Question

Comprehension:

A conducting rod of mass $m$ and length $l$ is released from rest on a smooth metallic rails placed in vertical plane in a uniform horizontal magnetic field $\left(B\right)$ as shown in the figure. When rod falls, it would cut magnetic field lines and motional emf will be induced. Velocity and acceleration of rod will change with time and after a long time rod will achieve a maximum velocity named as terminal velocity.


Answer the following question based on above discussion.

$\left(ii\right)$ As the rod falls down, (well before achieving terminal velocity)
Which of the given options is correct?

• A. Acceleration increases, velocity increases
• B. Acceleration decreases, velocity increases
• C. Acceleration remains constant, velocity decreases
• D. Acceleration decreases, velocity constant

Answer 1

The correct option is B Acceleration decreases, velocity increases

At any instant $t>0$,


Acceleration of the rod will be,

$a=\frac{mg-F_{mag}}{m}=g-\frac{F_{mag}}{m}$

So $a<g$ , but acceleration does not change its direction.

Thus, velocity will increase.

<!--td {border: 1px solid #ccc;}br {mso-data-placement:same-cell;}--> Hence, $\left(B\right)$ is the correct answer.