[Music] hi and welcome back to free science lessons. co.uk by the end of this video you should be able to calculate momentum you should then be able to use the idea of momentum to describe the movement of objects and all of this is for higher tier students only here's a first key fact all moving objects have momentum but if an object is not moving then its momentum is zero now it's really important that you learn this idea objects that are not moving have no momentum and that's often asked in the exam we calculate momentum by using this equation momentum equals the mass multipli by the velocity and I've given you the triangle for this in case you prefer to use these the unit of momentum is kilogram m/s now you're not given this equation in the exam so you do need to learn it okay let's take a look at a couple of simple questions a car has a mass of 1,000 kg and is not moving what's the momentum of the car so pause the video now and try this yourself okay in this case the car is not moving at all that means that its momentum must be zero here's another question a car has a mass of 1,000 kg and is moving at 20 m/s calculate the momentum of the car so again pause the video and try this yourself okay in this case the car is moving so it must have momentum remember that momentum equals the mass multipli by the velocity the mass is 1,000 kg and the velocity is 20 m/ second multiplying 1,000 by 20 gives a momentum of 20,000 kg m/s okay now there is one key fact about momentum which I'm showing you here and this is called the conservation of momentum in a closed system the total momentum before an event is equal to the total momentum after an event this sounds tricky but it's actually quite straightforward we're going to look at two examples and you'll soon get the idea I'm showing you here a van traveling towards a stationary car because the van's moving it has momentum in the forward Direction the momentum of the van is its mass multiplied by its velocity the stationary car has no momentum as it's not moving now the van collides with the car both the van and the car move together at a lower velocity than the initial veloc of the van the total momentum of the van and the car together is now the same as the initial momentum of the van by itself so as you can see the total momentum before the Collision is the same as the total momentum after the Collision in other words momentum has been conserved here's another example this shows a cannon containing a cannon ball before the cannon fires both the cannon and The Cannonball are not moving so the total momentum before fire firing equals zero when the cannon fires The Cannonball is moving at a very large velocity so the cannon ball has momentum in the forward Direction at the same time the cannon recoils in other words it moves backwards this means that the cannon has momentum acting in the backwards Direction the backwards momentum of the cannon equals the forwards momentum of the cannon ball so this means that the total momentum after firing is zero so as you can see the momentum before firing was zero and the momentum after firing is also zero in other words the momentum has been conserved remember you'll find plenty of questions on momentum in my vision workbook and you can get that by clicking on the link above [Music]