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Answer: An 8 kg book at a height of 3 m has the most gravitational potential energy.
Explanation:
Gravitational potential energy is the product of mass of object, height of object and gravitational field.
So, formula to calculate gravitational potential energy is as follows.
U = mgh
where,
m = mass of object
g = gravitational field = 
h = height of object
(A) m = 5 kg and h = 2m
Therefore, its gravitational potential energy is calculated as follows.

(B) m = 8 kg and h = 2 m
Therefore, its gravitational potential energy is calculated as follows.

(C) m = 8 kg and h = 3 m
Therefore, its gravitational potential energy is calculated as follows.

(D) m = 5 kg and h = 3 m
Therefore, its gravitational potential energy is calculated as follows.

Thus, we can conclude that an 8 kg book at a height of 3 m has the most gravitational potential energy.
Answer:
v = 12.86 km/h
v = 3.6 m/s
Explanation:
Given,
The distance, d = 13.5 km
The time, t = 21/20 h
= 1.05 h
The velocity of a body is defined as the distance traveled by the time taken.
v = d / t
= 13.5 km / 1.05 h
= 12.86 km/h
The conversion of km/h to m/s
1 km/h = 0.28 m/s
12.86 km/h = 12.86 x 0.28 m/s
= 3.6 m/s
Hence, the velocity in m/s is, v = 3.6 m/s
When Sam presses the brake lever, a pair of rubber shoes clamps onto the metal inner rim of the front and back wheels. As the brake shoes rub against the wheels, friction is caused and the kinetic energy possessed by the vehicle is converted into heat which slows down the vehicle.