Physics Teacher From Hell!!

[Cogman]

Well, I figured that I would give you guys a HT rant. As a bit of background, I am taking a physics 121 course (basic calculus based physics). Man, my teacher give use assignments that are unbelievably hard stuff that is impossible to do without a) A physics degree b) A tutor. Im just getting fed up with it.

So where is the HT part? Here is one problem that we where given A long uniform rod of mass M and length L is pivoted about one end by a horizontal, frictionless pin. The rod is released from rest in a vertical position. At the instant the rod is horizontal, find a) its angular speed, b) the magnitude of its angular acceleration, c) the x- and y-components of the acceleration of the center of mass, and d) the magnitude of the reaction force at the pivot.

Ok, first off, I know this is not a homework board, the assignment is due tomorrow so just don't say anything for however long you want, I really am not coming here to get answers, however if you want to take a stabe at it and then tell me, how long it took you to do the problem, and what you physics knowledge is I think it would be pretty interesting.

I am able to do 1 and 2 with the information provided in the book, but after that things just get hairy and I don't understand what the heck the teacher is talking about. We have just covered BASIC rotational kinematics in class. ie torque = r F sin (theta) and such. We get homework like this once a week and it is driving me insane (btw the is just problem 2 on the assignment, I dread to think what the last one is going to look like.)

So is this too hard for 121 level physics students? Or am I just whining? Oh, and good luck for those of you who dare to attempt it.

 

[CycloWizard]

That sounds remarkably similar to my first college physics course. It always seems hard at the time, but you'll think it was a joke looking back in a few years.

I would answer that it's a trick question: if the rod is perfectly vertical and the pin is located along its axis, then the rod will remain in the vertical position.

 

[Cogman]

Yeah, that is what I thought at the first moment that I saw it. but I guess we are assuming that it is varied by an infinitesimally small amount. This would not be bad, except for the fact that I don't know how to do it, and have not been taught how to do it. Yes, it will probably be a cake walk after I learn it all, but how can I learn what is not being taught? Algebra is pretty easy to me now, but to start with it was not fun, I was just lucky to have a good teacher that made everything easier.

 

[BrownTown]

meh, I took calculus based physics with problem just like that in high school, so don't come to me for sympathy .

 

[Cogman]

Ok, I'm feeling better now, that post was made after 4 hours of working on the physics assignment (it kind of rubs you a bit raw, especially when you realize "We've never been taught this and there are no examples of it in the book...") I still would like to see how long it takes one of you guys to do part c and d of the problem. part a and b are not that bad, but c and d become a bit hairy.

 

[CycloWizard]

Originally posted by: Cogman
Ok, I'm feeling better now, that post was made after 4 hours of working on the physics assignment (it kind of rubs you a bit raw, especially when you realize "We've never been taught this and there are no examples of it in the book...") I still would like to see how long it takes one of you guys to do part c and d of the problem. part a and b are not that bad, but c and d become a bit hairy.

C and D are not bad. Just solve them only at the instant that the rod is horizontal. What is the x component of acceleration at this point? The force at the pivot is simple to calculate at this point as well.

 

[gsellis]

Not too bad. Where cyclowizard is at, the basic physics has an exam question about a cable stretched across the Mississippi (1 mile) with a certain elasticity. If a 180 lb tightrope walker walked across it, and it has a certain tensile strength, would it break? What was the maximum deflection at where it broke or the middle if it did not. I still know I had no clue. Failed it the first time, squeaked through the second. After Statics, I changed majors

 

[herm0016]

that question is not too hard. would have to draw it out and do it on paper though.

just draw the free body diagram and sum the forces and moments. or use energy, its just a big pendulm.

I also did this stuff in HS physics. now its on to dynamics, where we have to find the equation that describes its velocity, acceleration and force for every time. and mechanical vibrations where we find the natural freqwency of they system and the equation describeing the force transfered to the ground due to the rotation.

 

[imported_iNGEN]

Unless the assignment provides for conditions you have not given and are not implied then Cyclo Wizard's first post is accurate. The rod will remain verticle in accordance with Newton's First Law of Motion unless acted upon by a force not mentioned in the question, therefore it is not possible to solve for any of a), b), c), or d). If such a force were applied, but is unknown it is not possible to solve for a) or d), because the effect of unbalancing will affect their values.

If this is a physics course not a math course, you may be able to really impress the prof by expressing all assuming there was an outside force. a) and d) would, of course, have to be expressed as polynomial equations. Any physics professor worth his salt wants you to be able to use the concepts not just do the math. The calculations themselves are a prerequisite for being able to use the concepts. Of course, if your prof is an arrogant ass he just wants to you do what you are told even when it isn't possible. If that's the case ignore this entire post.

