- #1
Hamal_Arietis
- 156
- 15
Homework Statement
The uniform rod mass m, length L falling into mass M. M is attacked with string k. Find the velocity u of system when the top of rod falling into M.
Homework Equations
I find this equation, but it seems wrong:
Chose origin at equilibrium position.
The rod has been divided into small segments, each of length has ##dm=\frac{m}{L}dy##
At the time t, the mass M is effected of force F, with F is:
+ The total force of gravitation and the elastic force ##F_1=ky##(magnitude)
+ The change of momentum of dm. It equals ##F_2=\dfrac{dp'}{dt}=v\dfrac{dm}{dt}=v\dfrac{m}{L}\dfrac{dy}{dt}=v^2\dfrac{m}{L}## with v is the velocity when dm touches M
So ##F=-F_1+F_2=-ky+v^2\dfrac{m}{L}##
That force equals momentum changes with respect to time of system (M+dm)
$$ F=\frac{dp}{dt}$$
$$\Leftrightarrow -ky+v^2\frac{m}{L}=u\frac{dm}{dt}+(M+dm)\frac{du}{dt}=u\frac{dm}{dt}+M\frac{du}{dt}$$
The Attempt at a Solution
Where are wrong ? I don't have solution.
v is the velocity when dm touches M. Each of collision, String changes so I can't find v.
This way seems difficult