1. What is a force?
1.1. Push or pull
1.2. SI unit is Newton(N)
1.3. Can be measured with a force meter
1.3.1. Two types of spring balances
1.3.1.1. Extension spring balance
1.3.1.1.1. Spring stretches when force pulls on it
1.3.1.1.2. The greater the force,the more the spring stretches
1.3.1.2. Compression spring balance
1.3.1.2.1. Spring compresses when a force acts on it
2. What are the effects of forces?
2.1. Changes the shape or size of an object
2.2. Cause an object to start moving
2.3. Cause a moving object to stop moving
2.4. Speed up or slow down a moving object
2.5. Change the direction of a moving object
3. What are the important forces around us?
3.1. Gravitational force
3.1.1. Pulls objects down to the Earth
3.1.2. Without gravity,we will be floating around
3.2. Frictional force
3.2.1. If there is no friction,we will not be able to stop if we are on a bicycle,the brakes would not help at all
3.3. Magnetic force
3.3.1. Two types of magnetic forces
3.3.1.1. Repulsion
3.3.1.2. Attraction
3.3.2. Without it,we would not be able to have conveyer belts and sorting metals would be harder
4. How do we calculate pressure?
4.1. Pressure is proportional to the amount of force applied;the greater the force applied, the greater the pressure
4.2. Pressure is inversely proportional to the size of the area that the force is in contact with. When the area of contact is small, the pressure is high because the force is concentrated on a limited space. With a larger area of contact, the force is spread over a larger space, so the pressure is lower.
4.3. Pressure is the force per unit area applied on a surface in a direction perpendicular to that surface. The standard unit of measurement for pressure is the pascal (Pa). One Pa is equivalent to 1 N/m2. Pressure is described in the following equation:
4.3.1. Pressure=force/unit area Example: A box has a unit area of 0.2m2 and a weight of 100N Pressure=100N/0.2m2 =500N/m2 =500Pa//
5. What is pressure?
5.1. You have learnt that a force is a push and a pull. However, a large amount of force is not able to push or pull a small object.
5.1.1. You can easily push the pin into the wood. However, you cannot push your finger into the wood, even if you use a large force.
5.1.1.1. Why?
5.1.1.1.1. The difference between this 2 examples is with the area.This is because the point of the pin has a small area.The force acts on a small area to give a high pressure.
5.1.1.1.2. With the thumb, the same amount of force spreads over a larger area, and thus the pressure is lower.
5.2. There are 2 factors that pressure depends on
5.2.1. Force
5.2.2. Area
5.3. How do we increase the pressure?
5.3.1. By reducing area.When a smaller surface is in contact with an object, the force acts on this small area, the pressure is higher.
5.3.1.1. Example:A sharp knife has a small area in contact with an object.As the force acts on this small area, the pressure is high.With a blunt knife, the small force acts over a larger area, lowering the pressure.Thus, it is easier to cut things with a sharp knife.
5.4. How do we reduce pressure?
5.4.1. By increasing area.Low pressures are needed when people or machines move on soft ground or snow.When a larger surface is in contact with the ground, the pressure exerted on the ground is lower, so people or machines can move without sinking.
5.4.1.1. Example:Large surface area of the ski prevents skier from sinking into the snow.
5.4.1.2. Example:The large flat feet of the camel prevents it from sinking into the desert sand.