Question

At the end of the lecture 6, I know that if professor pull the string in the relative fast speed, the bottom string will break. But what is the reason for that? Someone help PLZ

Answer 1

Ok, let's look at the case where we pull on the system gradually. We expect the top string to break here before the bottom one will. Why? Simply because the tension in the top string will exceed the tension on the bottom string. The tension of the top string is the sum of the applied force and the weight of the block attached to it:\[T_{1}=mg+F_{applied}\]The tension of the bottom string is just the applied force. Since the top string has an extra component of force making up it's tension it will be greater than the tension in the bottom string. Thus, the top string will break before the bottom string. Why is this not the case when we apply a force very rapidly to the system? When the professor pulls rapidly on the system the bottom string breaks. We need to keep in mind Newton's Second Law F=ma. If the system transmitted the applied force instantaneously then the top string would always break first. In reality the applied force is NOT transmitted instantaneously. The block which sits between the two string greatly slows down the transmission of force to the top string due to it's large inertia (mass). We accelerate the bottom string downward and bang! the block makes it come to a compete stop. This large acceleration implies a large force on the bottom string, so it breaks. The same force is applied to the block of course, but since it is WAY more massive the acceleration involved is much smaller. It transmits this much smaller force to the top string which can easily absorb it. The bottom string snaps.

Answer 2

Very nice answer thank you, but i have another question. Even though the bottom string snaps, there is still a small quantitive force transmitted into top string. THus,is it possible that both strings break simultaneously?

Answer 3

No that is not possible. If that happens, the force has to come from the center and aplied in the opposite firection

Answer 4

Try it out :)

Answer 5

as longa s we have got that block in place the tension of both strings vary and hence when one snaps the other doesnt

Answer 6

hi hsuehy2,the reason is because his FORCE is greater than that of the string