Tag: machines

Questions Related to machines

A force of $250 N$ is required to lift a $75 kg$ mass through a pulley system. In order to lift the mass through $3 m$, the rope has to be pulled through $12m$. The percent efficiency of system is

physics machines levers and pulleys pulley pulleys

  1. $50$

  2. $75$

  3. $33$

  4. $90$

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Correct Option: B
Explanation:
Height lift was $h=3m$
Work done by the force is $W=Fd=250\times 12=3000Joule$

Work against the gravity i.e. $increase $ in potential energy of the  mass is $E=mgh=75\times 10\times 3=2250Joule $

so efficiency will be $\eta \times 100=\dfrac{E}{W}\times 100$% = $75$%

It is easy to draw water from a well when the rope passes over a pulley than drawing it directly. Because :

physics machines levers and pulleys pulley pulleys

  1. Pulley adds more force

  2. Pulley decreases the weight of the bucket

  3. Pulley has a mechanical advantage of more than one

  4. Pulley changes the direction of force

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Correct Option: D