- #1

middleearthss

- 13

- 1

## Homework Statement

A string is connected to a box A mass M at its left end and a box B mass 2M at its right end. Boxes A and B start pulling in opposite directions at the same time with forces which are time dependent and described by functions F(t)=Ct for box A and F(t)=Dt for box B. Values of A and B 1 N/s and 3N/s respectably. Given the maximum tension of the string which is 25 N find the value of t at which the string breaks.

## Homework Equations

## The Attempt at a Solution

Here the boxes are pulling in opposite directions so tension equals sum of forces emitted by two boxes and g in formula can refer to acceleration of chosen box rather than acceleration of gravity. Here gravity is not in use since this is set up on the ground and friction as well as air drag is negligible.

T=∑(i)Fi(t)

T=F1(t) + F2(t)

T=t(C+D)

t=T/(C+D)

t=6.25 seconds

This seems coherent when plugged into the equation and results in 25 N of force after 6.25 seconds and yet masses than make no use of themselves and my answer sure

doesn't equal the one in the book. Are my equations wrong or is my calculation wrong. This seems perfectly reasonable from my point of view, but yet it make be wrong. The problem seems easy so what's wrong? Can you share your thoughts on this?[/B]