The discussion centers on a sample AP Physics C question regarding the collision of a dart with a box. Participants debate the validity of an equation derived by students during the experiment, specifically questioning the inclusion of the term m/(m+M). The consensus is that while the formula may not be definitively correct, it is essential to analyze the problem using momentum conservation principles, especially since kinetic energy is not conserved in inelastic collisions. Key equations discussed include FL = 1/2 mv^2 and FL = D x (friction coefficient) x (M+m)g.
PREREQUISITESStudents preparing for the AP Physics C exam, educators teaching physics concepts, and anyone interested in understanding collision mechanics and energy conservation in physical systems.
DrClaude said:It is said that students doing the experiment came up with that equation. I wouldn't assume that it is correct: it could be part of the answer to argue why it should be wrong.
You are not expected to determine whether the formula is right or wrong. You are only asked whether the answer the formula gives to 1a is the same as your answer, and how you determined that.aspodkfpo said:What would the correct equation be? without an extra m/(m+M)?
haruspex said:You are not expected to determine whether the formula is right or wrong. You are only asked whether the answer the formula gives to 1a is the same as your answer, and how you determined that.
DrClaude said:It is said that students doing the experiment came up with that equation. I wouldn't assume that it is correct: it could be part of the answer to argue why it should be wrong.
It is correct.aspodkfpo said:I know what the question is asking for, I'm just curious about whether that formula is right/can be calculated.
haruspex said:It is correct.
What is the velocity of the dart when it reaches the block?
What is the combined velocity when the dart is embedded in the block?
What is the KE of that?
You are not taking into account the collision.aspodkfpo said:FL = 1/2 mv^2
FL = D x (friction coefficient) x (M+m)g
D= FL/((friction coefficient) x (M+m) g)
Where am I wrong?
haruspex said:You are not taking into account the collision.
It's a bit tricky because you don't know how long it takes for the dart to become stationary wrt the box, so assume that happens very quickly. I.e. treat it as three phases: acceleration of dart, dart and box coalesce, dart and box slide together.
How does one calculate any coalescence problem? Momentum!aspodkfpo said:FL = 1/2 mu^2
= 1/2 (m+M) v^2
= D (friction coeff) x (m+M) g
I don't understand how to calculate the dart and box coalescing?
haruspex said:How does one calculate any coalescence problem? Momentum!
... provided there is no net external force on the system.aspodkfpo said:momentum is always conserved.