Q 11-05-048JEE MainMedium
A bob hangs from a light string $2$ m long. It is given a horizontal speed of $\sqrt{60}$ m/s at the lowest point. Find the tension in the string, in newtons, when the string becomes horizontal, if the mass of the bob is $0.5$ kg. ($g = 10\ \text{m/s}^2$)
Numerical value type. Enter your answer.
Answer: 5
At the horizontal position the bob has risen $2$ m:
$$v^2 = 60 - 2(10)(2) = 20$$
Gravity is along the path there, so the tension alone provides the centripetal force:
$$T = \frac{mv^2}{l} = \frac{0.5 \times 20}{2} = 5\ \text{N}$$
Solution by Sreeraj P, M.Sc Physics