Chapter - 4 Work and Energy

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Q
Work and Energy

Question:

Look at the activities listed below. Reason out whether or not work is done in the light of your understanding of the term ‘work’.

• Suma is swimming in a pond.

• A donkey is carrying a load on its back.

• A wind-mill is lifting water from a well.

• A green plant is carrying out photosynthesis.

• An engine is pulling a train.

• Food grains are getting dried in the sun.

• A sailboat is moving due to wind energy. 

Answer:

Work is done whenever the given 2 conditions are satisfied:

(i) A force acts on the body.

(ii) There's a displacement of the body by the applying of force in or opposite to the direction of force.

  • While swimming, Suma applies a force to push the water backwards. Therefore, Suma swims within the forward direction caused by the forward reaction of water. Here, the force causes a displacement. Hence, work is done by Suma while swimming.
  • Whereas carrying a load, the donkey should apply a force within the upward direction. But, displacement of the load is within the forward direction. Since, displacement is perpendicular to force, the work done is zero.
  • A wind mill works against the gravity to elevate water. Hence, work is finished by the wind mill in lifting water from the well.
  • During this case, there's no displacement of the leaves of the plant. Therefore, the work done is zero.
  • An engine applies force to tug the train. This allow the train to move within the direction of force. Therefore, there's a displacement within the train in the same direction. Hence, work is finished by the engine on the train.
  • In this case during the process of food grains getting dried in the sun, no work is done as food grains do not displace in the presence of solar energy.

  • In this case wind energy applies a force on the sailboat to push it in a forward direction. Therefore, the displacement of the boat is along the direction of the force. Hence, the work is done by wind on the boat.


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