Sliding Friction - acting between surfaces where one is moving (e.g. sliding furniture across the floor, writing with a pencil on paper) 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 Reducing the force pushing both surfaces together Using a lubricant such as oil or water between the surfaces Using ball bearings or rollers between both surfaces Polishing both surfaces THE FORCE RULE Force is affected in 2 ways: An object of greater mass has a greater force (e.g. An adult baseballer will hit the ball further than a child) 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 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 Weight (N)= Mass(kg) X Gravitational Acceleration (9.8m/s2)W = m X g