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## Main Question or Discussion Point

Hello! I am a high school teacher and I am doing a lab on Newton's laws. I need help interpreting part of the lab because the results did not come out to what the laws would have predicted.

The lab consisted of setting up two carts, one with a spring which can be compressed and one without. The first part of the experiment says the students should depress the spring and put the carts together. When released, they both go opposite ways with the same acceleration. So far so good, Newton's 3rd law tells us there will be an action-reaction pair and the 2nd law tells us that a=F/m.

Next, a 1 kg mass is added to the cart with the spring. Again the spring is compressed, they are put together, and the spring is released. Uh-oh. Now here is the problem. We know that again, there is an action and reaction pair, so the force will be the same on both carts. Since the mass of the second cart is greater, its acceleration will be smaller. But what about the cart with no weight. Correct me if I am wrong, but the acceleration here is again equal to F/m where F = -kx (Hooke's law). The force has not changed, the mass has not changed, so the acceleration should be the same as it was in the first trial. Right? All of my students reported a greater acceleration in the second trial for the cart with no added mass than in the first trial. Also, as the mass on the other cart increased to 2kg, the acceleration on the cart with no mass added increased again. Am I missing something, or are there real world factors that would make account for these observations? I considered conservation of momentum, but this does not change anything as the increase in mass of the first cart should be offset exactly by the decrease in its velocity

Thank you so much if you took the time to read this and reply.

Peter

The lab consisted of setting up two carts, one with a spring which can be compressed and one without. The first part of the experiment says the students should depress the spring and put the carts together. When released, they both go opposite ways with the same acceleration. So far so good, Newton's 3rd law tells us there will be an action-reaction pair and the 2nd law tells us that a=F/m.

Next, a 1 kg mass is added to the cart with the spring. Again the spring is compressed, they are put together, and the spring is released. Uh-oh. Now here is the problem. We know that again, there is an action and reaction pair, so the force will be the same on both carts. Since the mass of the second cart is greater, its acceleration will be smaller. But what about the cart with no weight. Correct me if I am wrong, but the acceleration here is again equal to F/m where F = -kx (Hooke's law). The force has not changed, the mass has not changed, so the acceleration should be the same as it was in the first trial. Right? All of my students reported a greater acceleration in the second trial for the cart with no added mass than in the first trial. Also, as the mass on the other cart increased to 2kg, the acceleration on the cart with no mass added increased again. Am I missing something, or are there real world factors that would make account for these observations? I considered conservation of momentum, but this does not change anything as the increase in mass of the first cart should be offset exactly by the decrease in its velocity

Thank you so much if you took the time to read this and reply.

Peter