# A 0.5 kg ball is about to roll off the edge of a 1.5 m tall table. What is its current potential energy? (g = 9.8 ms²) ?

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- NCSLv 72 months ago
Depends on your frame of reference. If you consider the ground to be the point of 0 gravitational potential energy, then at the tabletop

GPE = 0.5kg * 9.8m/s² * 1.5m = 7.4 J

But if the tabletop is your 0 GPE height, then at the tabletop

GPE = 0 J

And if the ceiling 2 m above the table is your 0 GPE reference, then at the tabletop

GPE = 0.5kg * 9.8m/s² * -2m = -9.8 J

The 0 GPE height can be set anywhere; it's the CHANGES in height that matter.

Hope this helps!

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