# Courses How do I solve physics problems?

1. Sep 7, 2016

### xforeverlove21

I try doing the assignments and I just can't do it, like my approach is completely wrong. What do you think went wrong? What is the best way to approach this?

Thanks

2. Sep 7, 2016

### BvU

I notice you focus on the general 'how to' and 'what to do'.
Why don't you just post one or two exercises that you struggle with on a homework forum to see how your problem solving approach can benefit from helper guidance ? Read the homework guidelines -- that in itself is a start already.

3. Sep 7, 2016

### Dishsoap

That's how I feel whenever I do homework as well. I try something, realize it doesn't get me to an answer (or that I made some false assumption along the way), and then try again using a different approach. This was very discouraging when starting out in undergrad, but eventually you'll find that it really helps you learn! I do a lot of problems on whiteboards so that redoing is easy before transferring homework to paper.

4. Sep 8, 2016

### Student100

Concrete example?

There are two areas to a typical physics problem, understanding the physics and applying mathematics. Most students struggle with the latter, simply because they don't know, or remember, all the tools they've learned in their mathematics coures.

5. Sep 8, 2016

### xforeverlove21

Do you just have to keep going at it and re-trying until you finally get it?

6. Sep 8, 2016

### Staff: Mentor

No. Did you understand the reply by @Student100 ?

7. Sep 8, 2016

### xforeverlove21

The thing is this is an introductory physics course (Physics 1) since I am taking it in 2nd year I already did calc I and calc II, I even aced calc II so I'm assuming my math background is pretty strong. Maybe it's the understanding part? Am I supposed to go over several textbooks, lecture notes... before I even attempt questions? Usually I just write the main points and formulas down.

8. Sep 8, 2016

### Staff: Mentor

When you look at a problem, you first figure out what type of problem it is. Accelerating masses? Uniform circular motion? SHM? Projectile motion? etc.

Then you think about the "Relevant Equations" you have learned for the type of problem it is. What equations would you be thinking about for each of those types of problems?

Then you think about what you have been given, and what you are trying to find. I also like to picture a sort of simulation in my head for what is going on physically. Like, what if that object were a bit heavier? What if that angle were less, or more? What happens as I take the angle to 0 degrees? What happens when I take it to 90 degrees? What happens in the limit of the one mass getting very heavy, and the other mass getting very light?

And by this time I've written down the Relevant Equations and picked which ones I think I should use. I'll do a quick check of the problem to see if I have enough unique equations written to solve for the number of unknowns, and then solve the equation(s). I also carry units along in my equations to help me check my work and watch for calculation errors.

9. Sep 8, 2016

### Dishsoap

Not re-trying the same method. If I got the wrong answer, then clearly I didn't understand something. @berkeman gave an answer that I can't put in better words.
Think about what you know, and what you know may not be explicitly given in the problem. You're given an initial velocity of a ball thrown straight upwards and have to find the time it takes for it to come back down? Think about what else you know about the system. You know that the ball "turns around" at the top of the arc, right? So the velocity at that point is zero? We also assume it's on Earth, so we know g. And we also know that the time it takes for it to go up is the same as the time it takes to go down. If we take all of the things we know and seek out some relevant equations, and don't make any calculation errors, we're golden!

10. Sep 8, 2016

### Student100

Here, let me give you a few recent examples from my brother who's taking intro E&M:

This first example illustrates not completely understanding the physics (or at least not applying concepts correctly), say you have a charge at the origin of a coordinate system in $\mathbb{R^2}$ of strength Q, and another particle with mass m, charge q, a distance x from Q on the x axis and an initial velocity of v in the y direction. The problem wants you to find the charge Q so that the other particle q experiences uniform circular motion around Q. So, applying the math,
$$m\frac{v^2}{x}=\frac{1}{4\pi\epsilon_0}\frac{Qq}{x^2}$$
or,
$$\frac{4\pi\epsilon_0 mv^2x}{q}=Q$$

He plugged in his numbers and solved for Q, but forgot to apply the physics correctly. For this to happen, Q needs to be of an opposite charge with respect to q. In this case he should have used the original numbers given to determine the sign of Q. He goofed on the physics.

An example where he failed to apply math correctly:

Three charges, $q_1, q_2, q_3$ are held at a fixed distance d, with all three charges equidistant from one another. Find the strength of the electric force that acts on $q_1.$ This problem just requires you to use Coulomb's law to find the $\vec{F}$ on each, which he did, plugged in his numbers and got two Forces. The problem came when he was trying to break the force (one or two forces, depending on how you define things) into it's individual components. He had drew things so that one force was only in one direction to simplify things, but the other force needed to be reduced to components to find the total force. He didn't think to apply to apply the law or cosines, or split things up, instead he was trying to force $F_x=F\cos(60)$ and $F_y=F\sin(60)$. Which is wrong. He had failed to apply the math he already knew.

This is a kid that also has gotten A's in calculus 1,2,3, a B in LA, and is taking a course in ODE's. The majority of the problems I help him with though are of the latter type, he just fails to apply the mathematics correctly. Math he should already know. If this is your kind of problem with physics, rereading the book or lectures or whatever isn't going to help at all, because it's assumed you're bringing this math into physics with you.

If you have a concrete example of what you're struggling with, you should post it in the HW section, so others can help guide you to answer, and maybe point out where your weaknesses are. If you don't even get to either point above, you just need more practice working problems, it's a skill.

Generally, you should always:
Draw a picture.
Write down what concepts of physics apply.
Write down what you want to know.
Write down what you know from the problem.
Apply the correct mathematical model, leaving everything as variables.
When you're able to solve for what you want to know, in terms of what you know, plug in your numbers.
Work the calculation, give answer to correct significance.
Check that it makes sense, analyze the math model you used. What's it telling you, what does it say about the physics?
See if there's another way to work the problem.
Compare it to other problems you've done, was there any uncommonly used tools that helped you with this one compared to others?