Acceleration in conservation of momentum problem

In summary, when a moving point mass collides with a point mass at rest, the resulting velocities can be determined using the principles of conservation of momentum and kinetic energy. The accelerations during the collision are not infinite, as the time of collision is assumed to be small but non-zero. The forces between the two objects are large but short-lived. The duration of the collision depends on the physical properties of the objects and the total energy is conserved, not just the kinetic energy. Collisions between point particles occur over a range and are mediated by ranged interactions such as electromagnetism or gravity.
  • #1
dimitri151
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If a moving point mass collides with a point mass at rest then you can find the resulting velocities by conservation of momentum and conservation of kinetic energy. Are the accelerations in this case said to be infinite in the sense that the changes to the velocities happen instantaneously?
 
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  • #2
Nothing happens instantaneously. I think the 'time of collision' is assumed small, but non=zero in any theoretical treatment.
 
  • #3
The first object will impart a certain impulse to the second object. The time of collision will be very short, but will not be 0. The forces will be large, but short forces between the two objects.

See: http://en.wikipedia.org/wiki/Impulse_(physics)
 
  • #4
The time or distance over which the collision occurs typically depends on the physical properties of the objects. The shorter the time or distance over which the collision occurs the higher the forces and accelerations involved.

It's not always true that force * time = constant but that's a useful concept when solving some types of problem. You may not always know the duration of the impact.
 
  • #5
Is it like this: The conservation of momentum/kinetic energy tells you what the masses are doing before and after the collision, but not during, except perhaps that if a velocity is greater or less after collision then velocity is rising or decreasing during the collision in a continuous way.
 
  • #6
Sort of. Conservation of momentum always holds, even during the collision. The reason you can't use conservation of kinetic energy during the collision is because there is some potential energy between the two colliding bodies due to forces between them (and due to some deformation of the objects, if they are not point particles), and it is the total energy that is conserved, not the kinetic energy. Before and after the collision, the bodies are far apart, so the potential energy between them is small enough to neglect in the calculation.

Collisions between point particles always occur over some range.
 
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  • #7
For the sake of the OP, I would like to elaborate a bit on this last point.

Two point particles can never collide in the sense that two extended hard spheres can by coming into contact with each other, the classical cross section for this is zero. Instead, a collision of point particles must be mediated by a ranged interaction such as electromagnetism or gravity. In those cases, the EM/gravity forces are acting on the particles for non-zero duration and the involved forces are not infinite.
 
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FAQ: Acceleration in conservation of momentum problem

What is conservation of momentum?

Conservation of momentum is a fundamental principle in physics that states that the total momentum of a closed system remains constant over time. This means that the initial momentum before a collision or interaction will be equal to the final momentum after the collision or interaction.

How does acceleration affect conservation of momentum?

Acceleration is a measure of how quickly the velocity of an object changes. In conservation of momentum problems, acceleration is important because it affects the rate at which the momentum of an object changes. This change in momentum must be equal and opposite between two objects in a closed system in order to maintain conservation of momentum.

What is the equation for calculating acceleration in a conservation of momentum problem?

The equation for acceleration in a conservation of momentum problem is a = (m1v1 - m2v2) / (m1 + m2), where a is the acceleration, m1 and m2 are the masses of the two objects, and v1 and v2 are their velocities before the collision or interaction. This equation is derived from the principle of conservation of momentum.

How does mass affect acceleration in a conservation of momentum problem?

In a conservation of momentum problem, mass affects acceleration by determining how much force is needed to change the momentum of an object. The larger the mass, the more force is required to change its momentum, resulting in a lower acceleration. This is why heavier objects tend to have a slower acceleration compared to lighter objects in the same collision or interaction.

What are some real-world examples of acceleration in conservation of momentum problems?

Some real-world examples of acceleration in conservation of momentum problems include collisions between two cars, a ball hitting a wall and bouncing back, and a rocket launching into space. In all of these scenarios, the initial momentum of the objects involved must be conserved, so the acceleration of each object must be taken into account to ensure conservation of momentum.

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