## Conceptual Physics (12th Edition)

Published by Addison-Wesley

# Chapter 7 - Think and Rank: 51

#### Answer

a. B, A, C b. C, B, A c. C, B, A

#### Work Step by Step

a. Momentum is mv. Their momentum values, in the order ABC, are $$(800 kg)(1.0 \frac{m}{s})=800 \frac{kg \cdot m}{s}$$ $$(1000 kg)(2.0 \frac{m}{s})=2000 \frac{kg \cdot m}{s}$$ $$(90 kg)(8.0 \frac{m}{s})=720 \frac{kg \cdot m}{s}$$ b. KE is $\frac{1}{2}mv^{2}$. Their KE amounts, in the order ABC, are $$\frac{1}{2}(800 kg)(1.0 m/s)^{2} = 400 J$$ $$\frac{1}{2}(1000 kg)(2.0 m/s)^{2} = 2000 J$$ $$\frac{1}{2}(90 kg)(8.0 m/s)^{2} = 2880 J$$ c. The work needed to bring each object up to its respective speed from rest is the change in KE, or in this case, final KE - 0. The final kinetic energies were just calculated.

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