Lesson: Chapter 18 — Work, Energy and Power

18.2. Kinetic energy 7 of 7 Reset answers

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Question 7

An object of mass 800 g is thrown vertically upwards with a velocity of 20 m s^-1. What is its kinetic energy at the moment it leaves the ground? (Mixed exercise, question 2 i)

A 8 J B 16 J C 320 J D 160 J

The right answer is the one marked above.

160 J. 800 g is 0.8 kg, so E_k = ½ × 0.8 × 20² = ½ × 0.8 × 400 = 160 J.

320 J is what you get by leaving out the ½. Every term of the formula has to be used, including the one that looks least important.

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