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Q.

A body of mass 0.8 kg has initial velocity $(3 \hat{i} - 4 \hat{j}) m / s$ and final velocity $(- 6 \hat{j} + 2 \hat{k}) m / s$ . Find the change in kinetic energy of the body ?

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a

2 J

b

4 J

c

6 J

d

8 J

answer is C.

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Detailed Solution

Change in kinetic energy

$\begin{array}{l} ΔKE = \frac{1}{2} {mv}_{f}^{2} - \frac{1}{2} {mv}_{i}^{2} \\ where, v_{f} = \sqrt{6^{2} + 2^{2}} = \sqrt{40} \\ and v_{i} = \sqrt{3^{2} + 4^{2}} = \sqrt{25} \\ = \frac{1}{2} \times 0 .8 ((\sqrt{40})^{2} - (\sqrt{25})^{2}) \\ = 0 .4 [40 - 25] = 0 .4 (15) = 6 joules. \end{array}$

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