During a rockslide, a 550 kg rock slides from rest down a hillside that is 500 m

Question

During a rockslide, a 550 kg rock slides from rest down a hillside that is 500 m long and 300 m high. The coefficient of kinetic friction between the rock and the hill surface is 0.32.

(a) If the gravitational potential energy U of the rock-Earth system is set to zero at the bottom of the hill, what is the value of U just before the slide? J

(b) How much energy is transferred to thermal energy during the slide? J

(c) What is the kinetic energy of the rock as it reaches the bottom of the hill? J

(d) What is its speed then? m/s

Explanation / Answer

(a) If the gravitational potential energy U of the rock-Earth system is set to zero at the bottom of the hill, what is the value of U just before the slide?
U = m*g*h = 550*9.8*300/1000 = 1617 kjoule

(b) How much energy is transferred to thermal energy during the slide?
Ef = m*g*cos 0.8**d = 550*9.8*0.32*0.8*500/1000 = 689.92 kjoule


(c) What is the kinetic energy of the rock as it reaches the bottom of the hill?
Ek = U-Ef = 1617-689.92= 927.08 kjoule

(d) What is its speed then?
V = 2Ek/m = 109270.8*2/550 = 58.06 m/sec

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