S-Cool Revision Summary

S-Cool Revision Summary

The Basics

Forces are vectors, so we can find a resultant force on an object, no matter how many forces are acting on it. If the resultant force is zero, the forces must be balanced.

Balanced forces cause no acceleration (This means that the object will remain stationary or carry on moving at a constant velocity.)

If the resultant force is not zero the forces are unbalanced. Unbalanced forces cause acceleration in the direction of the resultant force.

Every force has a partner force that is the same size but acts in the opposite direction on another object.

Calculating force

F = ma

Where:

F = force (N)

m = mass (kg)

a = acceleration (m/s2)

Newton's first law of motion

"Bodies will continue to move with a uniform velocity (which includes being stationary) unless acted on by a resultant force."

Newton's second law of motion

"Resultant force is equal to the rate of change of momentum."

This forms the basis of Newton's equation, F = ma.

Newton's third law of motion

"Every force acting on an object has an equal and opposite force which acts on another object."

Friction

Friction is caused by rubbing. It can be the surfaces between two solids rubbing, a solid surface and a liquid or a gas, etc. Anything! When friction is caused by fluids (liquids or gases) we tend to call it drag or air resistance.

"Friction dissipates energy." That means that energy moves from kinetic energy to heat energy, where it is lost to the surroundings.

Terminal velocity

For a falling object, when the air resistance force up has grown so big that it matches the weight down there is no resultant force and therefore no acceleration. The object will travel at a constant speed. This is called the terminal velocity.

It's not just falling objects that have a terminal velocity. You have one when you run! Think about it.

Pressure

Pressure is caused by forces acting on a surface. The greater the force or the smaller the surface area, the greater the pressure produced.

We can calculate pressure using:

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Where:

P = pressure (N/m2 or Pa, Pascals) - Note: 1 Pa = 1 N/m2

F = force (N)

A = surface area (m2)

Transmitting forces

Solids can transmit forces easily. If you push one end of a metal bar the other end will push what ever is near to it in the same direction. Liquids and gases can't do this.

Pressure is useful because it can be used to transmit forces from one place to another using liquids and gases.

Note: Energy can't be created from nowhere, so the small area will have to move down much further than the larger area moves up!

This idea is used in hydraulic systems - for example, car brakes. A person puts a small force onto the foot pedal, which creates a large force on the car brakes pads.