Physics Questions & Answers - Free Online Practice

621.

Which of the following quantities are dimensionally independent?

I. Dielectric constant II. Momentum III. Efficiency IV. Refractive index

I and II

II, III and IV

I, II, III and IV

I, III and IV

View answer & explanation

Correct answer is D

The quantities Efficiency, Dielectric constant and Refractive index are dimensionless.

622.

A block is acted upon by TWO horizontal forces as illustrated in the diagram above. The block accelerates at 1.5\(ms^{-2}\). Calculate the mass of the block.

6kg

9kg

10kg

15kg

View answer & explanation

Correct answer is A

Total force exerted on the block = (10 - 1)N =9N

Force = mass x acceleration

9 = m x 1.5

m = 9/1.5 = 6kg

623.

A body of mass m moving around a circle of radius r with a uniform speed v experiences a centripetal force F. The work done by the centripetal force on it is

zero

Fmr

\(\frac{Fr}{m}\)

View answer & explanation

Correct answer is B

Work is done whenever the given conditions are satisfied. (ii) there is a displacement of the body caused by the applied force along the direction of the applied force. No displacement takes place in this case. So, work done by a centripetal force is always zero.

The work done by centripetal force in the case of a uniform circular motion is 0 joules.

624.

A dam is able to hold a large quantity of water because its wall is

thickest at the top

thickest at the middle

thickest at the bottom

of the same thickness throughout

View answer & explanation

Correct answer is C

The pressure of the water is much greater deeper down and the dam needs to be thick at the bottom so that it is strong enough to withstand this larger pressure

625.

During a training session, two footballers pass a ball repeatedly between each other. The 'to and fro' motion of the ball is not simple harmonic because the

I. Acceleration of the ball is not directed towards a fixed point

II. Restoring force is not directed towards the centre

III. Acceleration of the ball is not directly proportional to the displacement from a fixed point

Which of the statements above are correct?

I and III only

I and II only

II and III only

I, II and III

View answer & explanation

Correct answer is A

In SHM, the body must have acceleration in a direction opposite to the displacement and the acceleration must be directly proportional to displacement. Also, the acceleration must be directed towards its mean position. This is not the case with a football on the pitch.