# Homework Help: Bending formula for steel rod

1. Sep 6, 2014

### May

Sorry this isn't really intro to physics, it's high school physics...

I'm investigating the force vs angle of a mouse trap and I need to talk about the uncertainties one thing is that the lever (hammer/arm/part that hits the mouse) bends. So the angle isn't very accurate. What formula do I use to work out how much the steel bends? The rod has a diametre of 1.5mm and is 60mm in length. The material is steel... the steel they use in mouse traps (that's all I know).

Sorry if this is a really simple question, please go easy on me, simple words would be appreciated

Thanks

2. Sep 6, 2014

### Student100

Hmmm, is there enough force applied to to the steel part to bend it? Have you considered uncertainties in how much force is applied by different springs? Did you study hookes law? (They use springs right? I haven't seen that kind of mouse trap in decades.)

You'll need to know how much force there is before you can determine if the metal bends.

3. Sep 6, 2014

### May

Here's a picture of it:

The force sensor reads about 7.5 N but it feels like a lot more than that.

Would this type or spring use Hooke's law? I thought that was only for compression and stretch.

I'm only investigating one mouse trap and one spring. I just need to graph the angle vs force with the degree of uncertainty.

A bonus would be if I actually knew the formula for force of a torsion spring but that's too advanced for me (unless there's a simplified version I don't know about)

Thanks

4. Sep 6, 2014

### Student100

So as far as I know, the trap should work by compressing the spring, which would mean that's hookes law would apply. You need to use the angular version in this case, though.

How are you guys actually calculating uncertainty? Are you using standard deviation/standard error calculations?

5. Sep 6, 2014

### May

Here's what it says on the project sheet:
Sorry I couldn't be bothered typing it up

So uncertainties I have are that:
1. some energy is used to bend the rod (need to work out how much)
2. Angle is slightly off because the rod is bent (I might just give a rough estimate and be done with it)
2. the result on the force sensor fluctuates (+- 0.5 N)

It's very open ended "design your own prac" project so I don't really know what he wants or even what I'm looking for...

Anything I should consider or that I've got wrong please say

Thanks

6. Sep 6, 2014

### Student100

What data are you taking in the lab and what data are you deriving?

Anyway it seems like the setup he has is each measurement has an uncertainty, use these uncertainties to calculate uncertainties in your calculated data. Not the "what physical mechanism is causing the uncertainty in the actual physical item." If that makes sense. So the bending and stuff wouldn't be needed, but you might include it in your discussion in your lab report.

Last edited: Sep 6, 2014
7. Sep 7, 2014

### May

Hmm ok, I agree that it'll be better to talk about it in the discussion but i don't understand what you mean about my uncertainties. I'm a bit confused, what uncertainties are you suggesting I factor in?

Thanks

8. Sep 7, 2014

### Student100

For each measurement, like force. How uncertain, or precise, is the measurement from the gear you're using. Make several measurements and find out how much each measurement varies. This is your uncertainty. Then when you use your measured data to calculate your calculated data you can find those uncertainys as well.

If you want to play with some statistics, you can make several measurements then find the mean. Using the mean you can calculate the standard deviation, sigma. Then sigma over the number of measurements squared will give you the standard error. This is your +- and will tell you how many significant figures your data has.

9. Sep 7, 2014

### May

Ok so the first paragraph was helpful thanks

I don't understand the second paragraph... what does sigma mean? but I'll figure it out.
Thanks so much :)