 

[ZeroNine8]

the simplest way I would do this would be using an energy method - potential energy converted to kinetic energy (mgh = 1/2 I omega^2). Without numbers, you'll just end up with answers in terms of the mass and length of the rod as well as some constants.

Just because you don't understand the problem, trust me, it takes far far less than a physics degree to 'get it'. I remember this stuff being tough at first, but if you stick with it, you'll wish that you were still working on simple problems like this a year from now.

 

[imported_flounder]

Honestly, this would take about 5min if I were to draw it out. We have problems like this every night in high school AP physics

 

[eLiu]

Have you talked about coordinate systems other than cartesian? B/c using polar coordinates & the associated equations of motion make this problem pretty easy.

Edit: my hw questions were much harder

 

[blahblah99]

Originally posted by: Cogman
Well, I figured that I would give you guys a HT rant. As a bit of background, I am taking a physics 121 course (basic calculus based physics). Man, my teacher give use assignments that are unbelievably hard stuff that is impossible to do without a) A physics degree b) A tutor. Im just getting fed up with it.

So where is the HT part? Here is one problem that we where given A long uniform rod of mass M and length L is pivoted about one end by a horizontal, frictionless pin. The rod is released from rest in a vertical position. At the instant the rod is horizontal, find a) its angular speed, b) the magnitude of its angular acceleration, c) the x- and y-components of the acceleration of the center of mass, and d) the magnitude of the reaction force at the pivot.

Ok, first off, I know this is not a homework board, the assignment is due tomorrow so just don't say anything for however long you want, I really am not coming here to get answers, however if you want to take a stabe at it and then tell me, how long it took you to do the problem, and what you physics knowledge is I think it would be pretty interesting.

I am able to do 1 and 2 with the information provided in the book, but after that things just get hairy and I don't understand what the heck the teacher is talking about. We have just covered BASIC rotational kinematics in class. ie torque = r F sin (theta) and such. We get homework like this once a week and it is driving me insane (btw the is just problem 2 on the assignment, I dread to think what the last one is going to look like.)

So is this too hard for 121 level physics students? Or am I just whining? Oh, and good luck for those of you who dare to attempt it.

That sounds like a typical problem for a calculus based physics class.... The problem gets easy when you draw a FBD of all the forces involved, which in this case, is not very many.

 

[patentman]

Originally posted by: gsellis
Not too bad. Where cyclowizard is at, the basic physics has an exam question about a cable stretched across the Mississippi (1 mile) with a certain elasticity. If a 180 lb tightrope walker walked across it, and it has a certain tensile strength, would it break? What was the maximum deflection at where it broke or the middle if it did not. I still know I had no clue. Failed it the first time, squeaked through the second. After Statics, I changed majors

I had a similar question as you mentioned gsellis, only in my 4th year physical chemistry class. In essence we were asked to calculate the amount of heat generated by a one legged ice skater as he moved in a defined pattenr on the ice. There was more to it than that, but it given that I rememebr the question after almost 10 years it made quite an impression on me.

 

[Karot]

Try www.cramster.com if the problems are coming out of a textbook.

 

[Matt1970]

Originally posted by: patentman

I had a similar question as you mentioned gsellis, only in my 4th year physical chemistry class. In essence we were asked to calculate the amount of heat generated by a one legged ice skater as he moved in a defined pattenr on the ice. There was more to it than that, but it given that I rememebr the question after almost 10 years it made quite an impression on me.

LOL, I would like to see a one legged skater try and get going without any assistance.

 

[Stg-Flame]

Originally posted by: flounder
Honestly, this would take about 5min if I were to draw it out. We have problems like this every night in high school AP physics

It is posts like this that make me see how low the standards are/were set for my High School when I was a student. Back when I was a Senior, I heard honor students in my AP Chemistry Class complaining about them not understanding Trig.

Granted the highest level of Math we had was Trig/Intro to Calc and only Basic Physics; I find myself wondering: where are these High Schools that have such advanced (to me) courses?

 

[gsellis]

Originally posted by: patentman

Originally posted by: gsellis
Not too bad. Where cyclowizard is at, the basic physics has an exam question about a cable stretched across the Mississippi (1 mile) with a certain elasticity. If a 180 lb tightrope walker walked across it, and it has a certain tensile strength, would it break? What was the maximum deflection at where it broke or the middle if it did not. I still know I had no clue. Failed it the first time, squeaked through the second. After Statics, I changed majors

I had a similar question as you mentioned gsellis, only in my 4th year physical chemistry class. In essence we were asked to calculate the amount of heat generated by a one legged ice skater as he moved in a defined pattenr on the ice. There was more to it than that, but it given that I rememebr the question after almost 10 years it made quite an impression on me.

I graduated in 1983 and took physics in 1980