# Law of conservation of momentum

1. Jul 23, 2012

### freshbox

I am confused over the law of conservation of momentum.

The definition says that "if there is no external force on the system then there is no impulse and the momentum is unchanged"

However under the definition of Impulse Note "F is for all external forces including forces due to gravity and spring"

So what exactly is the external force they are referring to for the law of conservation of momentum?

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2. Jul 23, 2012

### sahilt

impulse can also be described as the difference in momentum at two different instances... and acc. to the law,for a closed system momemtum stays conserved,,so as the momentum is the same,there is no impulse.

3. Jul 23, 2012

### freshbox

May I ask what do you mean by closed system momemtum and how do you know whether the system is closed?

Thanks...

4. Jul 23, 2012

### jbriggs444

You draw an imaginary box around the system and ask yourself whether there are any forces or any mass flows going into or out of the box.

If there are some forces or mass flows then you say that "momentum is conserved in this box except for <this or that>". When you apply a conservation of momentum argument you account for the <this or that>.

5. Jul 23, 2012

### Latarian

don't let gravity grab your attention when the whole system is located on a horizontal plate (most times it is). the plate gives a matching force against gravity and cancels it off.

a spring (or springs) in a system, or ropes or whatever sh!t it might be, functions as an dealer of momentum in a system, enabling individual objects to exchange momentum between each other while the overall momentum of the whole system remains the same

6. Jul 23, 2012

### freshbox

"if there are forces and mass flows through then u say momentum is conserved in the box.". In this statement, is impulse present? Is the momentum unchanged?

Thanks.

7. Jul 24, 2012

### jbriggs444

If there are forces or mass flows across the boundary of the box then momentum may NOT be conserved in the box. That is to say that momentum may change.

I am somewhat disturbed to see quotation marks surrounding a phrase that is not an accurate quote and somewhat saddened to see the meaning of that phrase inverted in the process of failing to transcribe it accurately.

8. Jul 24, 2012

### freshbox

I'm sorry Mister jbriggs, your original quote is

"If there are some forces or mass flows then you say that "momentum is conserved in this box except for <this or that>".

My quote "if there are forces and mass flows through then u say momentum is conserved in the box." is impulse present? Is the momentum unchanged?

I actually don't understand the meaning that's why i tried to ask a question but accidentally putting my "question" into your quote" because I'm eager to know the concept of it but have difficulty understanding it. It was unintentional.

I hope you are feeling well, my apologies again.

Last edited: Jul 24, 2012
9. Jul 24, 2012

### freshbox

I would like to ask since the law of conservation says that if there is no external force on the system then there is no impulse and the momentum is unchanged.

But how come for part b of the question (screenshot attached below) the impulse = 0?

There is frictional and gravitational force, so how come it's 0? So what is the "no external force" the book is talking about?

And if you see under the impulse definition there is a note "F is for all external forces including forces due to gravity and spring"

Then once again can you explain to me what is the no "external force" under the law of conservation please...

Thank you.

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10. Jul 24, 2012

### Staff: Mentor

Often collisions are treated using the so-called 'impulse approximation': Assume that the collision is of such short duration and that the contact forces are large enough so that the effect of other forces (gravity and friction, in this case) can be neglected during the collision.

11. Jul 25, 2012

### freshbox

I see. Is it right for me to say that if I see any question involving collision between 2 object, impulse=0.

Or is there any other special circumstances whereby when 2 objects collide together, impulse is not 0?

Thank you.

Last edited: Jul 25, 2012
12. Jul 25, 2012

### Staff: Mentor

In most collision problems, that's a reasonable assumption. That the net impulse on the system is zero during the collision.

13. Jul 25, 2012

### freshbox

Is there any situation whereby 2 object collide with each other and the impulse is not 0?

14. Jul 25, 2012

### Staff: Mentor

You posted just such a situation a while ago: The package being dropped onto the flat car. You could think of that as a collision problem where the 'collision' is slow enough that friction has time to act.

It should be clear from the context whether the 'impulse approximation' is warranted. It usually is.

15. Jul 25, 2012

### freshbox

Ok thank you for the explanation.