AP Physics 1 Question 390: Answer and Explanation

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Question: 390

8. A student launched a small rocket with a mass of 50 kg into the air with an initial velocity of 10 m/s in the positive vertical direction. The student then turns on the secondary engines of the rocket to apply a constant upwards force that increases the velocity of the rocket to 15 m/s. If the force provided by the secondary engines did 4,500 J of work, how much work did the force of gravity do on the rocket?

  • A. -625 J
  • B. -750 J
  • C. -1,250 J
  • D. -1,375 J

Correct Answer: D



The total work done on the rocket can be calculated using the Work-Energy Theorem:

(50 kg)(15 m/s)2 - (10 m/s)2 = (50 kg)(225 m2/s2 - 100 m2/s2) = (25 kg)(125 m2/s2) = 3,125 J

This is the total work done on the rocket. As the rocket's secondary engines did 4,500 J of work, this means that the force of gravity must have done -1,375 J of work.