Question

A 100-kg boulder sits on the top of a 50-meter high cliff. It falls off. How fast is it traveling just before it hits the ground?
A. 31 m/s


B. 490 m/s


C. 98 m/s


D. 245 m/s

Answer 1

m = mass of the boulder = 100 kg

h = height of the cliff from which boulder falls = 50 m

g = acceleration due to gravity = 9.8 $\frac{m}{s^{2}}$

v = speed of boulder just before it hits the ground = ?

Using conservation of energy

Kinetic energy of boulder just before hitting the ground = Potential energy of boulder at the top of cliff

(0.5) m v² = mgh

dividing both side by "m" , "m" cancel out and we get

(0.5) v² = gh

inserting the values

(0.5) v² = (9.8) (50)

v = 31 $\frac{m}{s}$

Hence correct choice is A. 31 m/s