I need help with a physics question

Question

Lets say a pendulum turns all of its KE to PE at the top of its swing. If the bottom of the swing is at a height of 0 m, how high above the ground is the pendulum at the top of its swing?

mass= 3kg

Speed= 5m/s

A. 1.25m 

B. 3.75m

C. 5m

D.15m

E. 75m

Answer 1

Well, conservation of energy is involved, which means that all of the KE at the bottom becomes PE at the top.

Using formulas for each we can see that (0.5)mv^2 = mgh

But the m's on both sides cancel and doing some rearranging, we get h = v ^2 / 2g

Substituting v = 5 and g = 10, we get answer A. 1,25 m

Answer 2

From the speed and the mass, you are able to calculate the kinetic energy by using the formula:

KE= 1/2 (M)(V^2)

This would results in an energy of 37.5 J

Now you can use the calculated energy to find the height that the pendulum can swing to with the formula for potential energy:

PE=mgh

When you enter 37.5 J for PE and 3 KG for M, and 9.6 for the acceleration due to gravity, you get a height of 1.25 m

So, answer A