Question

State the True and False values of the given three statements:

$S_1$ : A man is standing on a circular platform which is free to rotate about a fixed vertical axis. If the man starts moving on the periphery of the platform, the COM of the system (man + platform), Starts moving w.r.t. ground.

$S_2$: A charged particle is kept at rest in a uniform magnetic field. If now the magnetic field starts increasing with time, the particle may start moving.
$S_3$: If the angular momentum of a system is constant in magnitude then the torque (if any) acting on the system must be perpendicular to the angular momentum.

• A. $S_1:False,S_2:True,S_3:True$
• B. $S_1:True,S_2:True,S_3:True$
• C. $S_1:False,S_2:True,S_3:False$
• D. $S_1:True,S_2:True,S_3:False$

Answer 1

Answer: (D)

$S_1:True,S_2:True,S_3:False$

$S_1$ - The center of mass of the platform is at the center of platform , therefore COM of man and platform will lie on the line joining the man and center . When man moves on the periphery of platform , this line also moves , hence the COM of system (man+platform) . True .

$S_2$ - According to ,Maxwell a changing magnetic field produces an electric field , which apply a force on a stationary charged particle so it may start moving . True

$S_3$ - The angular momentum $L$ and torque $\tau$ are related by ,

$\tau =dL/dt$ ,

for $L=$ constant , $\tau =0$ ,

Therefore, net torque on the system is zero, hence given statement is false.