AP Mechanics

biggerman1

It mainly concerns Tourqe (spelling??) YOu set the T of both equal and then use that to find out the ratio of Force. Formulas required: T=Frsin(90) t=ma-mg-I think...

fizman

you have two distinct situations here. first, the system is in equilibrium. after the string is cut, it's dynamic.

1. discuss the equilibrium condition, that's before the string is cut.

2. discuss the dynamic situation after the string is cut.

zaggle56

Ok, I know we've had this one before. Here it is again.

1. The tension in the string attached to the mass is going to be half the tension of the string attached to the wall because of the inverse relationship between Ft and radius of the wheel.

2. when the string attached to the wall is cut, the mass is going to accelerate downward with the acceleration of gravity minus the force of tension, Ft, which is equal to mgrsin(90). use these equations to find the actual.

fizman

i like the part about the tension before the string is cut and the other up to where you say Ft =mgrsin(90) the tension is related to the torque on the wheel. also, the units don't work: mgrsin(90) does not give units of force(N).

again, let's stay away from equations. for instance, after you say "..mass is going to accelerate downward." you might say, " the acceleration is calculated as the net force: gravity minus the tension in the rope."

what else is the tension in the rope doing?

zaggle56

Ok mister Mo.

The tension in the string attached to the wall is equal to 2x the tension in the string attached to the mass, but you already know that. The new thing is that the tension in the string is equal to the torque produced by the small wheel, which is affected by the force of gravity on the mass, so the system is in equilibrium, that is until the the string is cut, at which point the system is out of equilibrium and, as a result of the off-set equilibrium, the mass accelerates downward due to the force of gravity. bwahahahahahaha.

zaggle56

also, problem #5 isn't working.

fizman

i got that part. i'm asking about the dynamics after the string is cut.

what are the affects on the mass and the wheel due to the tension in the rope?

this forum is a bit whacky. it seems to work first time even when you don't think so.

i'm working on #5.

for this week, just make a comment on this problem.

6strikekickball

after the string is cut, the mass will accelerate downwards due to a net force (the effect of gravity on the mass minus the tension in the spring) that's straight forward enough but would the tension in the string be the amount of work it takes to overcome the inertia of the wheel?

saveferris

the tension isnt equal to the torque. the force of the tension is causing the torque in both of the wheels. also, when the rope is cut, the tension is slowing the acceleration of the block because it is in the opposite direction of gravity.

brookes might have said all that already but his posts are too long and i didnt read them.

fizman

sf is right! torque is the product of tension and radius. six is right about the "the effect of gravity on the mass minus the tension in the string." and as restated by ferris, the tension is slowing the acceleration of the block. this is what we're looking for; a description of what's happening that helps us understand or setup the physics. now we can describe mathematically what's going on. we set the net force equal to the difference in gravity and tension.

what if asked about the speed that the mass hits the ground? what do you need to know and how would you tie it together?

zaggle56

Hey I've posted on number 5 a few times, but they never seem to show up when i do.