1. A book of mass 1.5 kg is thrown out of a window at the height of 12 m with a
ID: 1283407 • Letter: 1
Question
1. A book of mass 1.5 kg is thrown out of a window at the height of 12 m with a speed 4.0 m/s. Assuming that the potential energy is zero at the ground level (y=0) and neglecting the air resistance, find
a) the intitial mechanical energy of the book when it is released;
b) the speed of the book before it hits the top of a vehice at the height 2.4 m from the ground
2. A rock of mass 2100 kg slides from rest down the hill from A, whre the height is y_a= 18 m, to point B at the bottom of the hill (i.e y_b=0). When it reaches point B, the speed is v_b = 12.0 m/s. Find
a) its potenital energy at point a and kinectic energy at point B;
b) the work done by the frictional force during the slide from A to B.
Explanation / Answer
1. a) P.E. = mgh = 1.5 x 9.8 x 12 = 176.4 J
b) change in P.E. = chang in K.E.
176.4 - 1.5 x 9.8 x 2.4 = 1.5v^2 /2 - 0
v = 13.72 m/s
2. a) P.E. at a = mgh = 2100 x 9.8 x 18 = 370440 J
K.E.f = 2100 x 12^2 /2 = 151200 J
b) P.E.i + K.E.i + work done by friction = P.E.f + K.E.f
370440 +0 +W = 0 + 151200
W = - 219240 J
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