Well after reading that there was a lot of torque problems on today's MCAT I am a little nervous since I am taking it Thursday. I understand the basic formula and some basic problems but I feel like I don't have a great grip on it. Even though I have a day I can always learn something new and was wondering if someone who understands it very well can give some tips or tricks that are good to look out for on Torque problems/concepts for test day.. Any help would be greatly appreciated. It seems to be something people don't concentrate on but it always shows up.
Torque tau = r x F = r*Fsin(theta), where theta is the angle between the force and r. If it's just a simple situation like a teeter-totter with 2 masses, 1 at each end, the force is the weight of the mass, and it points straight down. So then theta would be 90 degrees.
An easy way to get these problems is to always look for the component of the force that's perpendicular to r (i.e. perpendicular to the bar or whatever that's rotating). Some ppl prefer to do these problems with lever arms, i.e. torque tau = r x F = rsin(theta) * F, but I find that unintuitive.
Anyway, once you've figured out the component of the force that's relevant to generation of torque, now you just have to figure out direction. Since torque is a cross product, you can use the R-hand rule to figure out the direction of the torque, and then assign + to one direction and - to another direction, then sum up all the torques. OR, you could just kind of use intuition about which end is going up, which end is going down (i.e. which way the thing ends up rotating). Then set one set of torques = other set of torques. This way obviates the need to assign direction, since you're just setting one set equal to the other set.
The other thing to keep in mind is around what point the bar or lever or whatever is rotating. Once you choose a point of rotation, remember that r is the distance to that pt from the pt where the force is exerted. If a force is exerted on the pt that you chose as your point of rotation, you get to ignore that when considering the torque.
I don't know what else to say about torque. That much knowledge has gotten me through a lot of problems. Basically it's just that one equation. It's helpful if you understand the cross product and how the R-hand rule applies to that, but I don't think it's a big deal or anything.
