# Physics final tomorrow (HELP)

1. Jun 4, 2012

### Steezy

What's going on, guys. I have a Physics final tomorrow, and I don't understand these!

"What is the frequency of a wave that has a speed of 3 m/s and a wavelength of 8 m?''

"What is the period of a wave that has a frequency of 5 Hz?"

I'm assuming they're all simple, so if someone can please help me out, that'd be greatly appreciated.

Thanks

2. Jun 4, 2012

### cepheid

Staff Emeritus
Welcome to PF,

Take a look at the PF rules when you get a chance. We won't just do your homework for you, but we're more than willing to help you out if you show us what you've attempted so far (basically fill out the template that you deleted when you submitted your post). We don't make you do this just to be pedantic. This template is actually a really useful and systematic way to approach any problem, that is why it is there. In the mean time, I don't mind giving you a few hints.

Yeah, these are *really* basic problems. For the first one, you just need to know what the relationship between speed, frequency, and wavelength is. There is an equation that gives this, and it will be in your book or notes. Or, you can derive it by noting that a wave, by definition, travels a distance of one wavelength in a time interval equal to one period. Combine that with the fact that speed = distance/time, and you'll have the equation that I mentioned above.

For the second question it's the same approach: what is the relationship between period and frequency? You can easily find this in your book.

These are seriously just "plug and chug" problems. I.e. you can just find the right equation and plug numbers into it. You can get away with not even understanding any of the actual physics concepts (which you clearly don't). For future reference, it is a good idea to pay attention in class, and do the homeworks and readings. EDIT: and I'm not saying this to be a dick. I'm just advising you not to shoot yourself in the foot by putting yourself in a position where you don't know the most basic things the day before the final exam.

Last edited: Jun 4, 2012
3. Jun 4, 2012

### Steezy

I appreciate the help. Yes, this was at the beginning of the year, and I did understand it at the time, just now I'm having trouble understanding it.

4. Jun 4, 2012

### cepheid

Staff Emeritus
Well, can you find the equations that I mentioned above that will allow you to solve these problems? You need:

1. An equation that relates wavelength (λ), speed (v), and frequency (f, or sometimes the Greek letter nu: $\nu$ is used).

2. An equation that relates frequency, and period (period is usually symbolized using T, or P).

A wave is periodic: in other words, it consist of cycles that repeat over and over again. The period of a wave is just the time interval required for one full cycle. Hence, the unit for period is just the second (s).

The word frequency is just what it sounds like. It's the "oftenness" of the cycle. The frequency tells you the rate at which cycles occur (i.e. how many cycles occur in a unit of time). Hence the unit for frequency is cycles "per second": 1/s, which we call a hertz (Hz).

Based on these descriptions of period, and frequency, can you see how they are related to each other? If I had a wave with a period of a tenth of a second (0.1 s), then how many times would it repeat in one second? This is the frequency.

Last edited: Jun 4, 2012