Explanation:
Given that,
The mass of rock, m = 2.35-kg
It was released from rest at a height of 21.4 m.
(a) The kinetic energy is given by : 
As the rock was at rest initially, it means, its kinetic energy is equal to 0.
(b) The gravitational potential energy is given by : 
It can be calculated as :

(c) The mechanical energy is equal to the sum of kinetic and potential energy such that,
M = 0 J + 492.84 J
M = 492.84 J
Hence, this is the required solution.
Answer:
The time it took the bobsled to come to rest is 10 s.
Explanation:
Given;
initial velocity of the bobsled, u = 50 m/s
deceleration of the bobsled, a = - 5 m/s²
distance traveled, s = 250 m
Apply the following kinematic equation to determine the time of motion of the bobsled;
s = ut + ¹/₂at²
250 = 50t + ¹/₂(-5)t²
250 = 50t - ⁵/₂t²
500 = 100t - 5t²
100 = 20t -t²
t² - 20t + 100 = 0
t² -10t - 10t + 100 = 0
t (t - 10) - 10(t - 10) = 0
(t - 10)(t - 10) = 0
t = 10 s
Therefore, the time it took the bobsled to come to rest is 10 s.