Chapter - 4 Work and Energy

Q&A -Ask Doubts and Get Answers

Work and Energy


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. 


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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