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Q: Just before hitting the earth, the kinetic energy of an object of mass 2 kg is 400 J. At which height it was dropped?
  • A. 10m
  • B. 25m
  • C. 20m
  • D. 15m
Correct Answer: Option C - From conservation of energy, Kinetic energy of body before striking (hitting) the ground = potential energy of body at height h from the ground, P.E. = mgh here, m = 2 kg, P.E = 400 J g = 10 m/s² 400 = mgh 400 = 2 × 10 × h h = 20 m.
C. From conservation of energy, Kinetic energy of body before striking (hitting) the ground = potential energy of body at height h from the ground, P.E. = mgh here, m = 2 kg, P.E = 400 J g = 10 m/s² 400 = mgh 400 = 2 × 10 × h h = 20 m.

Explanations:

From conservation of energy, Kinetic energy of body before striking (hitting) the ground = potential energy of body at height h from the ground, P.E. = mgh here, m = 2 kg, P.E = 400 J g = 10 m/s² 400 = mgh 400 = 2 × 10 × h h = 20 m.