Test: Impulse & Newton’s Third Law

A large amount of force acting on an object for a short interval of time is called impulse or impulsive force. Numerically impulse is the product of force and time. Impulse of an object is equal to the change in momentum of the object.

Spring balance works on action reaction principle. Hence, according to Newton’s third law the force measured by Spring balance will be 10 N.

The important thing to notice is that action and reaction forces act on different objects in the interaction. Two equal and opposite forces acting on the same object do not make an action-reaction pair! This is why the two opposing forces don't cancel out.

The force exerted by the floor of an elevator on the foot of a person is more than the weight of the person if the elevator is going up and speeding up.

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

 Answer :- a

Solution :- F = ma 

F = m ( v-u /t )

here u = 0 

F = m v /t 

F = 0.150 x 15 / 0.1 

F = 2.25 / 0.1 

F = 22.5 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.

Engine of the bus exerts force on the wheel is frictional force which opposes the bus to move forward ( acts opposite to the direction of motion of the body). Road also exert equal force on the wheel which pushes the bus forward

 Answer :- c

Solution :- Impulse I = Fdt

Also force F= mdv/dt

​So, I = mdv/dt dt = mdv

= m(v2 −v1)

= mv2 − mv1

​=p2 − p1 = change in momentum

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

Impulse applied to an object produces an equivalent vector change in its linear momentum, also in the same direction. The SI unit of impulse is the newton second (N⋅s), and the dimensionally equivalent unit of momentum is the kilogram meter per second (kg⋅m/s).

The impulse experienced by the object equals the change in momentum of the object. In equation form, F • t = m • Δ v. In a collision, objects experience an impulse; the impulse causes and is equal to the change in momentum. In a collision, the impulse experienced by an object is always equal to the momentum change.

 Answer :- d

Solution :- "The property due to which we hurt when a stone is kicked is the Newton's third law of motion.

"Newton's third law of motion" states that each and every action, we get an "equal and opposite" reaction.

In upward direction, acceleration becomes = g+a
N = m(g+a)

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.