Pulleys and Wheels
There are 3 pulleys in this system each weighing 2 N. The blue pulleys can move up and down while the green is fixed. As the 30 N load is suspended by two strands, each strand will get only half the load. What is the effort required by Tommy? Fig. 2 .gives an interesting experiment with a fixed pulley. Two plastic cups are hanging from either side of the pulley. Consider one as the LOAD then the other becomes EFFORT required to raise it. To measure the force, we can use marbles, coins, nails or some other such material. (The pulley has to be very light and well oiled to reduce friction.) Fig.2. An experiment with a fixed pulley.
If you have done the experiment correctly you will find the distance moved by load and effort are always the same. What about the magnitudes? Can you lift three marbles by using less than that?
Another way to use a pulley is to attach the pulley to the load. Then the pulley travels up and down with the load. You might think that this is not a good method, as you have to lift the pulley also. In Fig.4 the box weighs 20N. What do you think the weight the hand gets? Is it 20 N, or more or less? If you have a spring balance you can straight away find out the foce the hand has to give. Or else you can do this experiment to get a good understanding of using a movable pulley.
Here in addition to the movable pulley , we are using a fixed pulley. This way we can use weights on the pans provided . L marked one is the Load while the E marked one is the Effort. You can find out for yourself the effort required to lift different loads. Record your results using a table as for the previous experiment. These are two important ratios which are very useful for the study of simple machines such as pulleys.
Fig. 6 Mechanics of a movable pulley. -
*A load of 10 N is raised using a pulley weighing 2N as shown in Fig.6. Neglecting friction and the weight of thread, find the following:*
= 2/1 = = 12 N The load is supported by two strands of thread. Therefore each thread will get a force = 12/2 = 3. Mechanical advantage = Load / Effort = 10/ 6 N =
What do you think is the highest possible Mechanical advantage?
Efficiency is another important ratio regarding machines. It gives the ratio of actual energy required to do some work is to the total energy that has to be used. This is usually given as a percentage. Here area few Formulae which may be used to find the Percentage efficiency of a machine.
## Belt drive
Here is an analysis of a system.
Will the green pulley rotate at the same speed, ? Silly says “same”. Ali says “Slow” Billy says “ fast” Who is right?
Large wheel turns slowly.
## Extension 3## COG WHEELS or GEARSCog wheels can work in two major ways; touching each other as in Fig. or if apart, with a chain drive. If the black wheel turns clockwise, which way will the brown wheel rotate?That’s easy. OK Now answer this. If the black one rotates at 45 rpm what will be the speed of the brown wheel?
Method 2 Find the following:- 2.0 If a machine has an efficiency of 80% and a velocity ratio of 2 how much will be the effort required for a load of 700 N? 3.1 if the man pulls 20 m of rope find the displacement of the log and thereby the velocity ratio. 3.2 Neglecting friction, find the minimum effort required for the man to raise the load.. 3.3 Neglecting friction but considering the weight of pulley, find the efficiency of the system. 3.4 If the efficiency is taken into consideration will the efficiency be higher or lower ? 3.5 If you use a much lighter load , will the efficiency be higher or lower? (20 marks) Two sets of smooth double pulleys have been used in two different ways Set A and set B. Copy this table and write the answers to these. One set of double pulleys weighs 18 N. The load is 482 for both
The data sheet,
1 When the pedal does one round how many rounds will the bicycle tyre will go? For solutions visit Answer page. |

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