Simple Machines
Experiment 9 – Simple Pulleys
Introduction
Simple pulleys are a form of simple machines that makes heavy loads easier to lift. They have a grooved wheel that is used for a rope or belt to ride, and allows a force that is acting on one direction, to be changed to another more useful direction.
Types of simple pulley:
• Fixed Pulley – A pulley that is stationary relative to the earth
• Movable Pulley – A pulley that is allowed to move freely, being cradled by a rope or belt.
• Compound Pulley – Is a combination of both of the previously mentioned types.
Aims and Objectives
• To test if tension in a pulley cord changes due to a change in direction of the cord.
• To determine if the Mechanical Advantage (M.A) of a compound pulley system.
Apparatus
• 2 Adjustable hooks
• 4 Knurled nuts
• 2 Pulleys
• 1 Single pulley block
• 2 Weight hooks
• 1 Set of weights
• 1 10N Spring balance
• 1 Cord – Approximately 600mm long
Test 1 Results
W(N) P(N) M.A = W/P
1 1 1
2 2 1
3 3 1
4 4 1
5 5 1

Test 2 Results
W(N) P(N) M.A = W/P
1 1.1 0.91
2 1.9 0.91
3 3.2 0.94
4 4.1 0.98
5 4.7 1.1

Test 3 Results
W(N) P(N) M.A = W/P
1 0.5 2
2 1.1 1.82
3 1.8 1.67
4 2 2
5 2.2 2.27

Test 4 Results
W(N) P(N) M.A = W/P
1 0.6 1
2 1.2 1
3 1.7 1
4 2.3 1
5 2.9 1

Observations
In test 1 the readings are the same for both the load (W) and the effort (P) due to the fact that fixed pulley was used for the experiment and so only the direction of the force of effort applied that was changed by the pull. Pulling down is easier as gravity helps and one can use one’s own weight also.
In test2 the tension in the cord was not altered by the change in direction but friction in the pulley spindles made the apparatus insensitive. Because friction is always present in pulley systems and the friction will be increased by the increase in load.
In test 3 the total weight (i.e. the load plus the weight of the pulley block) was supported by two cords, each carrying half the total weight. Therefore, the weight recorded on the spring balance is half the total weight for some values of the total weight. And this confirms the theory of the movable pulley.
In test 4, the load (W) was lifted by a smaller force or effort (P). The advantage gained by using the movable and fixed pulley block can be expressed as a ratio W/P. This is called the mechanical advantage (M.A).
Conclusion
By combining fixed and movable pulleys it is possible to lift heavy loads with reduced effort and so gain a mechanical advantage
Questions
a) The effect of the tension in a cord when passed round a pulley T1 = 2 T2
Neglecting the mass of the movable pulley ‘B’ it is shown that T1 = 2 T2, and the tension in the cord was not altered by the change in direction but friction in the pulley spindles made the apparatus insensitive. If the frictional force is neglected, the tension throughout the cord will be the same as the amount of effort applied.
b) In test 4 we are able to lift load with an effort of a lesser force, in some ways this can seem as if we are getting “something for nothing”, but in actuality for us to achieve this mechanical advantage, our effort must travel a greater distance.
c) Yes it does
d) No, it doesn’t give the same mechanical advantage as the method used in test 4 because there was less friction in test 4 compared to figure 5.
Extra Questions
If W = 2kN
1000N = 1kN ? = 2kN = 2000N
Therefore P = W/2 = 2000/2 =1000N