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Physics/Momentum conservation


In laboratory, by recording the movement of trolley crashing into one another, we try to prove conservation of momentum. But in this experiment, we often find that momentum is actually not conserved in the measurements we take. What might be the reason for that?

Hello shah,

There are 2 obvious ones that I'll start with: Errors in measuring the mass and velocity of the trolleys. The possible reasons for errors in the velocity data are difficult to list without knowing the means of measurement of the velocity.

Friction is impossible to avoid in the laboratory. It is possible that the measurement of velocity is such that the measurement is the average velocity during the second immediately before the collision and the average velocity during the second immediately after the collision. If so, your data would be equal to the instantaneous velocity 1/2 second before and 1/2 second after the collision. The friction would cause the instantaneous velocity immediately before and after the collision to be different from this average.

I hope this helps,


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Steve Johnson


I would be delighted to help with questions up through the first year of college Physics. Particularly Electricity, Electronics and Newtonian Mechanics (motion, acceleration etc.). I decline questions on relativity and Atomic Physics. I also could discuss the Space Shuttle and space flight in general.


I have a BS in Physics and an MS in Electrical Engineering. I am retired now. My professional career was in Electrical Engineering with considerable time spent working with accelerometers, gyroscopes and flight dynamics (Physics related topics) while working on the Space Shuttle. I gave formal classroom lessons to technical co-workers periodically over a several year period.

BS Physics, North Dakota State University
MS Electrical Engineering, North Dakota State University

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