You can calculate potential energy by:
U = m.g.h
Where, U = potential energy
m = mass
g = acceleration due to gravity
h = height
Hope this helps!
Answer:
(a) 
(b) 
(c) K.E. = 21.168 J
(d) 
Explanation:
Given:
- mass of a block, M = 3.6 kg
- initial velocity of the block,

- constant downward acceleration,

That a constant upward acceleration of
is applied in the presence of gravity.
∴
- height through which the block falls, d = 4.2 m
(a)
Force by the cord on the block,



∴Work by the cord on the block,


We take -ve sign because the direction of force and the displacement are opposite to each other.

(b)
Force on the block due to gravity:

∵the gravity is naturally a constant and we cannot change it


∴Work by the gravity on the block,



(c)
Kinetic energy of the block will be equal to the net work done i.e. sum of the two works.
mathematically:


K.E. = 21.168 J
(d)
From the equation of motion:

putting the respective values:

is the speed when the block has fallen 4.2 meters.
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If F = Gm₁m₂/d², and we change m₁ to 5m₁ and m₂ to 2m₂, then the new magnitude of the gravitational force is
F' = G (5m₁) (2m₂) / d²
F' = 10 Gm₁m₂ / d²
but this is really just F' = 10F. So J is the correct choice.
Answer:
g / 16
Explanation:
T = 2π 
angular frequency ω = 2π /T
= 
ω₁ /ω₂ = 
Putting the values
ω₁ = ω , ω₂ = ω / 4
ω₁ /ω₂ = 4
4 = 
g₂ = g / 16
option d is correct.