# Hamilton's principle with semiholonomic constraints in Goldstein

I am studying from Goldstein's Classical Mechanics, 3rd edition. In section 2.4, he discussed Hamiltion's principle with semiholonomic constraints. The constraints can be written in the form $$f_\alpha(q_1,...,q_n;\dot{q_1},...,\dot{q_n};t)=0$$ where $$\alpha=1,...,m$$. Using variational priciple, we get

$$\delta\int_{t_1}^{t_2}\left(L+\sum_{\alpha=1}^m \mu_\alpha f_\alpha\right)\mathrm dt=0 \tag{2.26}$$

where $$\mu_\alpha=\mu_\alpha(t)$$.

But how can he get the formula

$$\frac{d}{dt}\frac{\partial L}{\partial\dot{q_k}}-\frac{\partial L}{\partial q_k}=-\sum_{\alpha=1}^m \mu_\alpha \frac{\partial f_\alpha}{\partial\dot{q_k}} \tag{2.27}$$

for $$k=1,...,n$$ from the previous formula?

When I go through the steps as in section 2.3, I get $$\frac{dI}{d\beta}=\int_{t_1}^{t_2}\sum_{k=1}^n\left(\frac{\partial L}{\partial q_k}-\frac{d}{dt}\frac{\partial L}{\partial\dot{q_k}}+\sum_{\alpha=1}^m \mu_\alpha\left(\frac{\partial f_\alpha}{\partial q_k}-\frac{d}{dt}\frac{\partial f_\alpha}{\partial\dot{q_k}}\right)\right)\frac{\partial q_k}{\partial\beta}\mathrm dt$$ where $$\beta$$ denotes the parameter of small change of path: \begin{align} q_1(t,\beta)&=q_1(t,0)+\beta\eta_1(t)\\ q_2(t,\beta)&=q_2(t,0)+\beta\eta_2(t)\\ &\ \,\,\vdots \end{align} Using the same argument as in the part of holonomic constraint in section 2.4, I get $$\frac{\partial L}{\partial q_k}-\frac{d}{dt}\frac{\partial L}{\partial\dot{q_k}}+\sum_{\alpha=1}^m \mu_\alpha \left(\frac{\partial f_\alpha}{\partial q_k}-\frac{d}{dt}\frac{\partial f_\alpha}{\partial\dot{q_k}}\right)=0$$ for $$k=1,...,n$$.

What am I missing?

• I am pretty sure that the derivation is not as straightforward as Goldstein might be suggesting. Note that if you follow your approach carefully you get (just one constraint for simplicity) $\frac{\partial L}{\partial q_i}-\frac{d}{dt}\frac{\partial L}{\partial \dot q_i}+\mu\frac{\partial f}{\partial q_i}-\frac{d}{dt}\left(\mu\frac{\partial f}{\partial\dot q_i}\right)=0$, since the multiplier depends on time. This means you should know the time derivative of $\mu$ but this does not make much sense (at least for me). Jul 8, 2017 at 0:35
• As far as I can tell, your phrasing of the constraints is as in the third edition, but the result you quote seems to be as in the second edition (it seems to be equation 2.27 in the second edition rather than the third). The treatments (in particular the kinds of constraints considered) in the two cases seem to me to be actually very different, and that the confusion comes from mixing the two. In particular, in the third edition, where they use the constraints you use, they give the result that @Diracology has suggested. Jul 8, 2017 at 12:30