Given-

Proton is released from rest in a room, it starts with an initial acceleration a₀ towards west

Now, we know coulomb’s force F given by –

where

q = charge

E= electric field

Also,

And Newton's second law of motion

Where

m = mass

a= acceleration

From (1) and (2)

Electric field,

which acts towards west.

We know, Lorentz force F is given by -

where,

q = charge

v = velocity of the charge

B=magnetic field

and

and θ = the angle between B and l

Now, given that when the proton is projected towards north with a speed v₀, it moves with an initial acceleration 3a₀ towards west.

An electric force will act on the proton in the west direction, which produces an acceleration a₀ on the proton.

Initially the proton was moving with velocity v, so a magnetic force was also acting on the proton.

So, magnetic force acting will be the only cause behind the change in acceleration of the proton

Change in acceleration towards west due to the magnetic force acting on it is -

∆a = 3a₀ − a₀ = 2a₀

So from (2), the force will become -

Hence, required magnetic field