Forces, movement and work

When a force is used to move an object, energy is transferred (because the object has moved) and we say that Work is done.

A Force that causes movement in the direction of the force has done Work.

But a force applied to an object that does NOT move, then no work is done on the object. Zero movement, zero Work.

Or, if a force is applied but the object does not move in the direction of the force, then once again, NO work is done!

To conclude: the bigger the force that is used, or the greater the distance moved (in the direction of the force), the greater the amount of Work done.

So, from this statement we can derive a simple equation:

OK, let's have a go at a few example calculations.

Machines

Machines are devices which make it easier for us to do work. They can be very simple things.

One example of a very simple but incredibly useful machine is a lever.
This simple machine is known as a lever since it is often used to lever objects (ie lift objects) from the ground; it was probably first used by the ancient Egyptians when they were building their pyramids.

As you can see from the diagram, the lever can be a length of a solid material such as wood or metal with a pivot positioned near to the end with the Object or Load to be lifted.

The job will be made easier if the person applies the Force as far from the pivot as possible, as shown above.

If the force is applied closer to the pivot, then the job gets a lot harder, as shown below, even though the person might exert the same size of Force.

Another example of a type of lever is a spanner, used in garages and workshops around the world to tighten and untighten nuts and bolts. Without the spanner the mechanic (ie the user of the "machine") would not stand any chance at all of untightening a nut or bolt.

Below you can see the mechanics hand gripping a spanner and using it to turn a nut.

As with the basic lever, the further the force is from the pivot the greater will be the "turning effect" produced. This is why spanners come in many different lengths, with the longest used for the largest nuts/bolts, which will usually be the tightest and hardest to loosen.

When you turn a water tap you are making use of a simple machine; your thumb and fingers produce a force at a distance from the pivot (which is right in the centre of the tap top) creating a turning effect. Imagine taking off the tap top and trying to turn the thin spindle that you would find below it - it would be impossible to turn with just your fingers and thumb!

When you open or close a door you apply a force (a push or a pull) at a distance from the hinges; your force and the distance produces a large turning effect such that the door turns easily on its hinges. This is why door handles are always positioned far from the hinges. Try closing a door by pushing much closer to the hinges and you will find that it is not so easy to do!

An easy way to compare the usefulness of two lever machines is to look at the distance through which the user has to move or swing the end of the lever/spanner.
The further the user moves the end of the lever, the less force required to lift the load or to do the work.
Consider the following two diagrams:

In the diagram above, the user has to move or swing the long lever a significant distance in order to lift the (red) load, but it would be an easy effort (due to the long lever).

In the diagram below, the user has to move or swing the short lever a much smaller distance in order to lift the same load the same amount, but it would be a harder effort (due to the shorter lever).

So the rule is - The further the user moves the end of the lever, the less force required to lift the load or to do the work

Simple machine - the pulley

The final simple machine that we shall consider is the pulley system.

The simplest pulley system consists of just one pulley wheel attached to a cieling support, and a piece of rope.
As you can see in the diagram below, this simple pulley system can be used to lift a load.

You should be able to see that in this simple pulley system, to lift the load by 1m, the person pulls the rope the same distance, 1m. Also the Effort Force required will be the same as the Load (its weight).

So you might think- why not just lift the load without the pulley system?
The answer is that it is easier for the person to pull downwards making use of their own weight, than it is for them to lift the load directly with just the strength of their arms, against the downward pull of gravity.
Notice also that a pulley wheel is used so that the rope will move without any friction.

So, once again we find that this simple machine makes the work easier.

However, pulley systems can be made a little more fancy to make the work even easier!

If you consider the diagram below; an extra pulley wheel has been added.

Now, to lift the load by a certain amount two sections of the rope (labelled a and b) have to be shortened compared to one in the first diagram. This means that the person pulling the rope has to pull more rope in order to lift the load by a certain amount. However, as a consequence of this he/she will find that the effort force needed will half (indicated by the shorter force arrow).

So a reduced effort force means that a two pulley system makes the work even easier.

Having 2 pulley wheels doubles the amount of rope pulled, so it halves the effort force required
Having 3 pullley wheels would triple the amount of rope pulled, so it would reduce the effort force to one third.

The easy way to think of this is - the greater the amount of rope "pulled", the less force required to lift the load.

Keywords

Now that we have reached the end of this section we can focus on the keywords highlighted in the KS3 specification. You have met most of these already.

• Work: The transfer of energy when a force moves an object, in joules.
• Lever: A type of machine which is a rigid bar that pivots about a point.
• Input force: The force you apply to a machine.
• Output force: The force that is applied to the object moved by the machine.
• Displacement: The distance an object moves from its original position.
• Deformation: When an elastic object is stretched or squashed, which requires work.