Test: Impulse & Newton’s Third Law - JEE MCQ

As the moving elevator is a non inertial frame hence newton's laws can’t be applied directly to it. So to apply Newton's laws we need to add a pseudo force to the man's body equal to mass times the acceleration of lift in the opposite direction to that of acceleration. Thus the balancing normal force is equal to the weight of the man +  mass times the acceleration which is,
Reading = Normal force = 700 + 70 x 5
= 700 + 350
= 1050 N

We know that Impulse due to force, I = FΔt
As F = 100N
And Δt = 1sec
We get I = 100 Ns

Since impulse I = F x Δt

And also Impulse I = Δp (i.e. change in linear momentum)

Change in momentum

= 0.01 x 5 – (-5)

= 0.10 N s

Impulse = 0.10 N s

Therefore 0.10 = F x 0.01

Or F = 10 N

Rocket works on the principle of conservation of momentum. Rocket ejaculates gases in backward direction which creates momentum of the gases backwards and thus by conservation of momentum, the rocket gets a momentum in the forward direction making it to move forward.

According to Newton's 3 rd law, every action and reaction have same magnitude and opposite direction. So the angle between them should be 180^o.

To conserve the momentum each time a single bullet is fired,
the reverse speed gained by the gun from one bullet is 
V = 400 X .035 / 20
= 0.7 m/s
Thus total speed gained in a second is = 0.7 X 400 = 280 m/s
As total speed is gained in one second only the acceleration produced = 280 m/s²
Thus total force applied on the gun by the bullets = 20 x 280 
= 5600 N

"Every action has an equal and opposite reaction” -Newton's 3rd Law

Simply by conserving the momentum of the system we get that,
0 (initial momentum) = 12 x 4 + 4 x v (final momentum)
Thus we get v = -12 m/s

We know that I = P, where P is momentum
As subtracting initial momentum from the final momentum won't affect its unit, we get unit if I is the same as that of P.