Four forces of the same magnitude but differing directions act at and tangent to the rim of a uniform wheel free to spin about its center of mass (CM). Which statement about the wheel is correct?
A. None of these
B. Neither the net force on the wheel nor the net torque on the wheel about the CM
is zero.
C. The net force on the wheel is not zero but the net torque about the CM is zero.
D. The net force on the wheel and the net torque on the wheel about the CM are zero.
E. The net force on the wheel is zero but the net torque about the CM is not zero.

Explanation:

Four forces of the same magnitude with different directions may not be able to cancel each other .

But of they are applied in opposite directions they can cancel each other also . Because the exact point of application of the forces are not given . Thus we can not say any thing about the resultant forces .

Thus option ( a ) can be taken as correct .

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the motion of a pendulum is a classic example of mechanical energy conservation. a pendulum consists of a mass (known as a bob ) attached by a string to a pivot point. as the pendulum moves it sweeps out a circular arc, moving back and forth in a periodic fashion.