Question

A brick lies perilously close to the edge of the flat roof of a building. The roof edge is 50 ft above street level, and the brick has 560.0 J of potential energy with respect to street level. Someone edges the brick off the roof, and it begins to fall. What is the brick's kinetic energy when it is 35 ft above street level?

Answer 1

Hey there!:

The potential energy is U =  m*g*h

Substitute 560.0 J for U = m*g*h so,  560.0 = m*g*h

g = 9.8 m/s²

50 ft in meters :

1 ft = 0.3048 m

50 ft = 50 * 0.3048 => 15.24 m

m = 560.0 / g*h

m = 560.0 / 9.8 * 15.24

m = 560.0 / 149.352

m = 3.74 kg

35 ft in m : 35 * 0.3048 => 10.668 m

The change initial  in potencial energy from point A to B is:

mg ( h1 - h2 ) = 3.74* ( 9.8 )* (15.24 - 10.668 )

=>  36.652 * ( 15.24 - 10.688 ) =

36.652 * 4.552 => 167.57 J

According to the conservation of energy

The change in potencial energy should be equal to the change in kinetic energy , so the , the change in kinetic energy is :

ΔK = 167.57 J

Hope that helps!