# New to physics have a couple questions

1. Jan 29, 2012

### zellx2004

I'm on here because I need help with my physics homework, but I"m not searching for answers. Currently I"m taking physics 201 at Midlands Technical College in SC, and we're using Physics: Principles with Applications by Douglas Giancoli 6th edition.

My question regarding physics is, the word problems..does anyone have any suggestions as to approach them? I do not find the mathematics portion of this class hard by any means. It's just setting up the problems and going from there. I've got the solutions manual with this book so I have complete access to the solutions, however this does not help me when it comes around to test time. I'm prepared to put in a number of hours to understand the material in detail, however I'm lost as to where to start.

Are there any systematic approaches to physics problems? I tend to convert every unit of measurement to metric as the first step, but try to keep the significant figures as long as possible to avoid rounding error. Second I write out every piece of detail I know in the problem and the formulas for velocity, as well as displacement.

any and all help will be greatly appreciated. Thank you again, and sorry about such a long post.

2. Jan 29, 2012

### cepheid

Staff Emeritus
Welcome to PF!

A good approach is similar to the template for homework help given on this site.

- list given quantities
- list unknowns

- write down any relevant equations
- try to identify the key physical concept that is behind the problem

Why don't you post a word problem that you're working on, including what you have so far, and maybe we can guide you towards a solution?

3. Jan 29, 2012

### zellx2004

A runner aims to finish a 10,000 race in less than 30.0 minutes. At 27.0 minutes he still has 1100 meters to go. He must accelerate 0.20 m/s/s for how long to finish the race in the desired time?

4. Jan 29, 2012

### Harrisonized

Use variables for everything. Try to avoid looking at many different numbers. It's bad for your brain.

As far as units go, you only have to make sure that your equation doesn't result in nonsense. (For instance, having a length equal a time.) Conversion and simplification always comes last. The numerical answer is never as important as finding a general solution in the form of variables.

It doesn't really matter if your units are in SI or not. If your answer for a force is in units of in/s2, then so be it. You're not any more correct converting it to m/s2 unless the problem specifically asks for it, and even then, it's kind of pointless.

Of course, certain constants will disappear or appear depending on what units you use (eg. SI to Gaussian units in E/M), but you don't have to worry about that. You're probably never going to see this if you're not a physics or math major.

5. Jan 29, 2012

### PeterO

One thing many people get lost on is what they are trying to find. It may sound trite but look for sentences that begin:

Calculate .....
Find......
That will tell you what you are after.

When neither of those is used ... find the ? mark, move back to the previous punctuation mark, then read the words in between.

Peter

6. Jan 29, 2012

### zellx2004

I'm actually a Biochem/Biology Major. I'm only taking this physics class as a filler class for my undergrad degree. I'm sure I probably never will use this in medschool (possibly on the MCAT examination), yet I can see the importance of physics in science. It's not the answer I'm interested in, just the solution process and how to think about a problem to set up for the solution.

7. Jan 29, 2012

### Harrisonized

Okay. I'm going to show you how to set up your following problem.

First, rewrite it like this:

Now think it out logically.

In total, he has t time for the d distance race. That means that he has t-t0 time to go a distance of x. Furthermore, he had an average velocity v=(d-x)/t0.

(Note here that velocity has units of distance over time, so the above equation is correct unit-wise.)

If he continues to go at his average velocity of v, then he will cover a distance of v(t-t0) during the time interval remaining. We note that he can only accelerate at an acceleration of a. Acceleration is the second time derivative of distance. Therefore, the distance he covers by accelerating during this interval is ∫∫a ds, where s is the amount of time he is accelerating. Then the distance he covers is (1/2)as2. (I'm assuming here that you know how to take the integrals of polynomials.)

(Note here that (1/2)as2 indeed results in a length, so the above equation is correct unit-wise.)

Therefore, he needs to cover x=v(t-t0)+(1/2)as2. Now you can solve for s.

x=v(t-t0)+(1/2)as2
(1/2)as2=x-v(t-t0)
s2=2(x-v(t-t0))/a
s=√[2(x-v(t-t0))/a]

Note that this is correct unit-wise. After you reach this stage, you can plug and chug whatever numbers you have. Suppose you somehow end up with an imaginary number for s. Then you can go back and check whether the acceleration was necessary (most likely it's not). However, if you end up with a real number, you can be certain that the above equation will always give you the number you want to find, so in essence, you've solved all problems of this type.

This is more revealing than solving and simplifying numbers at each step, and it's the key to physics. Basic arithmetic and geometric skills.

Last edited: Jan 29, 2012
8. Jan 29, 2012

### PeterO

Using my suggestion earlier . here are the words you concentrate on.

A runner aims to finish a 10,000 race in less than 30.0 minutes. At 27.0 minutes he still has 1100 meters to go. He must accelerate 0.20 m/s/s for how long to finish the race in the desired time?

First I would be checking that you need to accelerate at all.

The runner has already covered 8900 m in 27 minutes - you could work out the average speed for that.
[In a 10 000 m race, the initial acceleration time is so short (compared to the time of the whole race), we can approximate the run to one where the runner explodes out of the blocks at a certain speed then maintains that].
Once you have established that an increased speed is needed, I would start by sketching a velocity-time graph to see what is needed. Remember, the area under that graph gives the distance covered - which has to be 1100, while the time has to be 3 minutes at a maximum.

You could always assume the runner takes on that constant acceleration for how ever long it takes to cover 1100m, but I fear that might imply the runner has reached a speed far in excess of the top sprinters - like Usain Bolt - and that is unlikely.

And don't forget you answer is probably a time: He must accelerate 0.20 m/s/s for how long to finish the race in the desired time? But could, at a stretch, be interpreted as over what distance does the acceleration take place, before assuming a new constant speed that will get him there before the 30 minutes is up.