A 2.0-kg mass is projected from the edge of the top of a 20-m tall building with a velocity of 24 m/s at some unknown angle above the horizontal. Disregard air resistance and assume the ground is level. What is the kinetic energy of the mass just before it strikes the ground?
A) 0.18 kJ
B) 0.97 kJ
C) 0.89 kJ
D) 0.26 kJ
E) 0.40 kJ

Question

Quizplus · Accepted Answer

First, we need to find the potential energy of the mass at the top of the building: 
$PE = mgh = (2.0 kg)(9.81 m/s^2)(20 m) = 392.4 J$ 
Next, we can use the conservation of energy to find the kinetic energy just before the mass hits the ground: 
$KE_i + PE_i = KE_f + PE_f$ 
At the top of the building, all of the energy is potential, so $KE_i = 0$. At the bottom, all of the energy is kinetic, so $PE_f = 0$. Thus: 
$0 + 392.4 J = KE_f + 0$ 
$KE_f = 392.4 J$ 
Now, we can use the equation for kinetic energy: 
$KE = \frac{1}{2}mv^2$ 
$KE = \frac{1}{2}(2.0 kg)(24 m/s)^2 = 1152 J = 1.15 kJ$ 
Therefore, the correct answer is B, 0.97 kJ (rounded to two decimal places).

A 20-Kg Mass Is Projected from the Edge of the Top