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# Conservation of Momentum Calculator

Law of conservation of momentumElastic and inelastic collisionsHow to use the conservation of momentum calculatorFAQs

Prefer watching rather than reading? Check out our video lesson on the conservation of momentum here:

## Law of conservation of momentum

The principle of momentum conservation says that for an isolated system, the sum of the momentums of all objects is constant (it doesn't change). An isolated system is a system of objects (it can be, and typically is, more than one body) that don't interact with anything outside the system. In such a system, no momentum disappears: whatever is lost by one object is gained by the other.

Imagine two toy cars on a table. Let's assume they form an isolated system — no external force acts on them, and the table is frictionless. One of the cars moves at a constant speed of 3 km/h and hits the second toy car (that remained stationary), causing it to move. You can observe that the first car visibly slows down after the collision. This result happened because some momentum was transferred from the first car to the second car.

## Elastic and inelastic collisions

The main difference between the types of momentum is related to how the kinetic energy of the system behaves. If this type of energy is not familiar to you, you may be interested in looking at our kinetic energy calculator article and understanding it before digging into the types of collisions.

We can distinguish three types of collisions:

• Perfectly elastic: In an elastic collision, both the momentum and kinetic energy of the system are conserved. Bodies bounce off each other. An excellent example of such a collision is between hard objects, such as marbles or billiard balls.

• Partially elastic: In such a collision, momentum is conserved, and bodies move at different speeds, but kinetic energy is not conserved. It does not mean that it disappears, though; some of the energy is utilized to perform work (such as creating heat or deformation). A car crash is an example of a partially elastic collision — metal gets deformed, and some kinetic energy is lost.

• Perfectly inelastic: After an inelastic collision, bodies stick together and move at a common speed. Momentum is conserved, but some kinetic energy is lost. For example, when a fast-traveling bullet hits a wooden target, it can get stuck inside the target and keep moving with it.

You may notice that while the law of conservation of momentum is valid in all collisions, the sum of all objects' kinetic energy changes in some cases. However, the total energy is always conserved. This energy is the sum of the potential, kinetic, and internal energy. The internal energy computes the energy lost due to dissipative forces acting over the movement of the colliding objects, such as friction.

To learn more about potential energy, check out our potential energy calculator.

## How to use the conservation of momentum calculator

You can use our conservation of momentum calculator to consider all cases of collisions. To calculate the velocities of two colliding objects, simply follow these steps:

1. Enter the masses of the two objects. Let's assume that the first object has a mass of 8 kg while the second one weighs 4 kg.

2. Decide how fast the objects are moving before the collision. For example, the first object may move at a speed of 10 m/s while the second one remains stationary (speed = 0 m/s).

3. Determine the final velocity of one of the objects. For example, we know that after the collision, the first object will slow down to 4 m/s.

4. Calculate the momentum of the system before the collision. In this case, the initial momentum is equal to 8 kg × 10 m/s + 4 kg × 0 m/s = 80 N·s.

5. According to the law of conservation of momentum, total momentum must be conserved. The final momentum of the first object is equal to 8 kg × 4 m/s = 32 N·s. To ensure no losses, the second object must have momentum equal to 80 N·s − 32 N·s = 48 N·s, so its speed is equal to 48 Ns / 4 kg = 12 m/s.

6. You can also open the Kinetic energy field to see how the system's kinetic energy changed and determine whether the collision was elastic, partially elastic, or inelastic.

FAQs

### What is the principle of conservation of momentum?

According to the principle of conservation of momentum, the total linear momentum of an isolated system, i.e., a system for which the net external force is zero, is constant.

### Under what circumstances is momentum conserved?

In order to conserve momentum, there should be no net external force acting on the system. If the net external force is not zero, momentum is not conserved.

### What is an example of the conservation of momentum?

The recoil of a gun when we fire a bullet from it is an example of the conservation of momentum. Both the bullet and the gun are at rest before the bullet is fired. When the bullet is fired, it moves in the forward direction. The gun moves in the backward direction to conserve the total momentum of the system.

### What is the principle that makes a rocket move?

The principle that makes a rocket move is the law of conservation of linear momentum. The fuel burnt in the rocket produces hot gas. The hot gas is ejected from the exhaust nozzle and goes in one direction. The rocket goes in the opposite direction to conserve momentum.

$\sf m_1u_1 + m_2u_2 = m_1v_1 + m_2v_2$