Derivatives in an Atwood Machine

[henryd]

Homework Statement

I have the professor's solutions for a homework we handed in. There is a part that is confusing me. We have the following equation:

$$E = \frac{1}{2}(m_1 + m_2)\dot{x}^2-(m_1-m_2)gx$$

Homework Equations

We want to find: $$dE/dt = 0$$

The Attempt at a Solution

The solution says the correct answer is:

$$dE/dt = 0 = (m_1 + m_2)\dot{x}_1\ddot{x}_1 - g(m_1-m_2)\dot{x}_1$$

Why does it contain \dot{x}\ddot{x} instead of just \ddot{x}?

Is it because of the chain rule?

Thanks!

 

[Nugatory]

For any function U(t), what is \frac{dU^2}{dt}?

\dot{x} is a function of t.

 

[henryd]

So then it's just

$$ \frac{dU^2}{dU}\frac{dU}{dt} = 2U\dot{U}$$ ?

 

[Nugatory]

So then it's just

$$ \frac{dU^2}{dU}\frac{dU}{dt} = 2U\dot{U}$$ ?

That doesn't quite work, because \dot{U} \equiv \frac{dU}{dt}

Try \frac{dU^n}{dt} = nU^{n-1}\frac{dU}{dt}