Angle of Change: Rocket Launch 10 Seconds Later

[nefliege]

Homework Statement

A rocket has launched straight up, and its altitude is h = 10t2
feet after t seconds. You are on the ground 1000 feet from the launch site. The line
of sight from you to the rocket makes an angle θ with the horizontal. By how many
Radians per second is θ changing ten seconds after the launch?

It's a problem form MIT OpenCourseWare, and I know the solution is given, but I did it my way and (suprisingly) it doesn't work. What did I wrong?
The answer is 1/10

It's not a homework question, but I didn't want to spoil your Mathematics section :)

Homework Equations

\frac{dh}{dt}=20t

The Attempt at a Solution

\theta=arctan\frac{h}{1000}
\frac{d}{dt}(\theta=arctan\frac{h}{1000})
\frac{d\theta}{dt}=\frac{1}{1+\frac{h^{2}}{1000^2}}\frac{dh}{dt}
which is 100 ;/

Thanks for any help :)

 

[Mark44]

Homework Statement

A rocket has launched straight up, and its altitude is h = 10t2
feet after t seconds. You are on the ground 1000 feet from the launch site. The line
of sight from you to the rocket makes an angle θ with the horizontal. By how many
Radians per second is θ changing ten seconds after the launch?

It's a problem form MIT OpenCourseWare, and I know the solution is given, but I did it my way and (suprisingly) it doesn't work. What did I wrong?
The answer is 1/10

It's not a homework question, but I didn't want to spoil your Mathematics section :)

Homework Equations

\frac{dh}{dt}=20t

The Attempt at a Solution

\theta=arctan\frac{h}{1000}
\frac{d}{dt}(\theta=arctan\frac{h}{1000})
\frac{d\theta}{dt}=\frac{1}{1+\frac{h^{2}}{1000^2}}\frac{dh}{dt}
which is 100 ;/

Thanks for any help :)

No, d \theta/dt is a variable quantity. At t = 10 sec. dh/dt = 200 ft/sec, h = 1000 ft, and theta = pi/4. By your calculation, d(theta)/dt is 100 when t = 10 sec.

Your mistake is in your derivative of arctan(h/1000). You forgot to use the chain rule, so your value for the derivative is too large by a factor of 1000.

 

[nefliege]

Your mistake is in your derivative of arctan(h/1000). You forgot to use the chain rule, so your value for the derivative is too large by a factor of 1000.

I'm so stupid ! the last part (dh/dt) should be:
\frac{d}{dt}(\frac{h}{1000})=\frac{1}{1000}\frac{dh}{dt}
Right ?
And thank you for help :) I'd been already really upset; I thought the whole solution was wrong and didn't know why.

And of course "100" was only when t=10s. I just hadn't written it.

 

[Mark44]

Yes, that's right.

 

[nefliege]

thanks :)