Impulse Sum and Conservation

[justadream]

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1) Does impulse always sum to 0?

2) Is impulse ever "conserved"?

 

[user12]

1) Does impulse always sum to 0?

2) Is impulse ever "conserved"?

not sure what you mean by these. impulse is equal to change in momentum (Ft = delta p). it's also a vector. for something like a collision- whether it be elastic or inelastic- momentum is always conserved, implying no net force is acting on the system-- implying no net impulse. so, to try to answer your question, in collisions, there is no net impulse because there is no net force. however, I have never seen a question asking about impulse being conserved.

 

[Czarcasm]

1) Does impulse always sum to 0?

2) Is impulse ever "conserved"?

This is something that confuses me as well. But apparently, the only time you use Impulse-Momentum theorem is when momentum is not conserved. Usually, momentum is conserved. The idea is this. If you have multiple objects in a system, such that they are all in equilibrium (net external forces in all directions is zero), then the momentum of the system initially is equal to the momentum of the system finally. If these two objects collide with each other, momentum is still conserved (provided net external forces is zero; the collision itself is an internal force, which is a little more complicated but we generally ignore it when dealing with those problems). If on the other hand an object collides with a wall (something that wasn't part of the initial system), then an external force is present and in this case momentum is not conserved. The appropriate equation to use is impulse-momentum, that is: Force x deltaT (Impulse) = mass object (deltaV). Because the force is some non-zero number and the time in which it occurs is also non-zero, then I would assume impulse would never be zero in this instance.

 

[justadream]

@Czarcasm

Why is momentum not conserved when an object hits a wall?

Is it simply because the wall does not move?

 

[Czarcasm]

@Czarcasm

Why is momentum not conserved when an object hits a wall?

Is it simply because the wall does not move?

The wall is considered a net external (nonconservative) force on an object in the system. If you have 5 objects traveling in a given path (no net external nonconservative forces), the total momentum of those objects is some value. But if one of those objects hits a wall, then we can no longer say that system has the same amount of momentum. Therefore, momentum is not conserved.

 

[justadream]

@Czarcasm

You state that the wall is considered a net external force but I'm wondering why you automatically designate it that?

Like how do you "just know" it is a nonconservative net external force? Is the reason because it cannot move?

 

[Czarcasm]

@Czarcasm

You state that the wall is considered a net external force but I'm wondering why you automatically designate it that?

Like how do you "just know" it is a nonconservative net external force? Is the reason because it cannot move?

3 Conservative Forces: Gravitational, Electric, Spring. Contact and frictional forces are considered nonconservative forces.

 

[justadream]

@Czarcasm

What's a normal ball-to-ball collision? Spring?

If so, why wouldn't a ball-to-wall collision be the same?

Like let's say you throw a block at another block. Here, you'd say momentum is conserved.

Now let's say you throw a block at a collection of blocks of the same material (e.g., Great Wall). Here momentum is not conserved. But the collision appears to me to be the same?

 

[Czarcasm]

@Czarcasm

What's a normal ball-to-ball collision? Spring?

If so, why wouldn't a ball-to-wall collision be the same?

Like let's say you throw a block at another block. Here, you'd say momentum is conserved.

Now let's say you throw a block at a collection of blocks of the same material (e.g., Great Wall). Here momentum is not conserved. But the collision appears to me to be the same?

I suppose, it might help to consider a system of objects (in which movement is possible). So a system of say, three balls colliding into each other could still maintain the same total momentum after collision occurs (provided no net nonconservative external forces on any of the objects themselves). But if instead, your system was I guess a ball approaching a wall, I'm not entirely sure if that would be a case of conservation of momentum, because I believe the wall would have to move. I suppose if it could move (making an assumption here), momentum would be conserved (because it's part of the system ).

Momentum isn't a strong point for me, so I apologize if I'm not helping much here 🙂

 

[techfan]

The wall and everything it is attached to DOES move even if it's a small amount (unless the mass of the wall is infinite). Linear momentum is always conserved, the internal forces of the objects don't matter with momentum. Momentum is best thought of being conserved when it occurs in the same plane (i.e. if you have to add an x and y component in there then there could be a place where an external force is acting).

 

[justadream]

@techfan

What about impulse? In a collision with unequal masses, do they experience the same impulse in magnitude?

 

[techfan]

Impulse is just a change in momentum. So yes. The "cool" thing about momentum is that you are only really looking at the center of mass of the objects so you can treat it as a point instead of an object (with energy you can't simply treat the objects as points since energy gets lost in the internal interactions, but momentum doesn't). It's why you when you have an inelastic collision (one where the two objects stick together) the momentum is conserved, but the kinetic energy isn't (lose energy to the cars scraping together, sound, heat etc. which are all INTERNAL). With momentum, the ratios stay constant.

 

[justadream]

@techfan

Okay so impulse in collisions is Equal in magnitude but opposite in direction (similar to the force)

 

[techfan]

Exactly! In fact impulse=F*delta(t).