- What is a Force
- Material Elasticity
- Mass, Weight and Gravity
- Moment, Balancing and Stability
- Pressure
- Hydraulic Pressure
What is a Force
A force is either a 'push' or a 'pull'. There are contact forces (objects are touching each other) and non-contact forces (gravity). A pull is a tension force and causes stretching. A push is a compression force which shorten a material. Bending is caused by forces acting in opposite directions.
Forces change the speed of objects. Forces change the direction of moving objects. Forces are measure in newtons (N).
Material Elasticity
Materials have elastic limits. A material extends/compresses when pulled/pushed. The change in length is called extension. If the extension is within the material's elastic limit, the material will revert back to its original length when the force is removed. However, if the extension is so large that the material's elastic limit has been exceeded, the material is said to be deformed and would not return to its original length.
Hooke's law - the extension (e) is proportional to the force (F) applied:
e ∝ F
This means the greater the force, the larger the extension.
Springs in series are easier to stretch and there is a greater extension for the same force. This is because the same force will extend each spring by the same amount. Springs in parallel are harder to stretch and there is a smaller extension for the same force. This is because the springs share the load.
Mass, Weight and Gravity
Gravity is the force of attraction between masses (or objects). Mass of an object does not change whereas weight of the object is the force acted on the mass of that object by gravity. The value of gravity depends on the mass of the 'other' object in the interaction.
Weight = Mass x Gravity (w = mg)
The weight of an object on the Moon is lighter because the gravity pull is lower. The Gravitational force between two masses is
Gravitational Force ∝ (m1m2)/r2
where r is the distance between the two masses m1 and m2.
Moment, Balancing and Stability
Moment is the turning effect of a force and is the principle of balancing. It is measure in Nm and calculate using the formula
Moment = Force (N) x Distance from fulcrum (m) (Moment = F x d)
The moment of a force depends on the size of the force and the distance of the force from the fulcrum (or pivot). A large force can be balanced by a smaller force by moving the smaller force further away from the fulcrum. This concept is known as force multiplier - a small force can move a large object but the small force needs to move a large distance for the large object to move a small distance. Note that there is nothing gained as energy is conservred (ignoring friction, energy in is equal to energy out).
An object is balanced if it is supported at its centre of gravity (c of g) or centre of mass (c of m). This is because the weights of all the particles on one side of the c of g are balanced by the weights of all the particles on the other side of the c of g.
Stability of an object depends on how far it can be tipped before its weight moves outside the base - when this happens the moment of the weight tips it over. Tall objects have a high c of g and is less stable. Objects with wide bases have low centres of gravity and are therefore more stable.
Pressure
Pressure is the effect of a force when it acts on a surface. It is calculated using the equation
Pressure = Force/Area (Preesure = F/A)
The unit of pressure is pascal (pa) or N/m2.
For the same force, pressure is increased when the area is reduced. An application is cutting is more effective when the edge of the knife is sharp (small area). Increase the area allow the force to 'spread' out. An application is snow-shoes stop the person sinking so deeply into the surface.
Pressure in a liquid depends on the depth of the liquid - it increases as you go deeper. While the shape of the container does not affect the pressure and pressure at any point in a liquid acts equally in all directions, the density of the liquid does affect the pressure - dense liquids have a greater weight for the same volume so they press down with greater pressure.
Hydraulic Pressure
Hydraulic pressure is pressure transmitted through a liquid inside a container. The pressure is transmitted because liquid is difficult to compress. Generally hydraulic pressure is used to move pistons inside cylinders. The force applied to a piston in the master cylinder (small area) give rise to high pressure which is transmitted to the larger (area) slave cylinder allowing the piston in the slave cylinder to push with a larger force. Hydraulic pressure acts as a force multiplier with the increase in force determined by the relative areas of the master and slave cylinders.
Hydraulic forces are used in the disc brakes of cars. When the brake is applied, a piston in the master cylinder presses on the brake fluid which transfer the hydraulic pressure to the pistons of the slave cylinders. These force the brake pads on the disc and friction then slows the wheel down.