In this article, we will look at how to calculate momentum. use the idea of momentum to describe the movement of objects and use the law of the conservation of momentum to describe momentum inelastic collisions.

First, try this question. a car is moving along a straight road at a constant 60 km/h the forward force from the engine is greater than equal to or less than the backward frictional forces. if you said greater than you may be confusing force and momentum. the moving car has a forward momentum defined by its mass times its velocity but because it’s not accelerating its velocity is a constant 60 km/h. the forces must be balanced so there is no net force on the car.

If the engine force were higher the car would be getting faster increasing its speed and momentum and if lower the car would experience a backward force and be slowing down. if you like momentum is a measure of how difficult it is to stop a moving object the more massive it is and the faster it is moving the harder it is to stop it.

Think of firing a cannonball at a castle wall if it were made of wood or traveling slowly the wall can easily stop it but if it’s made of dense metal and traveling fast it could go right through the wall. so try this one the car is slowing down the arrow shows its velocity which is getting lower what direction is the net force on the car is it forward or backward? yes, the force has to be against the forward motion to enable the car to slow down, and what about the momentum of the slowing down car? yes because the car is still moving forward even though it’s slowing down it still has forward momentum.

All moving objects have momentum in the direction of movement. a stationary object has zero momentum. we calculate momentum using this equation. momentum P equals mass M times velocity V. this can also be displayed in a formula triangle like this so mass equals momentum over velocity and velocity equals momentum over mass.

So what are the units of momentum mass measured in kilograms and velocity in meters per second? so momentum equals kilograms times meters per second.

Now try this example what’s the momentum of a 2000-kilogram car traveling 7 meters per second pause the article while you work it out. how about this one a stationary car has a mass of 2,500 kilograms what is its momentum pause the article and work it out. the car is not moving so its velocity is zero and so it has no momentum.

What is the law of conservation of momentum? In a closed system, the total momentum before the start is equal to the total momentum after the start. let’s look at how to apply this to cars of equal mass 800 kilograms collide before the collision car B is at rest while car a is traveling at 15 meters per second. calculate the momentum of car A. After the collision, the two cars collided. Calculate the speed of the cars after the clash. pause the article while you work it out.

Let’s try another – 2 toy passenger cars moving towards each other collide and stick together. use this information to calculate the momentum of the cars after the collision pause the article while you work it out. note because car B is moving in the opposite direction to car A its velocity is negative.

Let’s finish with this toy can you explain why the N ball moves off at the same speed as the colliding one and if we pull two balls out then move away conserving momentum if three are pulled up then three move away. a lovely illustration of the law of conservation of momentum.