# Homework Help: Calculating error

1. Feb 2, 2010

### temaire

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

The expression for the slope of the graph of $$(m_{1} - m_{2})$$ versus a is given by $$m = (m_{1} + m_{2} + m_{p})/g$$. What is the expression for the error in the mass of the pulley, $$m_{P}$$?

3. The attempt at a solution

My answer is $$\delta_{p} = g\delta_{m} + \delta_{m1} + \delta_{m2}$$

I don't think this is right. Can someone show me where I went wrong?

Last edited: Feb 2, 2010
2. Feb 2, 2010

### Staff: Mentor

None of this makes any sense, as far as I can tell. In your first sentence you say
If you are graphing m1 - m2 vs. a, why doesn't a appear in the equation?

3. Feb 2, 2010

### temaire

I'm not sure if this would help, but $$(m_{1}-m_{2})g = (m_{1}+m_{2})a$$

I think g might have been substituted for a.

4. Feb 2, 2010

### Staff: Mentor

Here's where I think this is going. You have m = (1/g)(m1 + m2 + mp), so
$$dm = \frac{\partial d m}{\partial m_1}\Delta m_1 + \frac{\partial d m}{\partial m_1}\Delta m_1 +\frac{\partial d m}{\partial m_1}\Delta m_1$$
$$= (1/g)[1 \Delta m_1 + 1 \Delta m_2 + \Delta m_p]$$

Now solve for $\Delta m_p$ in terms of the other quantities.