# Derivatives in an Atwood Machine

1. Nov 11, 2012

### henryd

1. The problem statement, all variables and given/known data
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:

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

2. Relevant equations

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

3. 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!

2. Nov 11, 2012

### Staff: Mentor

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

$\dot{x}$ is a function of t.

3. Nov 11, 2012

### henryd

So then it's just

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

4. Nov 11, 2012

### Staff: Mentor

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

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