How many joules of kinetic energy does the pendulum have when it has 50 J of potential energy?

We do not have the information of the pendulum, so i will answer in a general way.

First, as you know, the total energy can be written as:

E = U + K

Where:

U = potential energy

K = kinetic energy.

Now, by the conservation of energy, we know that E does not change.

This means that if U increases an amount x, then K must decrease by the same amount.

Now, at the maximum height of the pendulum, the velocity is zero, this means that in this point the kinetic energy is 0j.

E = 0 + U

E = U.

Another thing that you may know about pendulums is that, if the initial displacement is by an angle θ, then the angle between the string and the vertical can never be more than θ.

And we know that:

U = m*g*h

in this case, we can write h, the maximum height, as:

h = L*(1 - cos(θ))

Where:

L is the length of the string, g is the gravitational acceleration and m is the mass of the pendulum.

Then the total energy of the pendulum is:

E = m*g*L*(1 - cos(θ))

And we had that:

E = K + U = m*g*L*(1 - cos(θ))

Then, if in a given point we have U = 50j, the kinetic energy must be:

K + 50j = m*g*L*(1 - cos(θ))

K = m*g*L*(1 - cos(θ)) - 50j

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