NAEST 2019 Screening Test: Force on the Dog

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Dr. H. C. Verma and his team were discussing about NAEST in the summer of 2019. It was an informal discussion at the canteen of IIT Kharagpur. Two dogs were listening to this conversation in their dreaming state. On seeing these dogs, an idea came to the mind of Dr Verma. He suddenly standup and hit one of the dog with his notebook. The dog could not resist the impact and moved out. This event leads to the following question of NAEST 2019.

Question: The dog was at rest to start with and started moving. The motion in the horizontal direction started due to

  1. the force exerted by the notebook on the dog.
  2. the force exerted by the dog on itself.
  3. the contact force exerted by the floor on the dog.
  4. the gravitational force exerted by the earth on the dog.

Solution: There are many techniques to solve multiple choice questions (MCQs). I will use the elimination technique to get the answer.

The force exerted by the notebook cannot move the dog. To get you convinced, imagine the dog to be dead. Can you move a dead dog by hitting it with a notebook? No. Thus, option (A) is ruled out.

The dog cannot move his centre of mass by exerting a force on himself. The internal forces cannot change the linear momentum of the system. Thus, option (B) is also ruled out.

Let us see the option (D). The gravitational force on the dog is vertically downward. The vertical force cannot move the dog in the horizontal direction. Hence, option (D) is not correct.

By elimination technique, (C) is the correct option. The force exerted by the ground on the dog moves it horizontally. Note that the dog applies a force on the ground using his muscles. The reaction to this force acts on the dog and moves him horizontally.

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