**AP Physics 1 Question 390: Answer and Explanation**

### Test Information

Use your browser's back button to return to your test results.

**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

**Explanation:**

**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 m^{2}/s^{2} - 100 m^{2}/s^{2}) = (25 kg)(125 m^{2}/s^{2}) = 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.