the missing force is spring force.
The object is hanging from the spring and the spring is stretched by some distance from its equilibrium position. due to this stretch in the spring , a spring force starts acting on the object trying to regain its equilibrium position.
the spring force is given as
F = kx
where F = spring force ,k = spring constant , x = stretch in the spring.
the spring force balances the weight of the object in down direction and hence keeps the block from falling down.
To find the change in centripetal acceleration, you should first look for the centripetal acceleration at the top of the hill and at the bottom of the hill.
The formula for centripetal acceleration is:
Centripetal Acceleration = v squared divided by r
where:
v = velocity, m/s
r= radium, m
assuming the velocity does not change:
at the top of the hill:
centripetal acceleration = (4.5 m/s^2) divided by 0.25 m
= 81 m/s^2
at the bottom of the hill:
centripetal acceleration = (4.5 m/s^2) divided by 1.25 m
= 16.2 m/s^2
to find the change in centripetal acceleration, take the difference of the two.
change in centripetal acceleration = centripetal acceleration at the top of the hill - centripetal acceleration at the bottom of the hill
= 81 m/s^2 - 16.2 m/s^2
= 64.8 m/s^2 or 65 m/s^2
If l and m both are doubled then the period becomes √2*T
what is a simple pendulum?
It is the one which can be considered to be a point mass suspended from a string or rod of negligible mass.
A pendulum is a weight suspended from a pivot so that it can swing freely.
Here,
A certain frictionless simple pendulum having a length l and mass m
mass of pendulum = m
length of the pendulum = l
The period of simple pendulum is:

Where k is the constant.
Now the length and mass are doubled,
m' = 2m
l' = 2l



Hence,
If l and m both are doubled then the period becomes √2*T
Learn more about Simple Harmonic Motion here:
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