Physics Asked by planckton on August 15, 2021
If there is a train moving at 2 m/s with a cannon attached to it loaded with a ball, and then the ball is launched from the train, how would one find the kinetic energy produced by the explosion if given the final speed of the ball and the final speed of the train is 0 m/s ?
Note the question specifically wants the kinetic energy produced by the explosion not the net change in kinetic energy in the system; the net change in kinetic energy in the system wouldn’t necessarily correspond to this would it? The net change in KE of the system would be the change in the ball’s KE minus the train’s original KE. Would it make sense for the original KE of the train to be subtracted though? Wouldn’t this be the explosion "taking" KE away from the train? Also, by finding the net change in ME of the system (in this case only KE changes), this gives the net work done by the non-conservative force (the explosion) but wouldn’t the explosion be both adding and dissipating energy from the system (adding KE but dissipating energy as heat and internal energy). Hence the net change in KE of the system would give both the KE produced by the explosion AND the energy lost through the explosion. I think this is different from the situation where you have a stationary object and then it is exploded and then you find the sum of the KE of each of the pieces to find the KE of each piece in order to find the total KE produced by the explosion; in this case no KE is "taken away" from an object like the way that the train’s KE is reduced. Could someone please help clarify this if it is a misunderstanding?
Could the KE added by the explosion be found purely through the change in kinetic energy of the ball? Would this mean that the decrease in the train’s KE is caused by the dissipation of energy as internal energy and sound?
IF instead the KE added by the explosion is found through the net change in the ball+train KE (as a system), where does the KE of the train go (intuitively I can understand that if the explosion force acts in the opposite direction to its velocity then it will do work on the train, decelerating it. However, then this would still be KE produced by the explosion no? So wouldn’t this have to be added to the KE produced by the explosion?). I think my main difficulty is in separating the KE produced by the explosion from the change in KE of the system (even though the KE of the train may decrease, this is partly due to the KE produced in the explosion so it should be added…?).
If someone could help clarify this, I would really appreciate it.
This is really about quite a simple balance of energies. Assuming noise and other non-conservative energies (like friction in the barrel) associated with the shot are negligible, then the energy balance is given by:
$$E_{k,1}+E_c=E_{k,2}$$
where $E_c$ is the chemical energy released by the shot, and the $E_k$ the kinetic energies before and after the explosion ($1$ and $2$).
So we have:
$$frac12m_tv_t^2+E_c=0+frac12m_bv_b^2$$
$$boxed{E_c=frac12(m_bv_b^2-m_tv_t^2)}$$
where the indices $t$ and $b$ refer to the train and cannon ball respectively.
So $E_c$ is the kinetic energy the explosion added to the system.
Answered by Gert on August 15, 2021
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