Calculating Ratios

[VansBuilders]

My design for the Class 1 Lever is shown below. However, when I tried testing it, the ratio of weights wasn't the same the distance to the fulcrum ratio. In fact, it was far off. For the fulcrum, I drilled a hole and used a threaded screw to attach a meter stick cut to 79.5 cm length. The weight ratio was 71.3g : 143.9g and the distance ratio was 16 inches : 37.5 inches. Does anybody know why the ratios aren't the same? Any help is appreciated.
https://drive.google.com/file/d/0B1EbHX ... sp=sharing
https://drive.google.com/file/d/0B1EbHX ... sp=sharing

[xiangyu]

Your lever needs to be balanced and level beforehand. It looks like you need some counterweight on the right side of your lever (the short side)

Hope that helps,
Xiangyu

[VansBuilders]

Then, should you use the counterweight while testing, or take it off before putting the masses on? Thank you for taking time to look at my question!

[xiangyu]

Then, should you use the counterweight while testing, or take it off before putting the masses on? Thank you for taking time to look at my question!

The counterweight should be there at all times. Since you have your lever set up so that one side is longer, that longerside is also initially heavier. This higher weight essentially helps whatever load you put on the left side. Thus, to balance things out so that no force is "helping" your left load push down, you need a counterweight on the right side.

To figure out the exact mass of the counterweight needed, take everything off the lever and simply add weight like sticky clay or just regular weights to the shorter side until the lever is level in equilibrium at its starting position.

Hope that wasn't too confusing Let me know if you have further questions.

Xiangyu

[VansBuilders]

I recorded values with a 200-gram counterweight and received the correct ratios with a 0.3 margin of error. Thank you for all your help.
https://drive.google.com/file/d/0B1EbHX ... sp=sharing
https://drive.google.com/file/d/0B1EbHX ... sp=sharing

[xiangyu]

I recorded values with a 200-gram counterweight and received the correct ratios with a 0.3 margin of error. Thank you for all your help.
https://drive.google.com/file/d/0B1EbHX ... sp=sharing
https://drive.google.com/file/d/0B1EbHX ... sp=sharing

No problem, glad I could help.

Also just a word of caution, before you go to a competition, you need to fix that counterweight in place so that it doesn't move as per rule 3e.

Happy Holidays.
Xiangyu

[SOCoach]

I am the coach for our Machine team and I have a quick question.

At first I told my students to take weight A and B . . . . find the heavier of the two and put that on the short end of their lever, the other on the long end and determine the ratio. I know realize that is incorrect. The ratio is A to B so regardless of which is heavier . . . . "A" would go on the long end of their device with "B" going on the short end, thus giving you the A to B ratio. B to C then would mean moving the B weight to the long end of the lever and then placing the C weight onto the short end. Is that right?

Also I have been telling my students to give answer as a decimal, not as a ratio (so divide A by B). Is that everyone's assumption?

[AwesomeSauceis1]

I am the coach for our Machine team and I have a quick question.

At first I told my students to take weight A and B . . . . find the heavier of the two and put that on the short end of their lever, the other on the long end and determine the ratio. I know realize that is incorrect. The ratio is A to B so regardless of which is heavier . . . . "A" would go on the long end of their device with "B" going on the short end, thus giving you the A to B ratio. B to C then would mean moving the B weight to the long end of the lever and then placing the C weight onto the short end. Is that right?

Also I have been telling my students to give answer as a decimal, not as a ratio (so divide A by B). Is that everyone's assumption?

The ratio A:B equals (B distance from fulcrum)/(A distance from fulcrum). It doesn't matter which end the masses are on unless the lever is unbalanced with no masses.
I mean, decimals are cleaner for the ES, but fractions will probably give you more accuracy and speed so I might start doing that.

[SOCoach]

No the lever is balanced . . . . but obviously the ratio will be much different if it is B/A as opposed to A/B. So if they are giving the decimal answer it should be A on the left side of the fulcrum and B on the right (short) side. So the decimal would be A divided by B. Does that seem right?