Transfer of energy in collsions - Isaac physics

https://isaacphysics.org/questions/transfer_energy_ZMF_num

I have done parts a and b of this question, but I am stuck on part c.

I assumed that all the collisions were elastic, and just repeated the method I used for parts a and b (using the ZMF and then transforming back to the lab frame).

However, when i tried to do this saying that both m_2 and m_3 would be travelling a the speed of the zmf u_1, and the m_1 would be travelling at v-u_1, and then saying that after the first collision, m_1 would travel at -v+u_1, m_2 would travel at u_1 and m_3 would continue to travel at -u_1. Then, m1 would continue to travel at -v+u_1 and m2 would travel at -u and m3 would travel at u. transforming back to the lab frame gets me m3 travels at 2u. From this I worked out the KE. Where am I going wrong, is it the assumption that all the collisions are elastic?

Reply 1

Eimmanuel

Study Forum Helper

15

Original post by amamama0999

https://isaacphysics.org/questions/transfer_energy_ZMF_num

I have done parts a and b of this question, but I am stuck on part c.

I assumed that all the collisions were elastic, and just repeated the method I used for parts a and b (using the ZMF and then transforming back to the lab frame).

However, when i tried to do this saying that both m_2 and m_3 would be travelling a the speed of the zmf u_1, and the m_1 would be travelling at v-u_1, and then saying that after the first collision, m_1 would travel at -v+u_1, m_2 would travel at u_1 and m_3 would continue to travel at -u_1. Then, m1 would continue to travel at -v+u_1 and m2 would travel at -u and m3 would travel at u. transforming back to the lab frame gets me m3 travels at 2u. From this I worked out the KE. Where am I going wrong, is it the assumption that all the collisions are elastic?

You can try to attempt the question based on lab frame.
The collision is elastic. If it is inelastic, there are not enough equations to solve for the unknowns.

Reply 2

Eimmanuel

Study Forum Helper

15

Original post by amamama0999

https://isaacphysics.org/questions/transfer_energy_ZMF_num

I have done parts a and b of this question, but I am stuck on part c.

I assumed that all the collisions were elastic, and just repeated the method I used for parts a and b (using the ZMF and then transforming back to the lab frame).

However, when i tried to do this saying that both m_2 and m_3 would be travelling a the speed of the zmf u_1, and the m_1 would be travelling at v-u_1, and then saying that after the first collision, m_1 would travel at -v+u_1, m_2 would travel at u_1 and m_3 would continue to travel at -u_1. Then, m1 would continue to travel at -v+u_1 and m2 would travel at -u and m3 would travel at u. transforming back to the lab frame gets me m3 travels at 2u. From this I worked out the KE. Where am I going wrong, is it the assumption that all the collisions are elastic?

I think your problem lies at your assumption that m1 collides with m2 and m3 simultaneously.

There are 2 separate collisions: m1 collides with m2 first, and then m2 collides with m3.
You should have dealt the first collision at part B.
In part C, you just need to consider the second collision where m2 moves with certain velocity, says v2 (in the lab frame) and collides with m3.

Reply 3

amamama0999

OP

5

Thank you! I managed to do it in the lab frame! I think it was the converting into the ZMF that was confusing me

Transfer of energy in collsions - Isaac physics

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