Physics GCSE: Forces, Moments and Pressure

Newton's Laws

Newton's First Law

Newton's First Law states that:

'Every body continues in a state of rest or uniform motion unless acted upon by an external force.'

This sounds really complicated... but isn't.

Imagine that you are playing table hockey. If you have not put your money in and you give the puck a hit, it doesn't travel very far. Friction stops it.

However, if you put your money in, air is pumped out of the holes in the table and so when you give the puck a flick, it will travel to the other end of the table. There is much less friction to slow the puck down. You need the end of the table to stop it. In both cases, a force starts and stops the puck, but a force is not needed to keep it moving at a steady speed.

So in other words, something without a net force acting on it will either stay still or move at a constant speed in a straight line until you apply a force to it.

When you apply a force to it, it will either:

• Speed up,
• Slow down,
• Change direction,
• (Or change shape).

The following example will help you understand this:

F = ma

Also known as Newton's Second Law - you will have seen this equation during your course.

• F is the force in Newtons, N.
• m is the mass in kilograms, kg.
• a is the acceleration in m/s2.

This shows that if you keep the mass constant and double the applied force the acceleration will double.

If you plot a graph of force against acceleration it will look like this:

You can see here that force is proportional to acceleration. As you double the force the acceleration doubles, as you triple the force the acceleration triples.

If you plot a graph of acceleration against mass it will look like this:

You can see here that if you keep the force constant and increase the mass the acceleration will fall. Acceleration is inversely proportional to mass. If you double the mass the acceleration will halve.

It is helpful if you can rearrange this equation. The triangle for this is as follows:

Some examples:

1. A 500kg car accelerates at 3 m/s2.

How much force is exerted by the wheels to accelerate the car?

Answer:

• Write down the formula: F = ma
• Plug in the numbers: F = 500 x 3
• Write down the answer: F=1500N

Note: Don't forget the units!

2. A 500kg car is accelerated by a force of 2000N. What is its acceleration?

Answer:

Important point: The equation works in exactly the same way for deceleration as it does for acceleration!