# Homework Help: Three blocks connected by two strings: tensions?

1. Sep 12, 2011

### ddtozone

1. The problem statement, all variables and given/known data

Three blocks on a frictionless surface are connected by strings like so (from left to right):
Block 1 (m=10kg) to string with Tension 1 to Block 2 (m=20kg) to string with Tension 2 to Block 3 (m=30kg) to string with Tension 3.
The blocks are pulled to the right with force T3=60N.
Find T1 and T2.

2. Relevant equations

I'm not even sure... maybe
Fn=mg

3. The attempt at a solution

I would guess T1 is 20N and T2 is 40N, because it's proportional to T3. But this doesn't seem correct. Could I please get some feedback?

Thank you!

2. Sep 13, 2011

### Staff: Mentor

T3 accelerates 3 blocks, T2 accelerates 2 blocks, etc.

3. Sep 13, 2011

### quietrain

if i am not wrong,

consider the entire system of 3 blocks

force to the right 60N = mass of 3 blocks (60kg) x acceleration

'a' is the same for the entire system. so , 'a' is the same for each block

so your acceleration is = 60/60 = 1

so for tension 1, which pulls only block 1, to have a = 1, it must have a force acting on it by f=ma = (10kg)(1) = 10N, so tension is = 10N

for tension 2, which pulls block 1 and block 2, a = 1, f=ma = (10+20)(1) = 30N

for tension 3, which pulls block 1,2,3, a = 1, f= ma = (10+20+30)(1) = 60N

4. Sep 13, 2011

### ddtozone

Thanks a ton! That makes much more sense.