- #1

Rikudo

- 120

- 26

- Homework Statement
- A balance consists of a straight light rod A, a right-angled light rod B rigidly welded with rod A and a fixed fulcrum F. Four loads are suspended from the balance with the help of light threads. The rods have equidistant marks on them. Find the mass of the load C.

- Relevant Equations
- Torque and Newton's 2nd law

Ok. So, I already worked on this problem, and get ##m_c## = 2m/3, which is correct according to the book.

However, I also want to know the value of the tension (T) between rod A and B.

Note: Before we start working on my modified question, I want to point out that the force exerted by the fulcrum is F = 5m + 2m/3 (I get this by using Newton law)

If we only look at the forces which is working on rod A, then with using the Newton 2nd law, we will get:

$$F = mg + mg + T$$

$$T = 3m + \frac{2m} {3}$$

Strangely, I gained a different result when I tried using torque equation for rod A, with point F as the origin. (Here, L is the length between two marks)

$$0 = -3mgL + 4mgL-2TL$$

$$T = mg/2$$

Not only that, changing the location of the origin also changed the value of the tension. What is happening here?