# SHM of Pendulum

1. Homework Statement
A pendulum's angle is given by (.10 rad)cos(5t + pi)
where t is in sec. What is the initial angle?

2. Homework Equations

3. The Attempt at a Solution
Do I plug in t= 0 to get this angle, or is the phase constant this angle? I read that the phase constant specifies initial conditions, but I'm not entirely clear on what it means.

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Hootenanny
Staff Emeritus
Gold Member
Do I plug in t= 0 to get this angle, or is the phase constant this angle? I read that the phase constant specifies initial conditions, but I'm not entirely clear on what it means.
Sounds good to me . The initial conditions is simply a set of conditions which fix the values of a function at time t=0.

According to the equation above pi is my phase constant, but if I plug in t=0, I don't come up with pi as initial angle, so I'm not sure if phase constant and initial angle are the same thing

Hootenanny
Staff Emeritus
Gold Member
so I'm not sure if phase constant and initial angle are the same thing
They're not, the phase angle is chosen such that the initial conditions, i.e. the angle at t=0, is satisfied. Does that make sense?

I still don't understand. Could you describe it to me using the equation Theta(t) = theta max * cos (omega*t+ phase constant) with the values in the equation in the problem up above?

tiny-tim
Homework Helper
Hi bcjochim07!

You're confused because, in this case, the amplitude is an angle, just like the phase constant.

Usually, the amplitude is a number or a length or a speed, so there's no confusion … the question would ask for the initial value, or the initial distance, or the initial speed … and you couldn't get that confused with the phase constant!

The initial angle is θ(0), which is θmax * cos (phase constant).