One Mass And Two Threads

2015-09-06 20.57.25

A big mass is suspended from a thin thread. An identical thread is tied to the bottom of the mass. To break only the lower thread, one should:

a.
push upward on the lower thread.

b.
pull down on the lower thread with a quick, firm jerk.

c.
pull down on the lower thread with a gentle and slowly increasing force.

d.
All of these have the same effect.

e.
do none of these.


 

Solution.

When we pull down on the lower thread with a gentle and slowly increasing force, the inertial mass of the big mass and the thread determines the acceleration. Also, the lower thread is experiencing only a pulling down force. However, the upper thread is experiencing two forces, the pulling down force and the force of gravity of the big mass. Therefore, in this case, the upper thread breaks first.

When we pull down on the lower thread with a quick, firm jerk, the inertial mass of the thread determines the acceleration. If the acceleration is too great the lower thread breaks first.

In conclusion, in order to break only the lower thread, one should pull down on the lower thread with a quick, firm jerk.

Answer: b.

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