AP Mechanics

fizman

So, you laugh. well instead of the famous bisquit on a pogo stick problem, i thought you might have a look at this one.

given: frictionless except for the final 10m long green strip whose coef of friction is unknown. the 100g bullet enters at the left, embeds itself in the 1.0kg block. the resulting combo barely makes it to the top of a 30cm radius loop without losing contact with the track. Lets discuss this in order.

1. What major concepts come to mind. don't even explain how they apply. just list what you see and the point it occurs.

this time, read what has been said and respond accordingly.

6strikekickball

conservation of momentum when the bullet hits the block, energy is then conserved as it changes from kinetic to potential and back around the loop, then all energy is sapped by friction on the green strip

fizman

6 has some good thoughts here. momentum first, then it's energy all the way to the finish line.

someone tell me more about the energy while the combo is doing the loop. 6strike said, "energy is then conserved as it change from kinetic to potential." can anyone be more specific about the mechanical energy? what about it just staying in contact with the track. what's that about?

saveferris

by more specific i hope you mean that you want me to point out that potential energy at the top will be equal to the kinetic at the bottom. its an even trade because the loop is frictionless

fizman

saveit, If it stays in contact at the top, it is still moving and therefore has kinetic. So, the kinetic at the bottom, is not just the potential at the top ,but potential and kinetic at the top.

chelsealane

I have nothing more to add on energy and momentum, but there's F(c) pulling it inwards, linear F(f) at the end, tangental force pulling it outwards, F(g) pulling it downwards, and then alpha (fishie?).

when you say "barely makes it to the top without loosing contact," does that mean that the acceleration does not reach 0, but gets close? I always thought at the top the net acceleration was 0.

fizman

When it barely makes it w/o losing contact, it is moving at critical speed; gravity is providing the centripetal force. The normal force is zero.