A problem of blocks and pulleys

Classical Mechanics Level 3

Two blocks A A and B B are connected by a massless, inextensible string, as shown in the figure, with block A A having a larger mass, i.e. m A > m B . m_A > m_B. At t = 0 , t=0, block A A is released from rest and starts moving down. When the two blocks cross each other, what is the relationship between their speeds v A v_A and v B ? v_B?


Details and Assumptions

  • The radii of the disks are very small compared to the length of the string.
v A = v B v_A = v_B m A m B = v B v A \frac{m_A}{m_B} = \frac{v_B}{v_A} m A m B = v A v B \frac{m_A}{m_B} = \frac{v_A}{v_B} Cannot be determined from the given data

Aarabdh Tiwari by Aarabdh Tiwari , 20, India

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1 solution

Rohit Gupta
Jul 15, 2017

The motion of the blocks is constraint by the length of the thread. The length of the thread is constant. If one block moves down, the other one is bound to move up. At the instant of crossing both the blocks are identically hung by the threads which make equal angles with the horizontal, therefore, both must have equal speeds irrespective of their masses.

2 Helpful 0 Interesting 0 Brilliant 0 Confused

Why can’t we use conservation of momentum? Initial momentum = final momentum = 0?

Rohan Joshi - 5 months ago

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