# How and why does momentum get transferred? [closed]

Let us imagine an isolated system and there are two objects of whatever masses and size. Let one object move towards the other object with constant velocity. We know, that when they collide, the moving object applies impulse on the other object and the other object starts to move. We know this because there has been a change in momentum in the moving object , because some or full momentum is transferred to the second object. But this leaves me with a question. Q- I thought that something can move another object only when it applies force. Force is applied only when there is acceleration. Acceleration happens when there is change in velocity. I see that velocity is changed in the first object 'cause it has collided and transferred some of the motion (momentum) to the second object. But why and how is momentum even transferred?

## closed as unclear what you're asking by FGSUZ, Jon Custer, ZeroTheHero, user191954, HDE 226868Feb 14 at 16:10

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

• "Force is applied only when there is acceleration. Acceleration happens when there is change in velocity." The cause of the acceleration is a force, and acceleration is defined as change in velocity. The way you said it makes it seem like it is the other way around. For two objects made of regular matter the force that causes momentum transfer is simply the electric repulsion of the electrons of the atoms on the surface of the objects. For a deeper discussion of the contact force (touching) check: physics.stackexchange.com/q/23797 – Hugo V Feb 6 at 11:11
• I think an intuitive explanation is that momentum is transferred whenever 2 objects travelling at different velocities come into direct contact with each other. However, I am writing this purely from everyday observations so it might be wrong. – Luo Zeyuan Feb 6 at 11:26
• momentum is the result of the action of forces, what else do you imagine we should answer? – Wolphram jonny Feb 6 at 12:10
• Why is momentum transferred? – NightKruger Feb 6 at 12:31
• @NightcoRohak What do you possibly mean by "why"? That is usually not a well posed question in physics. What you should probably ask is "how?". – Hugo V Feb 6 at 12:33

As you can see in my comment to your question, momentum transfer is due to electric repulsion. What do you mean by "how does repulsion transfer momentum?" Repulsion is a force, and force is equal to change in momentum, it is defined as $$\vec F=d\vec p/dt$$.

Consider the simple case of two cubes colliding face to face, when one of them touches the other with velocty $$\vec v$$, both objects feel a force of repulsion (due to their electrons interacting via the electric force), the stationary object feels it in the direction of $$\vec v$$, while the other feels a repulsion in the opposite direction, but with same magnitude. This clearly makes the moving object slow down while making the stationary object speed up.

That is the transfer of momentum via contact forces, which you can see and check everyday, and it happens because of Newton's second law of motion: $$\vec F= \frac {d\vec p}{dt}$$

Because of the electric force: $$\vec F=\frac {k \ Q \ q}{r^2}\hat r$$

And because of the fact that matter is made of atoms with a negative electron cloud around them, making the outter surface of objects reppel.

• As I said, for a deeper discussion on contact forces, go check out: physics.stackexchange.com/q/23797 – Hugo V Feb 6 at 12:43
• I checked that. The person who answered it, must be an experienced person. It was a very rigorous and interesting answer. – NightKruger Feb 6 at 17:41

But why and how is momentum even transferred? That is a good question. Due to transfer of momentum, stuff move right? So we can imagine momentum as the energy possesed by moving objetcs. Well, that is not the textbook definition but it would be better to imagine it as that. So, when a moving object strikes a stationary object, why was energy transfered? The truth is, energy wasnt transfered. A better term would be, energy was induced. Let me explain. When the moving object strikes the stationary one, the electrons come really close to one another. Then they repel one another. The electrons in the stationary object repel the electrons in the moving one. The force of repulsion is directly proportional to the speed of the moving object. Why? The faster the object, the closer the electrons will be pressed together when it strikes the stationary one. So, the repulsion force will have to increase to compensate the exclusion principle. This repulsion force is just enough to lower the velocity of the first object to levels acceptable with the principle. So, the first object either slows down, or stops. In our terms, it loses its momentum. But we remember from Newtons third law. Force always acts in both ways. The electrons of the stable object push back against the moving object. But, this force also acts in the opposite direction in which the electrons are applying force, in other words, the direction the moving object was moving in before impact. This energy gained in this manner by the second object, is equal to the energy that was negated by the pushing electrons from the initial moving object. We can say, that energy in the first object was transfered to the second one. There we have it. Momentum has transfered, we think. But, thats not what really happened. But its similar, and weird at the same time. I hope i answered your question.

• I feel the same way as of now. – NightKruger Feb 11 at 10:02