Answer:
20 J
Step-by-step explanation:
The law of conservation of energy states that (if we neglect air resistance) the mechanical energy of an object in free fall is conserved:
where
E is the mechanical energy, which is the sum of
U = potential energy
K = kinetic energy
When the ball is held 10 meters above the ground, its gravitational potential enegy is U = 20 J, while its kinetic energy is K = 0 (because the ball is at rest). Therefore, its mechanical energy is
E = U + K = 20 J + 0 = 20 J
Just before hitting the ground, its potential energy is zero (because its height is now zero), and since the mechanical energy must be conserved, we still have that E = 20 J. So, we can find the kinetic energy just before hitting the ground:
