- A smooth horizontal surface.
- A smooth inclined surface.
- A rough horizontal surface.
- A rough inclined surface.
Explanation:
A sphere cannot roll on a smooth inclined surface and on a smooth horizontal surface because there is no backward force (force of friction) to prevent its slipping.

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$(A)$ The vertical component of reaction force at $O$ does not depend on $\alpha$
$(B)$ The horizontal component of reaction force at $O$ is equal to $W$ for $\alpha=0.5$
$(C)$ The tension in the rope is $2 W$ for $\alpha=0.5$
$(D)$ The rope breaks if $\alpha>1.5$
| Student No. | Length of pendulum $(cm)$ | No. of oscillations $(n)$ | Total time for oscillations | Time period $(s)$ |
| $1.$ | $64.0$ | $8$ | $128.0$ | $16.0$ |
| $2.$ | $64.0$ | $4$ | $64.0$ | $16.0$ |
| $3.$ | $20.0$ | $4$ | $36.0$ | $9.0$ |
(Least count of length $=0.1 \,{m}$, least count for time $=0.1\, {s}$ )
If $E_{1}, E_{2}$ and $E_{3}$ are the percentage errors in $'g'$ for students $1,2$ and $3$ respectively, then the minimum percentage error is obtained by student no. ....... .


