Force, Mass and Weight

Force

A force can be defined as a push or a pull. It can change the size, shape or motion of a body. There are several different types of forces, some of which are:

• Gravitational force
• Electrical force
• Magnetic Force
• Nuclear force
  

Measuring Force

The spring balance is used to measure force. The spring balance is based on the fact that the extension of the spring is proportional to the force applied. The unit of force is the newton (N).

Stretching a Spring

We hang a wire spring from a retort stand. A pointer and a small pan are hung at the bottom of the spring. We set up a scale, marked in millimetres vertically by the side.

We add a series of small, equal weights which stretch the spring. We measure the extension i.e. the extra length, of the spring. Typical results are shown in the table below:

Force F/N	0	1.0	2.0	3.0	4.0	5.0
Extension e/cm	0	1.6	3.2	4.8	6.4	8.0

If a graph of extension against load is plotted, a straight line passing through the origin will be obtained. This means that extension if proportional to the load.

Elastic Limit of Springs

If we remove the weights the spring contracts to its original length. However, if we continue to add greater weights, eventually the spring stretches by different amounts. At this point, the extension is no longer proportional to the load.

When we remove the lager weights the spring does not return to its original length. We have gone beyond the elastic limit of the spring. The new graph looks like this:

Hooke's Law

Hooke's law states that the extension of a spring is directly proportional to the force applied, provided the elastic limit has not been exceeded.

We can obtain similar graphs for elastic bands and for straight metal wires. Elastic bands need smaller forces for a measurable extension and metal wires need much greater ones. 

The following video explains more on Hooke's Law:

Mass and Weight

Mass

The mass of an object is the measure of the amount of matter it contains. The mass of an object is also a measure of a resistance to a change in its motion. This resistance is known as the inertia of the body.

We measure mass using a triple beam balance as shown in the picture below. The unit of mass is the kilogram (kg). The mass of an object is constant everywhere.

Weight

The weight of an object is the force of gravity on the object. It acts towards the centre of the earth.

We measure weight using a spring balance as shown in the diagram below. The unit of weight is the newton (N). A mass of 1 kg has a weight of approxiamtely10N on earth. The weight of an object changes is the force of gravity changes.

In outer space there is no gravity, a 1kg mass has no weight. On the moon, where the gravity is about a sixth i.e. 1/6 that on the earth, a mass of 1 kg has a weight of about 1.6 N, as shown in the calculation below