Q1. IDEAS & ACTIVITIES

PLEASE CHECK OUT THIS ACTIVITY BEFORE YOU CONTINUE.

For this experiment, it may help if you think of a simple pulley as being like a hook that has no friction (a frictionless hook).

If you cannot find enough bits and pieces to perform this experiment, you can still try it in your imagination (this is called a 'thought experiment'). It is not as good as doing the real experiment, but you can still learn a lot:

Imagine that you perform an experiment to understand how a simple pulley works when two equal weights are suspended from it (if, for the sake of the experiment, we agree to ignore friction and the weight of the string):

1. Take a 1Kg weight (weight A) suspended by string.
2. Pick up the string and hold your left arm out so that the weight is suspended above the floor.
3. How much upward force do you need to apply so that you do not raise or lower the weight?
4. To rest your arm, you tie the string to a hook in the cieling
5. How much upward force does the hook need to apply so that the weight does not fall?

1. Untie the string
2. Pass the untied end of the string through the hook, and pull down on the end of the string to raise the weight.
3. How much upward force does the hook need to apply so that the weight does not fall?

1. To rest you arm. take another, identical 1Kg weight (weight B) and tie it to the end of the string near the hook.
2. Pull this weight down so that the first weight is raised off the floor again
3. Now let go of the string…
4. The weights should not move!

The weights do not move because (ignoring friction), both weights are pulling each-other upwards with the same force (because both A and B weigh exactly the same). Because of this, they balance each other exactly. In this case, the hook in the ceiling is working like a frictionless pulley (we should repeat the experiment with a real pulley to make sure that we get the same result):

If you think about it for a minute, you should see that due to the ceiling hook/pulley:

1. Weight A is pulling up by exactly the same amount that weight B is pulled down by gravity.
2. Weight B is pulling up by exactly the same amount that weight A is pulled down by gravity.
3. Because their weights are the same, the forces are perfectly balanced and neither weight will move.

You may wonder why the length of the string (ignore the small differences in the 'weight' of the string) does not affect the balance.

In this case, the length of string attached to each weight does not change the force like a lever would do - a lever changes the amount (amplitude) of the force, but a pulley only changes the direction of the force.

In our example experiment above, the pulley has the effect of making both weights 'pull up' equally against each other - it's a bit like trying to pull yourself up by your shoe laces!

ALSO CONSIDER THIS:

Figure: Transition from lever/balance to pulley

If you follow the images from a, b, c to d…

• How is the pulley in figure (d) similar to the balance in figure (a)?
• Refresher on Simple Machines (including pulleys)

Figure: Lever as balance

Could a single pulley just be a simple lever in disguise (a lever with equal size arms)?

Some examples:

• PBSKids Simple Lever Interactive
• levers and pulleys

Remember - A simple pulley changes the direction of a force - it does not change the amplitude of a force.

It is possible to design pulley systems that, like a lever, do change the amplitude of a force.

The reason that they amplify the force is that the pulleys are arranged in such a way that they do actually work like a lever: It may not be obvious, but these type of pulley systems are just a type of lever: