posted by me on .
THree blocks of masses 3m, 2m and m are connected to strings A,B, and C. The blocks are pulled along a rough surface by a force of magnitude F exerted by string C. The coefficient of friction between each block and the surface is the same. Which string must be the strongest in order not to break?
d) they must all be the same strength
e) It is impossible to determine without knowing the coefficient of friction
I think the answer is c) because it's the string that is pulling the largest amount of mass (which is all of the masses attached to this string).
If A pulls only mass 3m and B is between 3m and m, like this:
Then string C has tension force F, which equals or exceeds the friction force acting on A, B, and C together, assuming there is motion.
F - u (3m + 2m + m) g = 6 m a
F = 6m (a + ug)
The tension in A is TA, and
TA - u (3m g) = 3m a
TA = 3m (a + ug)
One could write a similar equation for string B
The tension in any string is proportional to the total mass pulled by all weights behind that string. Thus the answer is C.
So you are right again!