Wrong Formula Given for Notebook 4 HW

[jmr15122]

In problem 2 part B of the homework associated to the module 3 homework 4, a hint is given that we should use the ratio of the "last velocity from above (before the bounce) to the initial velocity calculated here (after the bounce)." to find the coefficient of restitution for the second bounce. In notebook one though, we use the inverse of this formula and get an answer that makes more sense (a value between 0 and 1 rather than a value slightly greater than 1). I think that the ratio we want to take is the inverse of the one suggested.

[mdb15106]

It's written somewhat unclearly. You're correct; the coefficient of restitution is always less than 1 for this type of collision, so use the velocity ratio that gets you a value less than 1.

[acg14006]

Jake, you are correct. notebook 4 did give the wrong formula. notebook one says:

During a collision with the ground, the coefficient of restitution is e=-v′y/vy. Where v′y is y-velocity perpendicular to the ground after impact and vy is the y-velocity before impact

So for 2b., on this homework, the coefficient of restitution should be ratio of the the velocity after the second bounce divided by the velocity before the second bounce. This will result in a value between 0 and 1.