Calculating Change in PE and KE of a 100 kg Rock | Energy Homework

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The gravitational potential energy (PE) of a 100 kg rock at 5 meters above the ground is calculated to be 4900 J using the formula MGH. When the rock falls to the ground, its PE becomes 0 J, resulting in a change in PE of 4900 J. The change in kinetic energy (KE) is also calculated to be 4900 J, as it equals the change in PE due to the conservation of energy principle. There is a suggestion to include +/- signs to indicate the direction of change in energy. The calculations are confirmed to be correct, assuming the Earth's surface is the zero reference point.
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Homework Statement


. Consider the energy of a 100 kg rock.
a) What is the gravitational potential energy (PE) of the rock that is 5 m above the ground?

b) The rock falls to the ground. What is its PE?

c) What is the change in PE of the rock?

d) What is the change in the kinetic energy of the rock?


Homework Equations



MGH

The Attempt at a Solution



a) 100*9.8 m/s2*5 = 4900 J
b) 0J
c) Change in PE = 4900 J
d) Change in KE = ½ * 100kg * (9.9 m/s)2 = 4900 J

Can anyone look at my answers?
 
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Looks correct assuming that the surface of the Earth is taken as the zero reference point. Should include +/- signs for the "change in XX" to indicate whether its an increase or a decrease. What's with the strange working for change in KE though? Since no external forces act on the system, the change in KE is simply equal to the change in PE magnitude-wise.
 
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