Student Login

Home›Chapters›Rotational Motion PYQs›Angular Momentum & its Conservation›Question 56

JEE Main 2020 (3 Sep, Shift 1) · Angular Momentum & its Conservation

Angular Momentum & its Conservation question from JEE Main 2020 (3 Sep, Shift 1)

Q56JEE Main 2020 · 3 SEP · SHIFT 1Medium

Angular Momentum & its Conservation

A person of 80 kg mass is standing on the rim of a circular platform of mass 200 kg rotating about its axis at 5 revolutions per minute (rpm). The person now starts moving towards the centre of the platform. What will be the rotational speed (in rpm) of the platform when the person reaches its centre _________.

Numerical answer type. Work it out, then open the answer.

Show answer and solution

Answer: 9

Treat the platform as a disc: $I_p = \frac{1}{2}(200)R^2 = 100R^2$ .
Person on rim: $I_i = 100R^2 + 80R^2 = 180R^2$ ; at centre: $I_f = 100R^2$ .
Conservation of $L$ : $I_in_i = I_fn_f \Rightarrow n_f = \frac{180}{100}\times 5 = 9\ \text{rpm}$
Answer: 9 rpm

Report a mistake

More questions on Angular Momentum & its Conservation

Consider a uniform rod of mass M = 4m and length pivoted about its centre. A mass m…JEE Main 2020 (8 Jan, Shift 1)
A thin rod of mass 0.9 kg and length 1 m is suspended, at rest, from one end so that it…JEE Main 2020 (5 Sep, Shift 2)
The time dependence of the position of a particle of mass m = 2 is given by r(t) = 2ti -…JEE Main 2019 (10 Apr, Shift 2)
A thin smooth rod of length L and mass M is rotating freely with angular speed ω 0 about…JEE Main 2019 (9 Apr, Shift 2)
If the angular momentum of a planet of mass m, moving around the Sun in a circular orbit…JEE Main 2019 (9 Jan, Shift 1)
Two coaxial discs, having moments of inertia I1 and I1/2, are rotating with respective…JEE Main 2019 (10 Apr, Shift 1)
Two particles A, B are moving on two concentric circles of radii R1 and R2 with equal…JEE Main 2019 (12 Jan, Shift 2)
A person of mass M is, sitting on a swing of length L and swinging with an angular…JEE Main 2019 (12 Apr, Shift 1)