Pulley Systems

A simple machine consisting essentially of a wheel with a grooved rim in which a pulled rope or chain can run to change the direction of the pull thereby lifting a load is called a Pulley. Pulleys can be used as Force Magnifiers. If there were no friction, using one pulley to lift a load of 1 N would need an effort of 1 N. The use of one pulley only changes the direction of the applied force. If two pulleys were used, only an effort of 0.5 N would be needed. If three pulleys were used, only an effort of 0.33N would be needed. The efficiency of a pulley system is less than 100% due to friction.

Velocity ratio = Number of pulleys used / Number of ropes supporting the load

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Types of Pulleys

There are two types of pulleys, which are given below with their definitions-

* Fixed Pulley: A fixed pulley has a fixed axle and is used to redirect the rope. A fixed pulley has a mechanical advantage of 1.

* Movable Pulley: A movable pulley has a free axle, and is used to transform forces – when stationary the total force on the axle balances the total force provided by the tension in the rope.

Important Formulae’s

Energy Output = Load X Weight Lifted

Energy Input = Effort X Distance Moved

Efficiency = Energy Output / Energy Input

Method

* I had set one pulley system and hung 1N load (100g) from it.

* Using a spring balance I had measured the effort which was needed to lift the load vertically through 10 centimetres.

* After the above, I had set up a two pulley system and hung a 1N load from it.

* Using a spring balance I had measured the effort needed to lift the load vertically through 10 centimetres.

* After the two-pulley system, I had set up a three-pulley system and hung 1 N load to lift the load vertically through 10 centimetres.

Equipment

* Pulley systems (3).

* Clamp Stand.

* Weights.

* Spring balance.

* Rope.

* Pen.

* Paper.

* Ruler.

Safety

There were only a few safety measures which were observed while accomplishing the experiment-

* Firstly, I was careful while using the spring balance as it was a delicate piece of equipment.

* Lastly, as I had desired to achieve accurate results I had taken extra care with all the equipment while carrying out the experiment.

Results (With 1N Load)

Number of Pulleys

Effort (N)

Accurate results from Computer

1

1.2N

1 N

2

0.4 N

0.5 N

3

0.38 N

0.33 N

Conclusion

From my results I have found out that the effort decreases when more pulleys are used. This is because the load is spread on all the pulleys.

Evaluation

I believe I have successfully accomplished the experiment with achieving the desired results although not the perfect results. I think my results were not accurate as compared to the results, from the computer, but according to the other groups, I have achieved respectable results. I think the main difference, which caused a barrier, leading me to inaccurate results, was Friction. I believe that I could have made my experiment accurate by lubricating the ropes and the wires that were to be used while accomplishing the experiment. If the friction was not present, I believe everyone have achieved the desired results. Besides this, I could have made my experiment more accurate by carrying out the experiment a few more times and by taking the average.

The second main reason, I did not achieve accurate results was while taking the results from the spring balance. This was because when I had set up everything properly I could not make the balance at the same place and it kept on moving.

Mechanical Advantage

Mechanical Advantage = Load / Effort

For e.g. for 1 pulley the mechanical advantage would be = 1 N / 1.2= 0.84

For 2 pulleys the mechanical advantage would be = 1 N / 0.6 = 1.67

For 3 pulleys the mechanical advantage would be = 1 N / 0.4 = 2.5 times easier.

With the one pulley it would be 0.84 times easier, with two pulleys it would be 1.67 times easier and lastly with three pulleys it would be 2.5 times easier.

Velocity Ratio

Velocity Ratio = Distance moved by Effort / Distance moved by Load.

The distance between the load distance and the effort distance is called the Velocity Ratio.

The smaller force always moves the longer distance. The Velocity Ratio does not depend on friction.

Efficiency

Efficiency (%) = Mechanical Distance (M.A) / Velocity Ratio (V.R) X 100

Disadvantages of Friction

1) Due to friction in between our shoes and the floor, the soles of our shoes wear out.

2) Tyres of vehicles wear out due to friction.

3) There is a lot of fuel wastage in overcoming friction.

4) Travelling becomes more expensive as fuel consumption increases order to overcome friction.

5) Parts of machinery wear out due to friction between them as they slide over or rub over each other.

6) Due to friction, in between the moving parts, a lot of heat is wasted. It heats up the machine and decreases the efficiency.

7) A complete absence of friction would make it difficult for us to walk. One tends to slip on banana skin or ice, as friction becomes negligible.

Advantages of Friction

1) Without friction it is not possible to walk. We shall therefore, not be able to walk on ice.

2) Writing and drawing would be impossible in absence of friction between the pen and the paper.

3) Without friction in between the saw and the wood, it would be impossible to cut the wood.

4) The brakes of bicycles and automobiles would not work without friction between the rim and the brake shoe. To increase friction the brake shoes are made rougher.

Friction can be Increased or Decreased

Friction can be increased in specific cases by following methods-

1) Sand is spread on slippery roads to increase friction and to avoid slipping of wheels.

2) A greater force of friction is required between the ground and the soles of shoes worn by athletes and players. To increase the friction in the soles of the shoes, spikes are provided in the soles.

3) In certain moving parts of the machinery such as the belt and the wheel, a greater friction is required; therefore special materials can achieve it.

Friction can be decreased-

1) Lubrication, oiling, and greasing the parts of the machinery can reduce friction.

2) By using ball bearings, the friction between the moving parts of machinery is considerably reduced.

Udit Aggarwal