A body of mass 2 kg is thrown upward with an initial velocity of 20 m/s. After 2 seconds, its kinetic energy will be: (g = 10 m/s²)
- A.100 J
- B.0 J
- C.400 J
- D.200 J
Correct answer
Explanation
The correct answer is B, 0 J. The body is thrown up at 20 m/s and gravity takes away 10 m/s of speed every second, so after 2 seconds its speed is zero. Using v = u − gt, we get v = 20 − 10 × 2 = 0 m/s. Kinetic energy is half m v squared, and with v = 0 the kinetic energy is also zero. At that instant the body is at the highest point of its path and all of its energy is potential energy, mgh = 2 × 10 × 20 = 400 J, since the body has risen 20 m. A moment later it begins to fall and the kinetic energy grows again. Option A is wrong because 100 J needs a speed of 10 m/s, which the body has after only one second. Option C is wrong because 400 J is the kinetic energy at the moment of throwing, and the potential energy at the top. Option D is wrong because 200 J matches no point of this motion. Exam tip: at the highest point of a vertical throw the speed, and so the kinetic energy, is zero.
Practice Questions
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