Pendulum ODE

[ValeryOchkov]

Mathcad 15 - an error.

Prime 5 - a constant not function as an answer!

See the attach and help me please!

[Werner_E]

Two problems in your MC15 sheet:

1) wrong syntax of Odesolve (you don't have to provide the function name theta in MC15)

2) the expression on the right hand side of your ODE takes non-real values if theta goes above 45 degree
I coped boldly with the last situation by applying the Re() function 😉

Now you get a solution, but its constant 45°

[ValeryOchkov]

Thanks!

But why I get a constants, not a function - angle of the pendulum as function of time?

[AlanStevens]

Total energy is constant throughout, so the value under the square root sign of the numerator of the RHS of your ODE is always zero; hence theta goes nowhere (as you can see from Werner's graph)!!

Alan 

[AlanStevens]

If you wish to get the equations of motion from conservation of energy Val, you need to do the following:

Alan

[ValeryOchkov]

Thanks Alan!

I see my bug!

See please Problem 3 in the Manuscript!