So does this mean I can't do physics?

I’ve been trying to teach myself math for about a year now, and generally, I’ve succeeded. I’ve gotten basic algebra down with a good dash of trig to boot. I’ve begun studying a bit of physics, but I’ve gotten lost at force vectors and FBD diagrams. I keep screwing up the forces and such, and I’m now thinking of just skipping this part and coming back later.

Does the fact that I can’t do force vectors mean that I’m screwed when it comes to learning the rest of physics, or are force something that can be skipped over? I’m not really looking to master physics. I’m just going for the basics. Thanks.

If you can figure out a way to learn physics without forces and vectors, my students want to hear about it. :slight_smile:

Is there any chance you can sit down with somebody who has a physics background? Have you tried GQ? Have you tried looking at a couple different textbooks? Sometimes just reading the same concepts but worded differently can be very helpful. I’ll betcha anything that it’s not that you can’t do physics, yov’ve just latched on to some misconception that’s tripping you up.

I also had troubles with force vectors when I first started studying physics. Having been there, I’d recommend spending a little more time trying to understand force vectors before moving on - it makes understanding the later concepts loads easier.

Maybe you could post what your sticking point is - I’m sure there are several Dopers that could help you out.

I think that teaching yourself physics is damned near impossible. You’re going to need someone to help you with this. There might be a Doper willing to teach you by email or something, or to direct you to online tutorials.

But vectors, yeah, you can’t do jack without them.

Thank all of you for your input.

I understand the basic meaning of vectors. I can add them, subtract them, get the dot product, cross product, etc., etc. I know the difference between a scalar and a vector. I know that if a horse pulls on a sled the sled pulls on the horse, just like I know that the sled still moves forward, because all that matters are the forces on the sled, and as long as the friction which moves the horse forward is greater than the friction moving against the forward motion of the sled, the sled will move.

Where I get hung up on is drawing the actual vectors and the diagram and getting the math right. It seems I always draw one vector extra or forget one. I can’t tell which vectors to count and which not to count. In other words, I get the theory, but not the practice.

I’m beginning to agree with dwalin. It’s probably not really worth the effort to teach myself the basics of physics, especially if I want to do something with it later on–don’t want to be making mistakes right off the bat. I’m thinking about going back to teaching myself math (something I have no problems with). I’m going to get through math on my own all the way through basic calculus and then audit a physics course at my college (oh, the joys of being an academic librarian :slight_smile: ). Physics has absolutely nothing to do with work. It’s just one of those things which I’ve always been a bit ticked off not being able to get and seeing how far I can go toward conquering it before I die.

Once again, thanks for your responses. They were very helpful.

What vectors do you draw besides the forces on your free body diagrams?

Whatever you answer that question with stop doing that.

But what do you get if you cross a moutain climber and a mosquito?

You can’t cross a scaler and a vector.

I love it! :smiley:

Linty Fresh, I disagree with dwalin - it’s possible to teach yourself almost anything. I’m not saying that it wouldn’t help to have outside help, but I think its admirable to be curious enough about physics to start learning it on your own.

If you’re still interested, could you provide an example of where you’re having a problem? There’s far too many engineers and physicist lurking about to not have your question answered!

I probably should expand on that. In physics you probably are going to only see friction, rope, member, normal, gravity and applied forces. Friction forces are placed on the FBD in the middle of where the body touches the surface. These are parallel and always oppose motion. Rope forces are always directed away from the body in the direction of the rope becuase its impossible to push with a rope. If you get a negative force when you have drawn it correctly you have calculated something wrong.

Member forces act the same way as rope forces but its possible for them to be directed in towards the body. Treat these the same way as ropes but its ok if you get a negative answer. Normal forces are those that surfaces apply on the body. These always are perpendicular to the surface and point towards the bodies center. Gravity is always applied at the center of mass straight down. Applied forces are placed however they are given in the problem.

The first step in any physics problem is to draw a diagram with all of your ropes, members and surfaces intact. Next you cut all of yours ropes/members and replace them with forces. Do the same with surfaces and add the normal and friction forces. Add all of your gravity and applied forces and you should have the FBD. After those are all drawn you want to draw an equal sign except with 3 lines (stands for defined as) to a body with an acceleration vector labelled ma. If the mass is stationary then its just 0 and you don’t need to do that step. From there you can write your E(sum) Forces=M*a.

I don’t mean to be blunt but the bad news is that if you can’t draw a FBD you might as well give up on learning physics. The good news is that once you learn how to draw a FBD you basically have learned how do every mechanics problems. Once you get the hang of FBDs it becomes 2nd nature. Drawing FBD and summing forces will get you half way to a mechanical engineering degree. All that different classes teach you is what process/tricks you use to solve the FBD. I am simplifying a bit but for the most part thats true.

Doesn’t the OP need to learn calculus before really being able to do physics?

Calculus is, of course, the fundamental language of physics; however, it is possible to grasp most of the basic concepts with just a “pidgeon” of algebra and trigonometry, with some linear algebra thrown in to make life easier. If the goal of the OP is just to get a basic understanding of classical mechanics and electrodynamics–to do what students call “plug’n’chug” textbook problems or to obtain a conceptual grasp of applied theory–then the most abstract understanding of calculus is all that’s required. If he wants to be able to solve actual, complex problems then a grasp of integral calculus and differential equations is necessary.

But vectors are pretty fundamental to being able to do any kind of problems; and creating and interpreting free-body diagrams seems to be one of those arts that confounds students and graduates of engineering and physics alike. (There’s an anecdote told by Dick Feynman about being an undergrad and questioning what would happen if you sucked water through a bent, axially-mounted tube–whether the tube would rotate in the direction of flow or against it–and even the professor he consulted kept getting confused about the mechanics of the problem.)

I think you just need more practice with free-body diagrams, and physics textbooks often don’t offer enough examples or exercises with that. Try picking up an engineering Statics and Dynamics textbook–Beer and Johnson are the well-worn standards, but you might look for a Schaum’s Outline or REA Problem Solver. And you can check out this site. (I used to work on this project, waywaybackaways, but it looks like eveything I did has been replaced by newer and more shiny code. :frowning: )

Don’t give up.


Once again, thanks for the input and thank you, Stranger for the book suggestions. I’ve decided to put off learning about vectors for two reasons, namely that I don’t really want to learn physics yet, but my algebra book had a section on vectors, which it didn’t explain very well, so that I had to get a physics textbook and also I want to wait until I can take a class so I have someone over my shoulder who will let me know if I’m doing something wrong. Teaching myself carries the risk that I’ll learn something wrong and royally screw myself when I try to learn the rest of physics.

Once I get around to taking the class, I’ll definitely pick up some of the suggested textbooks.

Spectre, my physics level is pretty basic, well before the point I need calculus. One of the books I used even had little jokes in the text that the author thought would inspire us to learn more quickly. As a comedian, he was a good physicist. :rolleyes: