Abrick of mass 2 kg is dropped from a rest position 5 m above the ground. what is its velocity at a height of 3 m above the ground?

We can solve the problem by using the law of conservation of energy.

Using the ground as reference point, the mechanical energy of the brick when it is at 5 m from the ground is just potential energy (because the brick is initially at rest, so it doesn't have kinetic energy):


when the brick is at h'=3 m from the ground, its mechanical energy is now sum of kinetic energy and potential energy:


where v is the velocity of the brick. Since E is conserved, it must be equal to the initial energy (98.1 J), so we can solve this equation to find v:

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