Release mass m on a string from rest at point A. As it passes the lowest point B, the tension in the string is
A) impossible to determine; the answer depends on the length of the string.
B) mg
C) 2mg
D) 3mg
E) None of these is correct.

Question

Release mass m on a string from rest at point A. As it passes the lowest point B, the tension in the string is
A) impossible to determine; the answer depends on the length of the string. 
B) mg 
C) 2mg 
D) 3mg 
E) None of these is correct.

Quizplus · Accepted Answer

As the mass passes the lowest point B, the tension in the string will be equal to the sum of the weight of the mass (mg) and the additional force required to keep it moving in a circle (centripetal force). Since the mass is at the bottom of its swing and moving horizontally, the centripetal force will be equal to the horizontal component of the tension, which is equal to the tension itself. Therefore, the tension in the string will be 2 times the weight of the mass, or 2mg.

Release Mass M on a String from Rest at Point