JEE Advanced 2017 · Paper 1 · Physics Q1

← JEE Advanced 2017 · Paper 1

PhysicsMultiple correct+4 / -2Question 1 of 54

A flat plate is moving normal to its plane through a gas under the action of a constant force $F$ . The gas is kept at a very low pressure. The speed of the plate $v$ is much less than the average speed $u$ of the gas molecules. Which of the following options is/are true?

The pressure difference between the leading and trailing faces of the plate is proportional to

uv

The resistive force experienced by the plate is proportional to

v

The plate will continue to move with constant non-zero acceleration, at all times

At a later time the external force

F

balances the resistive force

Answer and worked solution

Official answer: A, B, D

Momentum transfer. In the plate's frame, molecules hit the leading face with typical relative speed $u+v$ and the trailing face with $u-v$ . The pressure on a face goes as $\rho(\text{relative speed})^2$ , so
$\Delta P\propto\rho\left[(u+v)^2-(u-v)^2\right]=4\rho uv.$
(A) is true.

Resistive force. $F_{res}=\Delta P\cdot A\propto v$ . (B) is true.

Motion. $ma=F-kv$ . As $v$ grows, $a$ falls, so the acceleration is not constant. (C) is false.

Eventually $kv=F$ and the plate reaches a terminal velocity. (D) is true.

(A), (B), (D)

Topics:Impulse and momentum Molecular speeds Newton's laws

Next question →

JEE Advanced 2017 · Paper 1, Physics question 1