Tension on the rope (classical mechanics problem)

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SUMMARY

The discussion focuses on calculating the tension in a system of two mass points connected by a string, as outlined in Goldstein's Classical Mechanics problem 1.21. The Euler-Lagrange equations are utilized to derive the motion equations, specifically $$\frac{d}{dt}(m_1 d^2 \dot{\theta})=0$$ and $$(m_1+m_2)\ddot{\theta}=-m_2 g+m_1 d \dot{\theta}^2$$. The tension is defined in relation to the forces acting on mass m1, leading to the expression $$T=m_2 \ddot{r_1}+m_2 g$$. The discussion emphasizes the importance of free body diagrams in visualizing the forces at play.

PREREQUISITES
  • Understanding of Euler-Lagrange equations in classical mechanics
  • Familiarity with free body diagrams and force analysis
  • Knowledge of constraint forces in mechanical systems
  • Basic principles of dynamics, including tension and gravitational forces
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  • Study the derivation and application of Euler-Lagrange equations in classical mechanics
  • Learn how to draw and analyze free body diagrams for complex systems
  • Research methods for calculating constraint forces in mechanical systems
  • Explore textbooks on classical mechanics, such as "Classical Mechanics" by Herbert Goldstein
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allshaks
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Homework Statement


The situation is that of Goldstein's problem 1.21 (or 1.19 in some editions):
"Two mass points of mass m1 and m2 are connected by a string passing through
a hole in a smooth table so that m1 rests on the table and m2 hangs suspended.
Assume m2 moves only in a vertical line."
But the question is the following: what is the tension?

Homework Equations


The Euler-Lagrange equations of the system are:
$$ \frac{d}{dt}(m_1 d^2 \dot{\theta})=0$$
$$ (m_1+m_2)\ddot{\theta}=-m_2 g+m_1 d \dot{\theta}^2$$
The constraint equation is:
$$r_1-y_2=l$$
Where ##l## is the length of the rope.

The Attempt at a Solution


I started by using the fact that the only force applied on ##m_1## is the tension, and that this tension must be in the direction of the string; that is, always radial (using the hole as the frame of reference). This means:
$$F_{r_1}=\frac{a_{r_1}}{m_1}= \frac{\ddot{r_1}-r_1 \dot{\theta}^2}{m_1}=-T$$
Where ##T## is negative because I define it as being positive when it is pulling up mass 2 (the other mass) such that:
$$T=m_2 \ddot{r_1}+m_2 g$$
Because ##\ddot{y_2}=\ddot{y_1}##
I've been playing with all of these equations for a while, but I couldn't find the solution. I'm not sure if there is a way of obtaining an expresion ##T## that involves Lagrangian mechanics, or if there are any other techniques to do it.
I'm following a problem set from a Classical Mechanics course I'm taking at college, and there is another question in the problem set: "In order to calculate the constraint forces on a system, what are the methods that could be employed?". I'm not sure how to answer that question, and I think that maybe some info on that (maybe some textbook that covers the subject?) may help me solve the problem. Thank you very much.
 
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There is very little maths needed to solve this problem .

Draw free body diagrams for the two masses and just look at them .
 

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