Reasons for a gain of momentum during a collision

[greenrichy]

Homework Statement

An air track car A collides with a stationary air track car B (you may assume that the surface is frictionless). Two photogate timers are used to obtain data that allowed to determine the momentum of the system of cars before and after the collision. After the collision, the final velocities of the cars are in opposite directions. If the total momentum after the collision (0.3869 kg m/s) is greater than the total momentum before the collision (0.3782 kg m/s), what is the largest cause for a gain in momentum during the collision?

Relevant Equations

$$p = mv $$

I've come up with the following causes:
- air resistance
- parallax
- during the collision, some of the kinetic energy gets converted into thermal energy.
- invisible deformations

But I'm not sure which would be the biggest effect on the total momentum change.

Are there any other reasons that could have changed the total momentum?

 

[phinds]

If the total momentum after the collision (0.3782 kg m/s) is greater than the total momentum before the collision (0.3869 kg m/s),

Clearly you meant to say "is LESS than" not greater than.

 

[greenrichy]

Clearly you meant to say "is LESS than" not greater than.

Sorry, I just wrote the numbers incorrectly, please see my last edit. Can the momentum after the collision be greater than the momentum before the collision?

 

[jbriggs444]

Sorry, I just wrote the numbers incorrectly, please see my last edit. Can the momentum after the collision be greater than the momentum before the collision?

In theory, given an absence of all external forces and mass transfers, no. Momentum is conserved. Period.

In practice, even without any friction with the tracks, a sailboat could manage the feat.

 

[greenrichy]

In theory, given an absence of all external forces and mass transfers, no. Momentum is conserved. Period.

In practice, even without any friction with the tracks, a sailboat could manage the feat.

What if, in practice, the momentum after the collision were to be less than the momentum before the collision, would the conversion of kinetic energy into heat/sound energy be the main reason for the loss of momentum (given that there is no friction)? I'm just not sure why the conversion of energy would impact the total momentum of the system at all.

 

[jbriggs444]

What if, in practice, the momentum after the collision were to be less than the momentum before the collision, would the conversion of kinetic energy into heat/sound energy be the main reason for the loss of momentum (given that there is no friction)? I'm just not sure why the conversion of energy would impact the total momentum of the system at all.

I agree with you. There would be no reason for a kinetic energy loss during an inelastic collision between two objects to affect the total momentum of those objects at all. The force pair between those objects would be internal. No momentum change could possibly result.

 

[haruspex]

I've come up with the following causes:
- air resistance
- parallax
- during the collision, some of the kinetic energy gets converted into thermal energy.
- invisible deformations

With the exception of parallax, I do not see how any of those could lead to an increase in momentum in the direction of the initial motion. If it’s a force, presumably one that saps energy, you need it to be acting in that direction. The only obvious candidates would be dissipative forces - friction or drag - acting on the recoiling car. But why that should be greater than on the two forward motions is a mystery.

What exactly do you have in mind for parallax as a cause?

Other possibilities: a slight incline in the track; calibration errors in the gates.

Is the statement of the problem in post #1 as given to you or is it your description of the result of an experiment you conducted? If the latter, could you post the raw data in cut-and-pastable form?