Question 589

**Assertion: ** When a charged particle moves in a circular path. It produces electromagnetic wave.

**Reason:** Charged particle has acceleration

If both assertion and reason are true and reason is the correct explanation of assertion.

If both assertion and reason are true but reason is not the correct explanation of assertion.

If assertion is true but reason is false.

If both assertion and reason are false

Solution

A.

If both assertion and reason are true and reason is the correct explanation of assertion.

Magnetic forces can cause charged particles to move in circular or spiral paths. Particle accelerators keep protons following circular paths and magnetic force. The magnetic force is perpendicular to the velocity and so velocity changes in direction but not in magnitude.

Accelerated charges radiate electromagnetic waves.

Question 590

The radii of circular paths of two particles of same mass are in ratio 6 : 8 then what will be velocities ratio if they have a constant centripetal force?

$\sqrt{3}:4$

$4:\sqrt{3}$

$2:\sqrt{3}$

$\sqrt{3}:2$

Solution

D.

$\sqrt{3}:2$

The radii of circular paths of two particles of same mass m_{1} = m_{2} and ratio is given by

$\frac{{\mathrm{r}}_{1}}{{\mathrm{r}}_{2}}=\frac{6}{8}$

$\frac{{\mathrm{m}}_{1}}{{\mathrm{m}}_{2}}$ = 1

Centripetal force is a force that acts on a moving body in a circular path and is directed towards the centre around which the body is moving.

$\frac{{\mathrm{m}}_{1}{\mathrm{v}}_{1}^{2}}{{\mathrm{r}}_{1}}=\frac{{\mathrm{m}}_{2}{\mathrm{v}}_{2}^{2}}{{\mathrm{r}}_{2}}$

⇒ $\frac{{\mathrm{v}}_{1}^{2}}{{\mathrm{v}}_{2}^{2}}=\frac{{\mathrm{r}}_{1}}{{\mathrm{r}}_{2}}\times \frac{{\mathrm{m}}_{2}}{{\mathrm{m}}_{1}}$

= $\frac{6}{8}\times 1$

∴ $\frac{{\mathrm{v}}_{1}}{{\mathrm{v}}_{2}}=\frac{\sqrt{3}}{2}$

Question 591

**Assertion:** Use of ball bearing between two moving parts of a machine is a common practice.

**Reason:** Ball bearing reduces rolling friction and provides good stability.

If both assertion and reason are true and reason is the correct explanation of assertion.

If both assertion and reason are true but reason is not the correct explanation of assertion.

If assertion is true but reason is false.

If both assertion and reason are false.

Solution

A.

If both assertion and reason are true and reason is the correct explanation of assertion.

A bearing is a machine element that constrains relative motion to only the desired motion, and reduces friction between moving parts. The simplest form of bearing, the plain bearing, consist of a shaft rotating in a hole.

Question 592

**Assertion: ** Centripetal force is always required for motion in curved path.

**Reason: ** On a banked curved track, vertical component of normal reaction provides the necessary centripetal force.

If both assertion and reason are true and reason is the correct explanation of assertion.

If both assertion and reason are true but reason is not the correct explanation of assertion.

If assertion is true but reason is false.

If both assertion and reason are false.

Solution

C.

If assertion is true but reason is false.

While taking a turn on banked road, the necessary centripetal force is provided by the horizontal component of normal reaction and vertical component balances the weight of the vehicle. Also roads are usually banked for average speed of vehicles passing over them. However, ifthe speed of a vehicle is somewhat less or more than this, the self adjusting static friction will operate between the tyres and the road.