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Newtons Laws of Motion

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Newtons Laws of Motion

1st & 2nd Laws

Newton’s First Law

If there are no forces acting or if the forces acting on an object are balanced, the object will keep doing what it was already doing.

If the object is stationary, it will remain stationary; if it is moving at a constant speed, it will continue travelling at this speed (not speeding up or slowing down).

This statement is called Newton’s first law of motion, which is also known as the law of inertia.

All objects possess inertia, which means a tendency to resist change.

The larger the mass of an object, the harder it is to change its motion because it possesses more inertia than an object with a smaller mass. This is why it is much easier to stop an empty runaway shopping trolley than a full one.

D:\int\nwtLaws.exe

Newton’s Second Law

Describes how an object will move when it is acted upon by unbalanced forces.

A force is a push or a pull that causes a change in motion.

A change in motion is acceleration.

The size of a force is measured in newtons (symbol N). Some common measures of force are:

- switching a light switch, 5 N
- removing a ring tab from a can, 20 N
- hitting a tennis ball with a racquet, 2000 N
- the force of a jet engine, 200 000 N

A body will accelerate in the direction of the overall force that acts upon it.

The size of this acceleration depends on the mass of the object and the size of the force acting.

This relationship can be expressed as:

F = ma

Force is measured in newtons, mass is measured in kilograms and acceleration is measured in metres per second squared.

So using the equation;

- if you double the acceleration rate and the mass remains the same what happens to the force?
- If you triple the acceleration rate and the mass remains the same what happens to the force?
- If you double the mass and the acceleration rate is the same what happens to the force?
- If you halve the mass and acceleration rate remains the same what happens to the force?
- If you double the force and the mass remains the same what happens to the acceleration rate?

If we rearrange this formula, we can express acceleration as:

A car of mass 1200 kg that travels to the right, with a driving force of 3000 N, along a surface that provides a 600 N frictional force will experience a net force of:

.... So it will travel with an acceleration of:

A boy of mass 50 kg coasting along on a skateboard to the left, with a frictional force of 450 N, will experience an acceleration of:

Isaac Newton’s second law of motion:‘A large force produces a large acceleration, provided that the mass remains the same. A large mass reduces the acceleration of the same force’.