# What is the tesion between the two masses?

1. May 8, 2005

### UrbanXrisis

there are two masses connected by a string:

1kg-----2kg---->F

the 2 masses are being pulled by a force of F

what is the tesion between the two masses?

the total acceleration is a=F/3

this means that the tesion is either 2kg*F/3 or 1kg*F/3

not sure which one?

2. May 8, 2005

### Staff: Mentor

Correct.

Figure it out! Pick one of the masses and apply Newton's 2nd law. (You already know the acceleration.)

3. May 8, 2005

### UrbanXrisis

right, that's what I'm asking, do I apply it to the 1kg mass or the 2 kg mass?

4. May 8, 2005

### Staff: Mentor

You think Newton's 2nd law only works for one of them?

It better work for either mass. Now, of course, if you pick the correct mass to analyze the problem is easier.

Start by identifying the forces that act on each mass.

5. May 8, 2005

### UrbanXrisis

if I apply it to the 1 kg mass... it will be T=F/3

if I apply it to the 2 kg mass... it will be T=2F/3

6. May 8, 2005

### Staff: Mentor

Obviously that's not right since there is only one tension in the string.

How many forces act on the 1kg mass? How many forces act on the 2kg mass?

7. May 8, 2005

### UrbanXrisis

one force acts on the 1kg mass, 2 forces act on the 2kg mass

does that mean the tension force is only exerted on the 1kg mass?

8. May 8, 2005

### Staff: Mentor

You tell me. The string exerts its tension force on anything it's attached to.

9. May 8, 2005

### UrbanXrisis

oh, so only the 1 kg mass is effected, so that means the T=F/3

10. May 8, 2005

### whozum

Both masses are affected, but hte tension in the string generally results from a force applied on BOTH ends. In this problem, both ends have atleast a force of 1kg*F/3 on them, therefore that is the tension.

11. May 8, 2005

### Staff: Mentor

Analyzing the 1 kg mass:
$F_{net} = ma$
$T = (1)(F/3) = F/3$

Analyzing the 2 kg mass:
$F_{net} = ma$
$F - T = (2)(F/3) = (2/3)F$ ==> $T = F/3$