Momentum is conserved in a collision

[Khemical]

Momentum is conserved in a collision so, the momentum before collision = the momentum after the collision. But i have a hard time understanding this, as P=m*v in a collision doesn't velocity decrease? And if velocity decreases so does the momentum of the object as the mass is constant. It doesn't make sense how momentum is is conserved, can someone help me please? I'm quite confused.

 

[Simon Bridge]

Welcome to PF;
You have described the change in momentum of one object in a collision. For a collision to happen, though, there must be more than one object.

When one object in a collision loses momentum, some other object must gain it.
Conservation of momentum only applies to a closed system.

 

[Legaldose]

You are forgetting an important part. The total momentum of a system is conserved. Take this example, imaging an elastic collision where a moving object strikes a stationary object with equal mass and then the stationary object starts moving while the moving object stops moving. What happens? Well intuitively you would say that the stationary object would start moving with the same velocity as the original one, and you would be correct. For each object, their momenta certainly were not conserved before and after, but if you take the sum of their momenta, they would certainly be equal.

 

[Khemical]

Ahh, thank you Simon And Legaldose. It all makes sense now :)

 

[PJC01]

Hello,

I can understand your confusion about momentum conservation in a collision. Let me try to explain it in a simpler way.

First, let's understand what momentum is. Momentum is the product of an object's mass and its velocity. In a collision, two objects with different velocities collide and exert forces on each other. This causes a change in their velocities, but the total momentum of the system (the two objects) remains the same. This is because the forces acting on the objects cancel each other out, resulting in a net change of zero in the total momentum.

To understand this better, let's take an example of two cars colliding. Before the collision, both cars have different velocities and therefore different momentums. When they collide, the force exerted by one car on the other causes a decrease in its velocity, while the force exerted by the other car causes an increase in its velocity. However, the total momentum of the two cars remains the same.

So, even though the individual momentums of the cars have changed, the total momentum of the system (the two cars) remains constant. This is the principle of momentum conservation in a collision.

I hope this explanation helps you understand momentum conservation in a collision better. Please let me know if you have any further questions or if you need any further clarification.