- 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 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) × Acceleration (metre per second-squared)|
|F||= m × m/s2|
|F||= m × 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?
|= m × a|
|= 100 × 5|
|= 500 N (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 (newtons)||= Mass (kilograms) × Gravitational Acceleration (9.8 m/s2)|
|W||= m × g|