During a rockslide, a 770 kg rock slidesfrom rest down a hillside that is 500 m

Question

During a rockslide, a 770 kg rock slidesfrom rest down a hillside that is 500 m long and 300 m high. Thecoefficient of kinetic friction between the rock and the hillsurface is 0.29. (a) If the gravitational potential energyU of the rock-Earth system is set to zero at the bottom ofthe hill, what is the value of U just before theslide?
1 J

(b) How much energy is transferred to thermal energy during theslide?
2 J

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

(d) What is its speed then?
4 m/s (a) If the gravitational potential energyU of the rock-Earth system is set to zero at the bottom ofthe hill, what is the value of U just before theslide?
1 J

(b) How much energy is transferred to thermal energy during theslide?
2 J

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

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

Explanation / Answer

a. U = mgh = 770*9.8*300 = 2.26E6 J b. Q = mgd = 0.29*770*9.8*500 = 1.09E6 J c. Energy conservation: U = K+Q K = U - Q = 1.17E6 J d. K = (1/2)mv2 v = (2K/m) = 55.1 m/s

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