A 9 kg ball starts at the top of a 84 m high building. How much kinetic energy does it have three-quarters of the way to the ground?

1 Answer
Dwight · Nam D.
Jan 27, 2018

It will have #5556.6 J# of kinetic energy at a point three-quarters of the way down.

Explanation:

The simplest way to approach this problem is to realize that any increase in the kinetic energy has come as a result of an equal reduction in the potential energy.

This potential energy is gravitational, and given by

#PE = mgh#

where #h# is the height of the object, #m# its mass and #g# is the gravitational field, #9.8 N"/"kg#.

At top the top of the building, this gives us

#PE = 9xx9.8 xx84=7408.8 J#

Three-quarters of the way down, its height is #21 m#, and its remaining #PE# is

#PE = 9xx9.8 xx21=1852.2 J#

Therefore, it has lost #5556.6 J# of potential energy and has gained #5556.6 J# of kinetic energy.

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