All questions of Momentum and Impulse for JEE Exam

A mass ‘m’  moves with a velocity ‘v’ and collides inelastically with another identical mass . After collision the 1st mass moves with velocity  in a direction perpendicular to the initial direction of motion. Find the speed of the 2nd mass after collision.
  • a)
  • b)
    v
  • c)
  • d)
Correct answer is option 'D'. Can you explain this answer?

Arup Pura answered
Option (D) is the correct answer.

Here is the explanation :
Initial momentum of the particles = mv +0 = mv
which is along x-axis.
After collision the net momentum of the system should also be equal to mv and along x-axis only
Thus,
the 2nd particle should have two velocities, one equal to v/(root 3) opposite to the velocity of first particle after collision
And other velocity equal to v along x axis
Thus net speed of 2nd particle after collision is equal to magnitude of [ v/(root 3)j + v i]
Which is equal to 2v/(root3)

Two blocks A and B, each of mass m, are connected by a massless spring of natural length L and spring constant K.
The blocks are initially resting on a smooth horizontal floor with the spring at its natural length, as shown in fig.. A third identical block C, also of mass m, moves on the floor with a speed v along the line joining A and B, and collides elastically with A. Then
  • a)
    the kinetic energy of the A-B system, at maximum compression of the spring, is zero.
  • b)
    the kinetic energy of the A-B system, at maximum compression of the spring, is mv2/4.
  • c)
    the maximum compression of the spring is
  • d)
    the maximum compression of the spring is 
Correct answer is option 'B,D'. Can you explain this answer?

In situation 1, mass C is moving towards right with velocity v. A and B are at rest.
In situation 2, which is just after the collision of C with A, C stop and A acquires a velocity v. [head-on elastic collision between identical masses]
When A starts moving towards right, the spring suffer a compression due to which B also starts moving towards right. The compression of the spring continues till there is relative velocity between A  and B. When this relative velocity becomes zero, both A and B move with the same velocity v' and the spring is in a state of maximum compression.
Applying momentum conservation in situation 1 and 3,



∴ K.E. of the system in situation 3 is
This is the kinetic energy possessed by A – B system (since, C is at rest).
Let x be the maximum compression of the spring.
Applying energy conservation

The balls, having linear momenta  undergo a collision in free space. There is no external force acting on the balls. Let  be their final momenta. The following option (s) is (are) NOT ALLOWED for any non-zero value of p, a1, a2, b1, b2, c1 and c2.
  • a)
  • b)
  • c)
  • d)
Correct answer is option 'A,D'. Can you explain this answer?

Rounak Desai answered
KEY CONCEPT Use law of conservation of linear momentum.
The initial linear momentum of the system is  Therefore the final linear momentum should also be zero.

Option a :
 Final momentum.
It is given that a1, b1, c1, a2, b2 and c2 have non-zero values. If a1 = x and a2 = – x. Also if b1 = y and b2 = – y then the  components become zero. But the third term having  component is non-zero. This gives a definite final momentum to the system which violates conservation of linear momentum, so this is an incorrect option.
Option d:
because b1 ≠ 0
Following the same reasoning as above this option is also ruled ou

A particle of mass m is attached to one end of a mass-less spring of force constant k, lying on a frictionless horizontal plane. The other end of the spring is fixed. The particle starts moving horizontally from its equilibrium position at time t = 0 with an initial velocity u0. When the speed of the particle is 0.5 u0, it collides elastically with a rigid wall. After this collision
  • a)
    The speed of the particle when it returns to its equilibrium position is u0
  • b)
    The time at which the particle passes through the equilibrium position for the first time is 
  • c)
    The time at which the maximum compression of the spring occurs is 
  • d)
    The time at which the particle passes through the equilibrium position for the second time is 
Correct answer is option 'A,D'. Can you explain this answer?

Athira Datta answered
The particle collides elastically with rigid wall. Here 

i.e. the particle rebounds with the same speed. Therefore the particle will return to its equilibrium position with speed u0. option (a) is correct.
The velocity of the particle becomes 0.5u0 after time t.
Then using the equation V = Vmax cos wt we get 0.5u0 = u0 cos wt

The time period 
The time taken by the particle to pass through the equilibrium for the first time   Therefore option (b) is incorrect
The time taken for the maximum compression
  Therefore option c is incorrect.
The time taken for particle to pass through the equilibrium position second time
option (d) is correct.

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