If there is no net external force on a system, the total momentum of the system remains constant. This is the law of conservation of momentum. (also known as the principle of conservation of momentum). Here, by the term momentum, we mean Linear Momentum. When the net external force acting on a system is zero […]

# law of conservation of momentum

## Rocket Equation Derivation along with Rocket Acceleration formula

Rocket Equation Derivation is the objective of this post. We will also derive the Rocket Acceleration formula here as we go forward. When the fuel of rocket burns the fuel gas gets expelled backward at a high velocity. This results in a force being applied on the rocket in the forward direction and the rocket […]

## How do Rockets work?

How do rockets work? As the rocket engines operate, some kind of fuel (liquid or solid) is burnt. The engines continuously eject burned fuel gases. As these exhaust gases have both mass and velocity, they have some momentum as well. By the principle of conservation of momentum, the rocket’s momentum changes by this same amount […]

## Solution to problems – class 9 physics – Set 1 Q 59

Problem Statement 59) A man wearing a bullet-proof vest stands still on roller skates. The total mass is 80 kg. A bullet of mass 20 grams is fired at 400 m/s. It is stopped by the vest and falls to the ground. What is then the velocity of the man ? Solution When both man […]

## Solution to problems – class 9 physics – Set 1 Q 58

Problem Statement 58) A heavy car A of mass 2000 kg travelling at 10 m/s has a head-on collision with a sports car B of mass 500 kg. If both cars stop dead on colliding, what was the velocity of car B ? Solution Say, velocity of B before collision be V m/s Initial momentum […]

## Solution to problems – class 9 physics – Set 1 Q 57

Problem Statement 57) A ball X of mass 1 kg travelling at 2 m/s has a head-on collision with an identical ball Y at rest. X stops and Y moves off. Calculate the velocity of Y after the collision. Solution Initial momentum of X = 1×2 kg m/s = 2 kg m/s …………….(i) After collision […]

## Solution to problems – physics class 9 – Set 1 Q 56

Problem Statement 56)A truck of mass 500 kg moving at 4 m/s collides with another truck of mass 1500 kg moving in the same direction at 2 m/s. What is their common velocity just after the collision if they move off together? Solution Total initial momentum = 500 x 4 + 1500 x 2 = […]