this video we're going to look at recoil of gun so this is a third of our videos in on momentum and it says here on recoil of a gun or an explosion when a bullet is fired from a gun an explosion occurs in a gun barrel this exerts a force on the bullet and an equal an opposite force on the gun initially the momentum is zero and after the explosion the gain of momentum of the bullet is equal and opposite to the gain in the momentum of the gun but the only difference is the bullet is moving forwards and the gun is moving backwards okay so in this example it says a bullet is fired from a gun with a horizontal velocity of 400 m/ second the mass of the gun is 3 Kg and the mass of the bullet is 60 G so why it doesn't why the bullet moves so fast and the gun moves back Rel quite slowly is because um that the gun is so much heavier uh than the uh the buet so we'll do we before and after so before here we've got my fantastic diagram this is a whole thing so you've got your gun and inside the Chamber of the gun you've got the bullet so the total mass is going to be 3.06 kg then after your situation is going to be this your gun again my fantastic diagram for my gun and it was 3 Kg it is now moving backwards with speed of V whereas my bullet and the mass of bullet remember is 06 kg it's moving forwards with a speed of 400 m/s so again we're going to use momentum before is equal to momentum after now the momentum before that's this situation here and here there's no momentum cuz nothing's happened no one nothing is moving the bullet's not moving the gun's not moving then after and I'm going to consider the direction that V is going as positive just cuz that's what I'm trying to find so 3v so anything going to the left is positive that means minus n or I'll say plus and then it's going to be um minus 400 time the mass of the bullet which is 0.03 so 3v then plus 400 * that and that is going to be 0 L is equal to 3v and that's be minus 24 or means 24 is equal to 3v which means V is equal to 8 m/ second so if you've ever fired a gun or a shotgun on clay pigeon shooting or whatever you happen to be at um you'll realize it those sort of kicks back into your shoulder um so you can see that's um uh why then because technically or in theory then this thing is moving backwards with speed of 8 m/s uh but because you have your body exert sort a lot sorts a lot of force you don't shoot back in8 MERS in 1 second cuz you are standing uh standing firmly and you're holding thing strongly that's why you wouldn't move backwards but that is what's actually happening the instant that good bullet moves forward at a speed of whatever it was 400 m/s that instant that gun is moving backwards at a speed of 8 m/s but because of the force that you're exerting that's why you you don't backwards yourself okay one more example of this type says a gun of mass 5 4 kg far as a b of mass 50 the gun recoils with speed of 60 find the speed of the bullet so it's the other the other way around then we know the speed that the gun recoils we just need to find the speed of the bullet so again there's your gun and your bullet it's 0 m/s then after we've got our our gun and the mass of this gun is is 4 kg and it's moving backwards at a speed of 6 m/s we need to find the speed the that the bullet moves it's 05 50 g 05 kg that's moveing a speed of v v okay so we're just going to say momentum before equals momentum after so in this case zero that's my momentum at the start and I'm going to consider my uh speed to be positive my V to be positive so it's be 0.5 uh V that's positive so then this one's going against it is equal to 6 * 4 so that's 0 is equal to 05 vus 24 or 24 is equal to 05 V 24 / 05 is equal to V so V works out to be 4 80 m/ second and that is it done okay so it starts initially both of them have zero momentum and then one of them then uh is given a momentum and if because it's given momentum in One Direction the other one is giving an equal and opposite momentum in the other direction