A fly wheel having mass 3 kg and radius 5 m is free to rotate about a horizontal axis. A string having negligible mass is wound around the wheel and the loose end of the string is connected to 3 kg mass. The mass is kept at rest initially and released. Kinetic energy of the wheel when the mass descends by 3 m is

Q. A fly wheel having mass 3 kg and radius 5 m is free to rotate about a horizontal axis. A string having negligible mass is wound around the wheel and the loose end of the string is connected to 3 kg mass. The mass is kept at rest initially and released. Kinetic energy of the wheel when the mass descends by 3 m is ______ J. (g = 10 m/s2)

Correct Answer: 30

Solution

Loss of gravitational potential energy of the falling mass is:

mgh = 3 × 10 × 3 = 90 J

This energy is converted into translational kinetic energy of the mass and rotational kinetic energy of the flywheel.

Moment of inertia of the flywheel (solid disc) is:

I = ½ MR2 = ½ × 3 × 25 = 37.5 kg m2

Using equations of motion for a mass–pulley system:

a = mg / (m + I/R2)

a = 30 / (3 + 37.5/25) = 30 / 4.5 = 6.67 m/s2

Velocity of the mass after descending 3 m:

v2 = 2as = 2 × 6.67 × 3 = 40

Translational kinetic energy of the mass:

K = ½ mv2 = ½ × 3 × 40 = 60 J

Hence, rotational kinetic energy of the flywheel is:

90 − 60 = 30 J

Therefore, the kinetic energy of the wheel is 30 J.

Related JEE Main Physics Questions

A beam of light consisting of wavelengths 650 nm and 550 nm illuminates the Young's double slits with separation of 2 mm such that the interference fringes are formed on a screen, placed at a distance of 1.2 m from the slits. The least distance of a point from the central maximum, where the bright fringes due to both the wavelengths coincide, is ______ × 10⁻⁵ m.
An inductor stores 16 J of magnetic field energy and dissipates 32 W of thermal energy due to its resistance when an a.c. current of 2 A (rms) and frequency 50 Hz flows through it. The ratio of inductive reactance to its resistance is ______. (π = 3.14)
Number of photons of equal energy emitted per second by a 6 mW laser source operating at 663 nm is ________. (Given : h = 6.63 × 10⁻³⁴ J.s and c = 3 × 10⁸ m/s)
In an experiment, a set of reading are obtained as follows - 1.24 mm, 1.25 mm, 1.23 mm, 1.21 mm. The expected least count of the instrument used in recording these readings is ______ mm.
For a transparent prism, if the angle of minimum deviation is equal to its refracting angle, the refractive index n of the prism satisfies.
JEE Main Chapter-wise Previous Year Questions

Updated for JEE Main 2026: This PYQ is important for JEE Mains, JEE Advanced and other competitive exams. Practice more questions from this chapter.

Scroll to Top