A 6.0-kg block is released from rest 80 m above the ground.When it has fallen 60 m its kinetic energy is approximately:
A)4700 J
B)3500 J
C)1200 J
D)120 J
E)60 J

Question

Quizplus · Accepted Answer

When the block is released from rest at a height of 80 m, it has potential energy given by:

PE = mgh = (6.0 kg)(9.81 m/s^2)(80 m) = 4,705 J

As the block falls, gravitational potential energy is converted to kinetic energy. When the block has fallen 60 m, its height is h = 80 m - 60 m = 20 m. At this height, its potential energy is:

PE = mgh = (6.0 kg)(9.81 m/s^2)(20 m) = 1,177 J

Since the total mechanical energy of the block is conserved (ignoring air resistance), the remaining energy must be kinetic. Therefore, the kinetic energy of the block when it has fallen 60 m is:

KE = total energy - potential energy = PE(initial) - PE(fall) = 4,705 J - 1,177 J = 3,528 J

Rounding this to the nearest hundred gives an answer of approximately 3,500 J, which corresponds to choice B.

A 60-Kg Block Is Released from Rest 80 M Above the the Ground.When