A 1.5 kg block is pushed against a spring (block is not attached to the spring)

Question

A 1.5 kg block is pushed against a spring (block is not attached to the spring) on 37 degree-inclined plane, until the spring is compressed 35 cm and released from rest. The coefficient of kinetic friction between the block and the inclined plane is mu k = 0.35; and the spring constant is 150 N/m. What is the change in gravitational potential energy of the block when it momentarily stops? What is the corresponding increase Delta Eth in the thermal energy of the block-floor system

Explanation / Answer

1. The problem statement, all variables and given/known data Ok, I am deriving the change in gravitational potential energy at the surface of the earth and have met with some difficulties. 2. Relevant equations g=GM/r2 P.E.=-GMm/r 3. The attempt at a solution dP.E.=-GMm/rf+GMm/ri Where rf>ri Since gravitational field strength is approaximately a constant for small changes of r, dP.E.=-mgrf+mgri But this gives me a negative answer! How can my change in gravitational potential energy be negative when I bring my object from a lower position to a higher poistion in the gravitational field. I have probed the mathematics of the above proof, but it all makes sense to me since g is a constant.

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