Law of Conservation of Momentum Derivation

The law of conservation of momentum is one of the most prominent laws in physics. The principle of conservation of momentum law tells us that the total momentum of a system is always conserved. Let us learn more about the conservation of momentum along with derivation and solved problems.

Momentum Conservation Principle

Law of conservation of momentum states that

For two or more bodies in an isolated system acting upon each other, their total momentum remains constant unless an external force is applied. Therefore, momentum can neither be created nor destroyed.

The principle of conservation of momentum is a direct consequence of Newton’s third law of motion.

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Derivation of Conservation of Momentum

Newton’s third law states that for a force applied by an object A on object B, object B exerts back an equal force in magnitude, but opposite in direction. This idea was used by Newton to derive the law of conservation of momentum.

Consider two colliding particles A and B whose masses are m1 and m2 with initial and final velocities as u1 and v1 of A and u2 and v2 of B. The time of contact between two particles is given as t.

\(A=m_{1}(v_{1}-u_{1})\) (change in momentum of particle A)

\(B=m_{2}(v_{2}-u_{2})\) (change in momentum of particle B)

\(F_{BA}=-F_{AB}\) (from third law of motion)

\(F_{BA}=m_{2}*a_{2}=\frac{m_{2}(v_{2}-u_{2})}{t}\) \(F_{AB}=m_{1}*a_{1}=\frac{m_{1}(v_{1}-u_{1})}{t}\) \(\frac{m_{2}(v_{2}-u_{2})}{t}=\frac{-m_{1}(v_{1}-u_{1})}{t}\)
\(m_{1}u_{1}+m_{2}u_{2}=m_{1}v_{1}+m_{2}v_{2}\)

Therefore, above is the equation of law of conservation of momentum where \(m_{1}u_{1}+m_{2}u_{2}\) is the representation of total momentum of particles A and B before the collision and \(m_{1}v_{1}+m_{2}v_{2}\) is the representation of total momentum of particles A and B after the collision.

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Examples of Law of Conservation of Momentum

Following are the examples of law of conservation of momentum:

  • Air-filled balloons
  • System of gun and bullet
  • Motion of rockets

Solved Problems on Law of Conservation of Momentum

Q1. There are cars with masses 4 kg and 10 kg respectively that are at rest. A car having the mass 10 kg moves towards the east with a velocity of 5 m.s-1. Find the velocity of the car with mass 4 kg with respect to ground.
Ans: Given,

m1 = 4 kg

m2 = 10 kg

v1 = ?

v2 = 5 m.s-1

We know from the law of conservation of momentum that,

Pinitial = 0, as the cars are at rest

Pfinal = p1 + p2

Pfinal = m1.v1 + m2.v2

= 4 kg.v1 + 10 kg.5 m.s-1

Pi = Pf

0=4 kg.v1+50 kg.m.s-1

v1 = 12.5 m.s-1

Q2. Find the velocity of a bullet of mass 5 grams which is fired from a pistol of mass 1.5 kg. The recoil velocity of the pistol is 1.5 m.s-1.
Ans: Given,

Mass of bullet, m1 = 5 gram = 0.005 kg

Mass of pistol, m2 = 1.5 kg

The velocity of a bullet, v1 = ?

Recoil velocity of pistol, v2 = 1.5 m.s-1

Using law of conservation of momentum,

m1u1 + m2u2 = m1v1 + m2v2

Here, Initial velocity of the bullet, u1 = 0

Initial recoil velocity of a pistol, u2 = 0

∴ (0.005 kg)(0) + (1.5 kg)(0) = (0.005 kg)(v1) + (1.5 kg)(1.5 m.s-1)

0 = (0.005 kg)(v1)+(2.25 kg.m.s-1)

v1=-450 m.s-1

Hence, the recoil velocity of the pistol is 450 m.s-1.

Frequently Asked Questions – FAQs

What is momentum?

Momentum is the product of mass and velocity. It is the quantity of measure of the mass of the object and velocity of the object.

Give the formula for law of conservation of momentum

The formula for the law of conservation of momentum is:
\(m_{1}u_{1}+m_{2}u_{2}=m_{1}v_{1}+m_{2}v_{2}\)

List some examples of law of conservation of momentum.

Examples of law of conservation of momentum are:
  • Motion of rockets
  • Air-filled balloons
  • System of gun and bullet
  • Momentum is a scalar or vector quantity?

    Momentum is a vector quantity since it has both magnitude and direction.

    State TRUE or FALSE: As friction increases, momentum decreases.

    TRUE

    Test your knowledge on Conservation of momentum derivation

    12 Comments

    1. Thanks so much for the derivation

    2. Thank you

    3. Very Nice notes and explain

    4. thank you

    5. Jhilam Chattopadhyay

      thank you very much

    6. Very well explained and thanks for giving Questions. This makes it easier 2 understand the concept

    7. Thanks for the Explanation

    8. Thank you so much for this derivation.

    9. Extremely helpful

    10. thank you so much i understood it really well

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