who really uses this stuff...this is why we have computers, i wish my old teachers could see me now!!

brett

 

What's frusterating is it's such an easy concept, just see when the wheel stops, but yet it confuses me when I try to make a model of it...

 

why dont you trace it onto cardboard, cut it out, then roll the silly wheel and see where it stops!!!

maybe i should go back to school so i can be a teacher...

brett

 

whoo yeah! I think I got it! Well, at least... I got an answer. using .4 for kinetic coefficient of friction and .5 for static, I found the wheel stops slipping at .945 seconds. Total distance before stopping and going up the ramp was .745 meters. I'm curious to see what you got HOHN. I'm a little confused though, because we were supposed to use a differential equation, but I used work-energy. We'll see on the test for sure if it's the right way!

 

From what I calculated, your answers are in the ballpark, however its been awhile since I've been in dynamics. I too had to blow the dust of my TI-89. You can work a problem like this backwards:




And obviously the differential equations come from these equations below. They can be solved simply using calculus. Just modify them with the friction added. The uppercase gamma is torque should you choose to go that route.




My books are so thick thats hard to get a good scan, I apologize. The work energy theorem will get you there, but you are bypassing the DEs.

 

I am glad I just chase queer light beams betwixt morrors and around the room. All this mechanical stuff makes be want a beer.

thinkingaboutPT(poppingatop)Shortround out

 

Now that's what I'm talkin ab out right there!

 

That's funny right there. Funny because it's so far from true.

Anyway, I didn't get a chance to look at this really last night-- sorry. Maybe later today.

Conceptually, here's what I would do (right or wrong):

You can calculate the rotational inertia of the "drum" that's on the ramp. As is climbs the ramp, gravity will apply a force in the opposite direction of the drum's rotation, causing it to decelerate.

First, calculate the "stopping" accleration acting on the drum. This will be the angular component (theta) of acceleration due to gravity (32.2 ft/s/s or 9.8m/s/s). Since theta is 30°, you have SIN (30)= .5, or 50% of g (32.2ft/s^2) acting against the rotation of the drum. Use F=MA to calculate the Force applied to the drum. Since you know the Vinitial, you can now find the TIME it takes for the mass to decelerate from Vinitial to V=0.

Then you can calculate the distance, or how far it travelled up the ramp.


I'm probably misunderstanding the problem, so I'm sure this is probably wrong.

I'm not seeing the relevance of the static friction-- the drum is rotating, right?

 

OK, I guess I was understanding the question all wrong.

Are you saying that the wheel "hits the brakes" when it hits the ramp?]

If that's the case, then start by finding the force from friction. So you have Cf* normal force. Normal force in this case is COS(30)(gravity)(mass of drum).

Since you now know the force of friction trying to slow down the drum/wheel/whatever, you can calculate the time and distance.

Justin

 

"I was just checking the specs on the endline for the...rotary...girder... I'm retarded."

 

That's funny right there!

HOHN, I think I got it figured out, but screwed up on the test... I forgot a negative sign and stuff started cancelling out that shouldn't be cancelling out... but as far as the problem goes, I might have used bad numbers, as it had no initial velocity. Without that it would just keep going in whatever direction it decides to go (balancing forces). So sorry for the confusion, I guess it was a trick question

 

I never really got anywhere on this one because I wasn't sure what I was supposed to find out.

Your lead post had all kinds of "givens" in it, but I didn't see a question!

Justin (my eyes are geting worse)