The string of a pendulum is horizontal.The mass of bob attached to it is m.Now, the string is released.The tension in the string in the lowest position is

Options

(a) mg
(b) 2 mg
(c) 3 mg
(d) 4 mg

Correct Answer:

3 mg

Explanation:

When string is released, then according to conservation of energy,
(1/2) mv² = mgh
v² = 2gh = 2gR
Now, when it is at the bottom,
T – mg cos 0² = mv²/R
or T – mg = (m/R)(2gR)
T = mg + 2mg = 3mg.