# Conservation of linear momentum understanding

1. Oct 23, 2004

### faisal

I need help understanding the priciple of conservation of linear momentum heres the definition.
priciple of conservation linear momentum is when there is no net external force (what does net external force mean?) acting on a system (what system?)of particles the total momentum of the system is conserved.
how does this law coincide with newtons 1st 2nd and 3rd law?
p.s.

Last edited: Oct 23, 2004
2. Oct 23, 2004

### Pseudo Statistic

Salam alaikom bro. ;)
Newton's 2nd law is basically an object accelerates in the direction of the force applied to it..
Think about playing tennis with a friend, when you apply the force for the ball to go all the way to him with your racket, if he merely held his racket up and the ball hit it with this force, the ball would then accelerate in the direction of your force!
Momentum is... different I guess, basically, let's say two cars are moving at different speeds, for example one is moving at 40ms^-1 and another is at 20ms^-1, the first car has a mass of 1000kg and second car has a mass of 1200kg, if you summed the momentum of these up, basically times 40 by 10000 and 1200 by 20 and add them, you would have the total momentum of the system.
Usually you're given info about what happens AFTER the collision, after they hit, so if I say car A is now going at 20ms^-1, you can EASILY find out how fast the other one is going algebraically, so the formula would be:
40(1000) + 20(1200) = 20(1000) + v(1200)
And solve for v!
It's overly simplistic, hopefully what I said should make sense to you..
A net force or external force is any force applied which is outside nature, in other words any force by someone or something..
When it says system, it means the objects or bodies that are colliding, in this case the two cars...
Newton's first law says that an object will not change its state in motion unless a net force is applied, and this relates to the conservation of linear momentum in a way that these two laws both state that the values would be equal as long as no net force is applied.
Second law, too tired to think right now and give you a straight answer. :-\
Third law, same as the above..
Hoped some of this helped!

3. Oct 23, 2004

### Pyrrhus

The 2nd Law is related as

$$\sum_{i=1}^{n} \vec{F}_{i} = \frac{d \vec{P}}{dt}$$

If $\sum_{i=1}^{n} \vec{F}_{i} = 0$ then linear momentum doesn't change, and thus it remains constant. That's why It's called the conservation of linear momentum.

Also, it's good to notice in collisions, momentum is not necessarily conserved, the problem is Impulsive Forces are so big compared to the other forces acting on the body, that you could discard them, and use conservation of linear momentum in collision problems.

4. Oct 30, 2004

### faisal

Thanks for the informative replies (shukran Pseudo Statistic ), u cleared up alot of problems for me.
P.s.
hope u'r have have a great Ramdan