Momentum Lab Help: Understanding Collisions in AP Physics B Course

[bleedblue1234]

Homework Statement

So this isn't necessairly a direct problem, but we are currently doing a momentum lab in my AP Physics B course and the lab is basically dealing with 4 cases of collision between two carts. In all cases we are to measure the velocity just before collision of the two carts and just after collision. We are using sonic detectors on both sides of the track to do this. In case 1, the carts are collided (they are nearly the same mass) with one cart sitting motionless on the track and the plunger cart hitting it going approx. 3.0 m/s. In case 2, the conditions are the same except an approx 1/2 kg mass is added to the passive cart. In case 3, the carts are the same except they are turned so the velcro sides face each other so they stick together after the plunger cart rams into the passive cart. The fourth case is the same except for the same mass is added once again.

Homework Equations

None

The Attempt at a Solution

We are asked what type of collision is each case, and I have stated it is elastic for case 1 and 2, and completely inelastic in case 3 and 4... but I don't think this is possible because no collisions are perfectly elastic or inelastic? I may have to ask my instructor on this question...

The second question I have is regarding how well my results support the expected results, but I don't know what my expected results should be? Should my momentums be related in some way? Would this be percent error or percent difference?

And what key concept is this showing? conservation of momentum?

 

[Delphi51]

What does "perfectly inelastic" mean?
Elastic means no kinetic energy is lost, and this is not likely to be perfectly true, but it could well be pretty close. I would go with that prediction in the first two cases, and then use conservation laws to work out the expected speeds after the collision. Compare those with the experimental results. It would be interesting to compare the predicted kinetic energy with the actual to see how close they are. I think a % difference would be more appropriate because it could perhaps be applied to other situations where the masses are much larger or smaller.

 

[kerimek]

I would like to commend you for recognizing the importance of understanding the type of collision in each case and the expected results. Your initial responses seem to be on the right track, but let me provide some additional insights that may help you better understand the concepts involved in this momentum lab.

Firstly, it is correct to say that no collision is perfectly elastic or inelastic. In reality, all collisions involve some degree of energy loss due to factors such as friction, deformation, and sound. However, we often use the terms "elastic" and "inelastic" to describe the relative amount of energy lost in a collision. In an elastic collision, the two objects bounce off each other and the total kinetic energy is conserved. In an inelastic collision, the two objects stick together and the total kinetic energy is not conserved. Therefore, it is appropriate to describe case 1 and 2 as elastic collisions, and case 3 and 4 as inelastic collisions.

Regarding the expected results, it is important to understand the concept of conservation of momentum. This principle states that the total momentum of a system remains constant unless an external force acts upon it. In other words, the initial momentum of the system is equal to the final momentum of the system. In case 1 and 2, the initial momentum of the system is equal to the final momentum, as the two carts are moving in opposite directions with equal but opposite velocities. In case 3 and 4, the initial momentum of the system is also equal to the final momentum, as the two carts stick together and move with a common velocity. Therefore, your results should support this principle of conservation of momentum.

In terms of percent error or percent difference, it would depend on what you are comparing. If you are comparing your experimental results to the expected results, then it would be appropriate to calculate a percent difference. However, if you are comparing your results for each case to the expected results for that specific case, then it would be more appropriate to calculate a percent error.

Overall, this momentum lab is a great way to understand the concept of conservation of momentum and how it applies to different types of collisions. It also helps to reinforce the idea that no collision is perfectly elastic or inelastic, and there will always be some energy loss. I would encourage you to discuss your questions and results with your instructor to gain a deeper understanding of the concepts involved. Good luck with your lab!