Calculating Displacement with Sine Waves

[ScienceGeek24]

Homework Statement

y=0.2msin[(0.18rad/m)x+(90 rad/s)t - pi/4]

What is the displacement at position x=0 and t=0?

Homework Equations

y(x,t)=Asin(kx-wt+phi)

The Attempt at a Solution

y=0.2m[0.18-90]=-0.199m

the answer is -.141 m help?

 

[Doc Al]

y=0.2m[0.18-90]=-0.199m

Where did the 0.18 come from? x = 0.

And don't mix radians and degrees. And don't forget the sine.

 

[hypnosam]

As the above post said, the displacement is 0. This is because at x=0, t= 0, so everything in the sin function becomes 0. Sin of 0 is 0. Hence the displacement =0

 

[ScienceGeek24]

the displacement can't be 0 because the answer sheet says is -.141m and when i put x=0 i still get the same asnwer -0.199m don't get it.

 

[Doc Al]

The value of x is 0, not the displacement (which is the y value).

and when i put x=0 i still get the same asnwer -0.199m don't get it.

Show exactly what you did.

 

[hypnosam]

Silly me, forgot the phase of the thing. Since everything else in the sin =0, then sin(-pi/4) =-1/(sqrt(2) multiply this by 0.2 and you get your answer of -0.141m.

 

[ScienceGeek24]

Never mind what i was doing was wrong i was subtracting .18-90 and the sine of that time 0.2m is .199m

but what hypnosam is doing seems right

I did y= 0.2sin[0.18(0)+90(0)-pi/4] but i just don't know how he got 0.707 when inmy calculator i get 0.013 for sin(-pi/4) wth.

 

[Doc Al]

I did y= 0.2sin[0.18(0)+90(0)-pi/4] but i just don't know how he got 0.707 when inmy calculator i get 0.013 for sin(-pi/4) wth.

Make sure you're using radian mode.

 

[ScienceGeek24]

got it thanks!