# Homework Help: Balancing a spring and arm at 45 degrees

1. Jul 13, 2012

### Ziv7

I need to find what spring will balance an arm at 45 degrees
the arm is 5m and can be considered as a UDL of 1.6kg/m
the situation is that the arm starts upwards at 90 degrees and as it goes down it puts tension on the extension spring

5*1.6 = 8kN
acting 2.5m along the beam
which is 1.77m in direction x (horizontal)
8000*1.77 = 14160Nm (the moment acting at 45degrees)

how do I find the details on the spring? (diameter, length etc)
Maybe i'm approaching it wrong, any advice would be appreciated :)

2. Jul 13, 2012

### CWatters

What two points is the spring connected between ? Diagram might help.

3. Jul 16, 2012

### Ziv7

Here is what i mean

#### Attached Files:

• ###### 123z_20120716164542.pdf
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4. Jul 16, 2012

### CWatters

Ok. I reckon the spring needs to produce a torque about the pivot of

= 8 * 9.8 * 2.5 * cos(45)
= 138.6 NM

PS: Normally we like you to have a go at the solution first.

Last edited: Jul 16, 2012