# why does a gun recoil when it is fired

Although guns may not be everyday things for many of us, gun recoil is
certainly something we're aware of (at least those of us that don't make Hollywood action films with guns whose recoil would instantly kill the person firing them! ). Gun recoil is a result of momentum conservation, which is an extremely important fundamental principle. Newton was talking about momentum conservation when he wrote "every action has an equal and opposite reaction". However, the physics behind all guns remain the same. Weapons such as cannons, shotguns and rifles, work on the basic idea of conservation of momentum and the change in energy from potential to kinetic. Momentum characterizes an object's resistance to change in motion. If this is motion along a straight line, we call it linear momentum; if it is rotational motion we call it angular momentum. The basic idea is the same: moving things like to keep moving, and to change their motion we have to apply a force. If no force is present, then the momentum doesn't change, i. e. it is conserved. Now, you might point out that a bullet coming out of a gun has a huge force on it from the exploding gunpowder. True enough, and that force is what propels the bullet forward. However, if you look at a bullet and gun together (say while the bullet is still in the barrel but already heading out at full speed), you can say there is no net force on the bullet-gun system.

So the momentum of the bullet plus gun should be conserved. out of the gun, it has momentum p in the forward direction. To balance this momentum (and keep the net momentum of the bullet-gun system zero), the gun recoils with momentum in the opposite direction: p, or Conservation of momentum is the law that is held true when the gun is fired and a "kick" is felt. When a bullet is fired from a gun, total momentum before is zero since nothing is moving. After firing the bullet there is a momentum in the forward direction. The gun must therefore have the same magnitude of momentum but in the opposite direction so that they cancel each other out leaving the total momentum still equal to zero. For this reason the gun must have a recoil velocity after the bullet is fired (i. e. the gun 'jumps' backwards and a 'kick' is felt). As the bullet is propelled through the barrel, it gains momentum. In order for the entire system of the gun and the ammunition to have equal momentum, the gun must gain momentum in the opposite direction from the bullet. Momentum is a vector quantity, having both a direction and a direction. The faster an object is moving or the more mass it has, the more momentum it has in the direction of its motion (momentum = mass velocity). Because momentum is a conserved quantity, it cannot be created or destroyed (momentum before = momentum after). It can only be transferred between objects.

Momentum is conserved because of Newton's third law of motion. When one object exerts a force on a second object for a certain amount of time, the second object exerts an equal but oppositely directed force on the first object for exactly the same amount of time. The momentum lost by the first object is exactly equal to the momentum gained by the second object. Momentum is transferred from the first object to the second object. In this case, if a gun exerts a force on a bullet when firing it forward then the bullet will exert an equal force in the opposite direction on the gun causing it to move backwards or recoil. Although the action and reaction forces are equal in size the effect on the gun and the bullet are not the same since the mass of the gun is far greater than the mass of the bullet. The acceleration of the bullet while moving along the gun barrel would be much greater than the acceleration of the gun (acceleration= force mass). Although guns may not be everyday things for many of us, gun recoil is certainly something we're aware of (at least those of us that don't make Hollywood action films with guns whose recoil would instantly kill the person firing them! ). Gun recoil is a result of momentum conservation, which is an extremely important fundamental principle. Newton was talking about momentum conservation when he wrote "every action has an equal and opposite reaction".

Momentum characterizes an object's resistance to change in motion. If this is motion along a straight line, we call it linear momentum ; if it is rotational motion we call it angular momentum. The basic idea is the same: moving things like to keep moving, and to change their motion we have to apply a force. If no force is present, then momentum doesn't change, ie. it is conserved. Now, you might point out that a bullet coming out of a gun has a huge force on it from the exploding gunpowder. True enough, and that force is what propels the bullet forward. However, if you look at a bullet and gun together (say while the bullet is still in the barrel but already heading out at full speed), you can say there is no net force on the bullet-gun system. So the momentum of the bullet plus gun should be conserved. out of the gun, it has momentum in the forward direction. To balance this momentum (and keep the net momentum of the bullet-gun system zero), the gun recoils with momentum in the opposite direction:, or Although the bullet's mass is small, its speed is quite large, so it released with large momentum. The gun has much larger mass, so the recoil speed is much smaller, but still large enough to give a serious kick against the shooter's shoulder. Example: Winchester. 308 Let's look at an example. A Winchester. 308 cartridge launches a bullet of mass 150 grains (1 grain = 64. 8 mg) with a speed of 2820 ft/s (1 ft = 30. 5 cm).

In MKS units, then, = 8. 4 kg m/s. This rifle has a weight of about 8 lbs, or a mass of = 3. 8 kg. That means the recoil speed of the rifle will be = -2. 2 m/s This primary recoil is noticeable, but not the main recoil that one feels. There are actually two distinct recoils from a gun: the first, primary recoil, which I've described above, conserves momentum of the gun-bullet system. However, a larger secondary recoil comes slightly later, when the bullet leaves the muzzle: then the hot expanding gas behind the bullet shoots out of the muzzle, and the muzzle recoils further like a rocket. This is, again, conservation of momentum, but in this case is is the gas momentum out of the barrel that makes the secondary recoil. Gun manufacturers make baffles that reduce the flow of gas out of the muzzle to reduce secondary recoil. Primary recoil cannot be reduced, since it is simply associated with the forward momentum of the bullet. The total momentum of a system is conserved if there are no outside forces acting on it. Gun recoil results from conservation of total momentum of the bullet-gun system: the backward recoil gun momentum balances the forward bullet momentum to maintain zero total momentum. Gun recoil actually has two parts: primary recoil from the escaping bullet and secondary recoil from the escaping gas behind the bullet.

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