# Wave functions and dynamic question

1. Jun 28, 2011

### yjk91

1. The problem statement, all variables and given/known data

The displacement from equilibrium caused by a wave on a string is given by
y(x, t) = (−0.00217 m)sin[(44.4 m−1)x − (728 s−1)t].

i need to find

(b) number of waves in 1 m
waves

(c) number of complete cycles in 1 s
cycles

(d) wavelength

(e) frequency

3. The attempt at a solution

i first thought the wavelenght was 44.4m because of the formula
Acos2pi(x/λ - t/T)

but then i realized that you had to use the other formula : Acos(kx - wt)

so k = 1/44.4m = 2pi / λ

so λ wave length is 278.97m ?? is this right?

then w = 2pif = 1/728s

so f = 2.186 * 10 ^-4 HZ

and with λ and f you can find the Velocity

not sure if i found λ and f right? can you please tell me if i did it right?

b) number of waves in 1 m
i'm guessing you find this by 1/λ

(c) number of complete cycles in 1 s
and you find this by doing 1/ T

i just want to make sure this is right. if the λ and f are right so i can get the rest of the answer

thank you!

2. Jun 28, 2011

### ehild

The units look weird. Instead of m-1 use m-1 or 1/m. The same for s-1.
So the wave is y(x,t)=(−0.00217 m) sin[(44.4 m−1)x − (728 s−1)t].

No, k=44.4 m-1. Otherwise your way of solving the problem will be correct.

ehild

3. Jun 28, 2011

### yjk91

so k =44.4m^-1

does that equal to k = 1/44.4m what does m^-1 mean?

4. Jun 28, 2011

### ideasrule

No, because k=1/44.4m is the same as 1/44.4 m^-1, which is 0.0225 m^-1. k is proportional to the inverse of the wavelength and wavelength is measured in m, so you'd expect the units to be m^-1.