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Rotational Motion MCQs for NEET — Physics Questions with Answers

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A disc is rotating With an angular speed of m. If a child sits on it, which of the following is conserved?

kinetic energy

potential energy

linear momentum

angular momentum

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Explanation

According to the principle of conservation of angular momentum, when the child sits on the rotating disc, moment of inertia changes, so angular velocity also must change so as to keep angular momentum, as there is no torque acting on the system.

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The torque of a force -6 $\hat{i}$ acting at a point r = 4 $\hat{j}$ about origin will bee

-24 $\hat{k}$

24 $\hat{k}$

24 $\hat{j}$

24 $\hat{i}$

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Explanation

$τ = r \times F$

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A particle moves in a force field given by $\vec{F} = \hat{r} F (r)$ , where r is the unit vector along with the position vector $\vec{r}$ , then which is true?

The torque acting on the particle is not zero

The torque acting on the particle produces an angular acceleration in it

The angular momentum of the particle is conserved

The angular momentum of the particle increases.

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Explanation

$\begin{aligned} \vec{t} = \vec{r} \times \vec{F} & = \vec{r} \times \hat{r} F (r) = 0 \\ Therefore, \vec{t} & = \frac{\vec{dL}}{dt} = 0; \vec{L} = constant \end{aligned}$

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The ratio of rotational and translatory kinetic energies of the sphere is

2/9

2/7

2/5

7/2

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Explanation

$\frac{Rotational kinetic energy}{Translatory kinetic energy} = \frac{\frac{1}{2} {mv}^{2} \frac{K^{2}}{R^{2}}}{\frac{1}{2} {mv}^{2}} = \frac{K^{2}}{R^{2}} = \frac{2}{5}$

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Two perfectly elastic particles A and B of equal mass traveling along the line joining them with velocities 15 m/s and 10 m/s. After the collision, their velocities will be

10 m/s, 10 rn/s

15 m/s, 15 m/s

10 rn/s, 15 m/s

15 m/s, 10 m/s

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A small disc of radius 2 cm is cut from a disc of radius 6 cm. If the distance between their centres is 3.2 cm, what is the shift in the centre of mass of the disc?

0.4 cm

2.4 cm

1.8 cm

1.2 cm

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From a circular disc of radius R, a square is cut out with a radius as its diagonal. The centre of mass of remainder is at a distance (from the centre)

$\frac{R}{(4 π - 2)}$

$\frac{R}{2 π}$

$\frac{R}{(π - 2)}$

$\frac{R}{(2 π - 2)}$

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Two bodies with moment of inertia $I_{1} a n d I_{2}$ $(I_{1} > I_{2})$ and have equal angular momenta. If KE of rotation are $E_{1} a n d E_{2}$ , then

$E_{1} = E_{2}$

$E_{1} > E_{2}$

$E_{1} < E_{2}$

$E_{1} \geq E_{2}$

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Explanation

$E_{1} = \frac{1}{2} I_{1} ω_{1}^{2}, E_{2} = \frac{1}{2} I_{2} ω_{2}^{2} A s I_{1} ω_{1} = I_{2} ω_{2} I_{1}^{2} ω_{1}^{2} = I_{2}^{2} ω_{2}^{2}; I_{1} I_{1} ω_{1}^{2} = I_{2} I_{2} ω_{2}^{2} ∴ \frac{I_{1} ω_{1}^{2}}{I_{2} ω_{2}^{2}} = \frac{I_{2}}{I_{1}} o r \frac{\frac{1}{2} I_{1} ω_{1}^{2}}{\frac{1}{2} I_{2} ω_{2}^{2}} = \frac{E_{1}}{E_{2}} = \frac{I_{2}}{I_{1}} < 1 E_{1} < E_{2}$

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A mass m with velocity u strikes a wall normally and returns at the same speed. What is the magnitude of the change in momentum of the body when it returns

4 mu

2 mu

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Two particles of equal mass have coordinates (2m,4m,6m) and (6m,2m,8m). Of these one particle has velocity $v_{1} = (2 \hat{i})$ m/s and another particle has velocity $v_{2} = (2 \hat{j})$ m/s at time t = 0. The coordinate of their centre of mass at time t = 1 s will be

(4m,4m,7m)

(5m,4m,7m)

(2m,4m,6m)

(4m,5m,4m)

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Explanation

After 1 s, coordinates of first particle will become (4m,4m,6m) and co-ordinates of second particle will become (6m,4m,8m)

$∴ X_{c m} = \frac{4 + 6}{2} = 5 m Y_{c m} = \frac{4 + 4}{2} = 4 m Z_{c m} = \frac{6 + 8}{2} = 7 m$

Note : In this problem answer is independent of the fact whether the first particle has velocity $v_{1} or v_{2}$ . Think why?

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Yes. You can attempt every Rotational Motion question on this page for free without logging in, and check the correct answer with a detailed explanation instantly.

No account is required to attempt questions and view answers. A free account adds bookmarks, personal notes, and progress tracking.

The bank mixes NEET previous year questions (PYQs) with practice questions, each tagged with its exam appearances where applicable.