<p>Where is everyone studying for Physics C? Could anybody check my answer above?</p>
<p>Also, does anyone know if there is usually a question involving motion on an inclined track (sort of like a NASCAR oval). I’m reviewing with the Barron’s book and they made it seem like a fairly important topic. I guess it actually isn’t too hard, but it’s just one thing I wasn’t already really familiar with.</p>
<p>1) I think the friction force is the only one causing rotation, so it is equal to Mgsin(22)
2) so T=2/5MR^2*alpha
3) T also = Mg sin (22) R
4) alpha = a/R
5) substitute, i got a= 5/2g(sin(22))
then v= sqrt (2ax) and i got 14.84 m/s</p>
<p>ChemE14 looks right. th3, the second part of your statement #1 is not correct. The friction force is the only force producing rotation, but that doesn’t imply that it equals mg(sin(22)).</p>
<p>Well the guy who posted the problem said there was an answer in the tenths place (not sure if used 9.81 or 10 for g). Also, I think if you say F<em>friction = F</em>gravity = mgsin(22), then that means that its acceleration is equal to zero. And since v_o = 0, then v=0.</p>
<p>The test is a week away! Let’s get this going again.</p>
<p>After reading through Barron’s, I have a question about rolling. Let’s say you have a solid ball, and it’s rolling without slipping at constant velocity with a=0 and alpha=0 across a horizontal surface. So the ball never stops and keeps on going at the same velocity regardless of whether or not the surface is frictionless? Just wanted this clarified.</p>
<p>EDIT: Okay, look’s like everyone busy studying for more urgent AP exams. I’ll bump this back up on the weekend, haha.</p>
<p>Great question, yes, the ball can keep rolling at a constant linear and angular velocity even without any friction as long as the surface remains perfectly horizontal w/r/t gravity.</p>
<p>I have a couple questions; I’ve been confused on these topics for nearly a month, and my teacher’s explanations don’t help.</p>
<p>What is the difference between linear momentum and angular momentum? How can I determine when each is conserved? For angular momentum, the part of the formula is mvrsin(θ). What is θ between? </p>
<p>Leading into conservation, how can I determine when mechanical energy is conserved? Conservation of mechanical energy always seems to be a crucial component of many problems we solve in class, and I can never pick up on where the hell the teacher pulled it out of.</p>
<p>I have inelastic collisions down; they’re easy. What about elastic collisions in two dimensions? I know there are momentum equations for the x-direction, the y-direction, and I think a conservation of kinetic energy equation? So for these types of problems, I’d set my three equations (px, py, and energy) up and solve a system?</p>
<p>Also, I don’t get torque. For static equilibrium problems, when setting up the sum of torques equation, why are some terms negative while others are positive?</p>
<p>I would love to pass this AP exam, but it just doesn’t seem likely. I’ve been confused the whole year. My teacher just can’t teach.</p>
<p>Linear momentum is just p = m*v. Angular momentum is p x r = |p||r|sin(θ). In other words, you only want the component of linear momentum that is perpendicular to r. Say you have a satelite orbiting a much larger mass. Draw a line connecting the mass and the satelite. That’s your r vector. Now draw a line (that starts at the satelite) that is tangent to the satelite’s path and points in the direction it’s going. That’s your velocity vector. The angle between these to vectors is θ. If the satelite has a circular path and the mass is in the center, then v (and therefore, p) will always be perpendicular to r, which is why its uncommon to tack on the sin(θ). If you can’t visualize this, let me know, and I’ll upload a cheap image out of Paint.</p>
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<p>Off the top of my head I can tell you mechanical energy is NOT conserved when you have an inelastic collision or when there’s a frictional force or any other nonconservative force.</p>
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<p>Yep! Unless there’s a problem like one I worked a few days ago where the elastic collision involved angular momentum.</p>
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<p>You have to realize which direction each force wants to make the object turn. Generally, counter-clockwise is the positive direction. For static equilibrium, you want to set up your torque equation such that</p>
<p>Thanks so much, ChemE14. Your post has been enormously helpful.</p>
<p>However, I’m still confused as to how to determine when angular momentum and linear momentum are conserved. Frequently on practice AP exams, I’ve seen questions that give a situation and ask about whether or not linear momentum and angular momentum are conserved. What’re the basic principle here to determine if momentum is conserved?</p>
<p>I think I’ve decided not to study. MIT only gives credit for a 5 on both and I got a 26% on the MCQ when we did it in class. I would have to completely devote all the rest of my time to physics, and I might still fail. So, yeah. Not worth it. I have three other APs. Sorry Mr. Physics Teacher, but I doubt you thought I would pass in the first place.</p>
<p>I had some questions about a few things, but forgot them. So instead… I’ve got a practice problem.</p>
<p>A person of mass “m<em>p” is standing in an elevator of mass “m</em>e” suspended from the top of a building by a spring with spring constant “k”.</p>
<p>a) What is the equilibrium extension of the spring?</p>
<p>b) Supoose the elevator is displaced from equilibrium a small distance. What is the resulting frequency of oscillation?</p>
<p>c) What range of accelerations can the elevator experience without losing contact with the person?</p>
<p>d) What is the maximum amplitude with which the system can oscillate without the elevator’s losing contact with the person?</p>
<p>e) Assuming the amplitude of the oscillations is maximized, where is the acceleration of the person maximum? Minimum? Zero?</p>
<p>Question - I know that in a standard horizontal spring system, the potential energy of the mass is just (1/2)<em>k</em>x^2, with x is distance away from equilibrium. But what is it in a vertical/tilted system, where gravity plays a role? Is it U<em>s - U</em>g, which I believe would be (1/2)<em>k</em>x^2 - mgxsin(theta)? And what does the “x” mean, in this case? Distance the mass is from the equilibrium point, or the distance the spring has stretched/compressed from it’s normal position (meaning, the amount the spring is compressed is x=0, the place where the spring would be without a mass)?</p>
<p>A person of mass “m<em>p” is standing in an elevator of mass “m</em>e” suspended from the top of a building by a spring with spring constant “k”.</p>
<p>a) What is the equilibrium extension of the spring?</p>
<p>b) Supoose the elevator is displaced from equilibrium a small distance. What is the resulting frequency of oscillation?
f=1/(2pi*sqrt((m<em>e+m</em>p)/k))</p>
<p>c) What range of accelerations can the elevator experience without losing contact with the person?</p>
<p>-g<</p>
<p>d) What is the maximum amplitude with which the system can oscillate without the elevator’s losing contact with the person?</p>
<p>e) Assuming the amplitude of the oscillations is maximized, where is the acceleration of the person maximum? Minimum? Zero?
a_max at the very bottom, min at top, zero at equilibrium</p>
<p>new question: A conducting loop of wire that is initially around a magnet is pulled away from the north end of a magnet to the right, inducing a current in the loop. What is the direction of the force on the magnet? Why?</p>