MCAT Physics Question 16: Answer and Explanation

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Question: 16

1. A weight lifter lifts a 275 kg barbell from the ground to a height of 2.4 m. How much work has he done in lifting the barbell, and how much work is required to hold the weight at that height?

  • A. 3234 J and 0 J, respectively
  • B. 3234 J and 3234 J, respectively
  • C. 6468 J and 0 J, respectively
  • D. 6468 J and 6468 J, respectively

Correct Answer: C

Explanation:

Because the weight of the barbell (force acting downward) is

or about 2750 N, it follows that the weightlifter must exert an equal and opposite force of 2750 N on the barbell. The work done in lifting the barbell is therefore W = Fd cos θ = (2750 N)(2.4 m)(cos 0) ≈ 7000 J. Using the same equation, it follows that the work done to hold the barbell in place is W = Fd cos θ = (2750 N)(0 m)(cos θ) = 0 J. Because the barbell is held in place and there is no displacement, the work done is zero. This is closest to choice (C).

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