A 1.55-kg rock is released from rest at the surface of a pond 1.00 m deep. As th

Question

A 1.55-kg rock is released from rest at the surface of a pond 1.00 m deep. As the rock falls, a constant upward force of 4.20 N is exerted on it by water resistance. Calculate the nonconservative work, Wnc, done by water resistance on the rock, the gravitational potential energy of the system, U, the kinetic energy of the rock, K, and the total mechanical energy of the system, E, for the following depths below the water's surface. Let y = 0 be at the bottom of the pond.

(a) d = 0.00 m

(b) d = 0.50 m

(c) d = 1.00 m

Explanation / Answer

Mass of the rock m = 1.55 Kg
Depth of the pond H = 1 m Total Mechanical energy E = mgH = 1.55*9.8*1 = 15.19 J Upward resistance force F = 4.20 N  [Non conservative force] Rock at d = 1 m depth
Work by water resistance W = F x d = (4.20)(1) = 4.2 J
P.E = mg(H - d) = (1.55)(9.8)(1 - 1) = 0 J
K.E = Total M.E. - P.E.- W
K.E. = 15.19 - 0 - 4.2 = 10.99 J
Total M.E. of system = 15.19 J (constant)

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