Physics GRE GR 1777, Problem 019 Solution

19. Classical Mechanics (Work-Energy Principle)

Solution
From the work-energy theorem,

W = \Delta K = \frac{1}{2} mv_f^2 - \frac{1}{2} mv_i^2
Since the work done by the box is
W = F \cdot d = F \cdot (5 m)
and the change of the kinetic energy is
\frac{1}{2} mv_f^2 - \frac{1}{2} mv_i^2 = \frac{1}{2} (10 kg) \cdot [ (2 m/s)^2 - (1 m/s)^2] = 15 kg \cdot m^2/s^2
Therefore, The force is
F = 3 kg \cdot m/s^2 = 3N

Answer
(C) 3N
Reference
https://en.wikipedia.org/wiki/Work_(physics)#Work–energy_principle

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