I don't understand the law of conservation of momentum?

Question

Maybe i'm getting something wrong here (plz correct me if i'm missing the concept of momentum) but for example:

In the situation of an isolated system we have two bodies, say a bullet and a wall. When the bullet travels with 'mv' towards the wall, it hits it and collides inelastically with it. The wall does not budge. How on earth can momentum be conserved when the velocity of the wall and bullet are now zero (not moving)???

Answer 1

You are forgetting a CRUCIAL condition of the law of conservation of momentum.



The law of conservation of momentum is ONLY TRUE if and ONLY if the net impulse caused by all forces acting on the system from outside bodies is ZERO (or roughly zero).



In your situation, when you consider the ball + wall as the system, you are forgetting that the wall is well-constrained to Earth. If you wish to use conservation of momentum to solve this problem, you must include all of Earth as well in your calculation. How massive do you think Earth is?

Answer 2

You might find it difficult because It is applicable for an isolated system ( ideal system ).where there are no force acting on a body.. On earth if a body is at rest... Don't say there is no force acting on it. There is gravitational force which is binding the molecules of wall........

Let's say you and me are present in that isolated system.. You run towards me and collide with me.. I was initially at rest and after collision i run with the force that you exerted on me and you now are at rest.. The law says that Initial momentum you and me had is equal to the final momentum you and me now have..

Answer 3

except between the electrons or few sub atomic particles most of the collisions(mechanical) are inelastic. so the bullet moves at a very slow final velocity,v = initial vel of bulletxits mass /sum of the two masses.. . the bullet will eventually stop as both the initial and final velocities are average velocities

Answer 4

The momentum is conserved because it is spread out through the wall.

What you don't see is the wall absorbing the impact, like how water ripples when you throw a pebble into a lake.

Answer 5

that energy is transferred through a shock wave in the wall, the fragmentation/distortion/friction of the bullet and reflected energy waves. for example if that bullet hit you with ballistic armour on, you'd feel like you were hit with a big hammer.

Answer 6

actually we all are in an earth's frame of reference,, u are really right in saying that wall is moving but we take it as inertial freame of reference...