Anyone know of good sites that teach you tricks to solving physics problems?

1. May 31, 2005

michaelw

With equations or facts that are not given in textbooks?

Just things that can help you solve problems faster with general basic physics

for example, the acceleration of a block on a frictionless incline is gsin(theta)
the volume displaced by a block in a water is (volume of block) * (specific gravity of block)
or, in hydraulic lifts, F1/A1 = F2/A2, F1d1 = F2d2
or, fraction submerged of something (x) in a water is = the specific gravity of x
or, the max height of a projectile can be solved by vsin(theta) = sqrt(2gh)

things like that :)

2. May 31, 2005

whozum

You can get those yourself by just deriving them from the concepts taught in the book... :uhh:

3. May 31, 2005

abercrombiems02

some hints

I think alot of your equations are too specific. Use more general principals and cancel out terms.

For example, with your equation to predict the height of a projectile you simply state an equation. I don't like this approach because it requires memorization of a formula and not a process.

Start problems with basic equations and assumptions. It seems for the physics you are taking that constant acceleration and neglegible air resistance are all good assumptions. With these assumptions we can write the displacement of a particle as X = X0 + Vot + 1/2at^2

Take this equation and lets apply it to the y direction
Y = Yo + Voyt + 1/2at^2

If we start at ground level, our initial displacement in the y direction is 0
Also if we take up to be positive then the acceleration term must be negative because we have downwards acceleration. So the taylored for of this equation becomes
Y = Voyt - 1/2at^2

Now you have an equation to decribe the y position of a particle for projectile motion starting at ground level with constant accleration and neglegible air resistance and non zero initial velocity pointing upwards.
Quite specific huh?

Typically you wont know everything though in this equation. Well whats true about the velocity in the y direction of a particle at the top of its motion? It has to be zero right? It cant go up any higher than its max height so the velocity in the y must be zero at this point. This gives us
0 = Voy - at

We can solve this for t and get t = Voy/a
Plug it into our very specific equation about 12 lines up and you'll get the same formula for max height. Using this principle you just need to think about what you have.

For force problems ALWAYS ALWAYS ALWAYS draw and FBD. If you draw your forces correctly all you need to do is model them and set them equal to m*a.

In momentum problem.....when is momentum a constant? Well momentum is the integral of external forces on a system with respect to time. So the integral of what gives us a constant? How about zero.... Integral of 0 is just equal to zero + C. So linear momentum is conserved when the sum of external forces on a system is zero.

Basically what im doing here in all these problems is just taking very general concepts and doing some basic math.

What about energy how do you do that. Well in most mechanics problems i've seen at this level the only energies you need to model are potential, spring, kinetic, and frictional. Remember Energy Initial = Energy Final + Energy Lost
I like to use this definition because then I use positive terms only. There are no negatives. So with energy it will always be true that....
KEo + PEo + Uo = KEf + PEf + Uf + |Wf|
KE = Kinetic energy
PE = Potential energy
U = Elastic Energy
|Wf| = Magnitude Of Work Done By Friction

4. May 31, 2005

pete worthington

michaelw, what text are you using - what is your major?

"no pain no gain"

5. Jun 1, 2005

James R

The trouble with remembering "tricks" to help you solve certain types of problems is that they won't help you at all when you're confronted with a problem you haven't seen before. It is FAR better to understand the relevant physical principles than to rely on "tricks" to solve problems.

Of course, that depends on your end goal. If you just want to pass exams, then tricks may be enough. If you really want to learn physics, you need to learn the concepts.

6. Jun 1, 2005

Tom Mattson

Staff Emeritus
I totally agree with the overall sentiments of the respondents when they say that you should go for understanding on a more general level. Cases in point...

Unless "theta" happens to be the angle that the incline makes with the vertical, rather than the horizontal.

Unless the particle is launched from an elevated platform, and the height h is measured from the ground.

Physics is not meant to be learned formulaically. It is meant to be learned from first principles. That's the difference between a scientist/engineer and a technician.

7. Jun 1, 2005

michaelw

I agree

For example, if there was a charged plate infinitely high and wide, then no matter how far you move a charged particle from the plate, it will always have the same electric field affecting it, and thus the same force (it becomes independant of radius, since the field lines coming out from the plate have nowhere to go, thus there is a constant electric field present)

Things like that :)
Anyone have any tips where to find such things?

8. Jun 2, 2005

Dr.Brain

I think , the tricks you are talking about should be learnt from the books itself.Some of the books on competitive exams provide a good insight on short tips . Moreover, you can develop a habit of first understanding the concept and then derive the formula and know when and where the forumula is applicable.

9. Jun 2, 2005

Tom Mattson

Staff Emeritus
I think that the "tips" are the laws of physics themselves, combined with the relevant principles of mathematics. There are no short-cuts to replace thinking and exprerience. That's how you acquire the knowledge of a large number of special cases.

Physics is best learned by going from the general to the specific.

10. Jun 2, 2005