A 5kg mass is attached to the end of a string 2 meters long. The other end of th

Question

A 5kg mass is attached to the end of a string 2 meters long. The other end of the string is fixed to a hook to make a simple pendulum. Initially, the mass is held so that the string is horizontal. The mass is then released. At the point when the string is vertical, what is the kinetic energy of the mass? A. 10 joules B. 49 joules C. 98 joules D. Cannot be determined because we do not know the velocity A 5kg mass is attached to the end of a string 2 meters long. The other end of the string is fixed to a hook to make a simple pendulum. Initially, the mass is held so that the string is horizontal. The mass is then released. At the point when the string is vertical, what is the kinetic energy of the mass? A. 10 joules B. 49 joules C. 98 joules D. Cannot be determined because we do not know the velocity A. 10 joules B. 49 joules C. 98 joules D. Cannot be determined because we do not know the velocity

Explanation / Answer

The initial kinetic energy when the string is horizontal, KE1 = 0

The potential energy at this point = mgL (considering the lowest point as reference)

The final kinetic energy when it is vertical KE2 = ?

The potential energy at this point = 0.

So from the energy conservation,

KE1 + PE1 = KE2 + PE2

0 + mgL = KE2 + 0

so KE2 = mgL = 5*9.8*2 = 98 J. 'C' is correct.

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