The tension in the line between A and B

[mariachy]

Homework Statement

THREE IDENTICAL BLOCKS (A,B AND C) ARE BEING PULLED BY AN IDEAL STRING BY A HORIZONTAL FORCE F( PULLES THEM TO THE RIGHT). IT THE TENSION BETWEEN BLOCK B AND C IS T=3N, mass of each block= 0.4 kg ,I DONT KNOW HOW TO UPLOAD A PICTURE BUT THE BLOCKS ARE IN THE ORDER OF
A B C pUllED BY F IN THIS DIRECTION >

Homework Equations

A) FIND THE FORCE F
B) FIND THE TENSION BETWEEN BLOCK A AND B

The Attempt at a Solution

I TRIED F=MASS OF C* acc. + tension but i got a wrong answer
and for part b idont have a clue please guide me!

 

[Astronuc]

Are the blocks moving at constant velocity, i.e. the acceleration is zero, or are they accelerating? What is the coefficient of friction?

The tension in the line between A and B will be one half of that in B and C, the then tension in the line attached to C must be 1.5 times that between B and C.

The friction force on each of A, B and C is $\mu$ mg, where m is the mass of A which is also the masses of B and C, and the friction force must = tension between A and B if the blocks are not accelerating.

 

[mariachy]

thanks for ur responce

there is no friction

the blocks are at rest but when the force pulls them they starts to move
any thing else??

 

[Astronuc]

Ah, OK, then we can assume that the blocks are accelerating.

So the tension between B and C is related strictly to the resistance to motion of blocks A and B. The tension, 3 N, is due to the masses of A and B (0.4 + 0.4 kg) resisting the acceleration a. Now to find the acceleration, take the tension 3 N and divide by the masses of A and B.

The tension between A and B is still half that of the string between B and C, since the tension there is only due to the acceleration of mass A.

Then the tension or force pulling C must be mass (A+B+C)*a or the sum of the tensions between AB and BC.

 

[mariachy]

thnx man