Science Buddies: "Ask an Expert"

My 8th-grade son and I are working on the "Jack It Up!" project (https://www.sciencebuddies.org/science- ... p048.shtml). We purchased all of the recommended syringes from Amazon as directed and rather than use a cumbersome vice, I build a wooden rack to hold all 6 syringes. Images of the setup can be found here: https://www.dropbox.com/sh/16566m7joivx ... hr6oa?dl=0. My son also meticulously insured that there was as little air in the lines as humanly possible (some bubbles have appeared over time, however).

The experiment as described on the site seems very problematic to me because it really doesn't tell you much about the mechanical advantage afforded by each setup--it seems to be more of a measure of the coefficient of static friction in each arrangement...you keep adding weight to the slave until the master "breaks" (starts to move). This doesn't really illustrate any hydraulic principles. But that aside for the moment, what we have found is that the resistance afforded by the friction of the plungers against their respective syringe walls is amazingly high:

* 12cc - 12cc : ~700g to "break"
* 12cc - 35cc : ~600g
* 12cc - 60cc : ~900g

And these numbers vary wildly depending upon how high the master cylinder's plunger is, and how long the plungers have been static--the longer they sit, the stickier they are (to a point).

What I really wanted to do with this experiment is demonstrate and calculate an actual mechanical advantage by applying a known force on the master cylinder (metric weights) and measuring the resulting force at the slave (hence the digital scale in the photos). However, the frictional losses are making this nearly impossible to show. For example, on the 12cc/12cc setup, I would expect 1000g of force applied to the master to result in 1000g of force measured at the slave...but it only registers about 600-700g. Even when we try to account for the static friction (the ~700g mentioned above), it doesn't work well at all.

So this has been a rather frustrating experience....hypodermic syringes are clearly not well suited for this kind of experiment. Does anyone have any suggestions on how to rescue this experiment?

Hi dkreft7,

Thanks for pointing this out. I just read through the experiment and you're right - as written, it doesn't really make sense. The procedure described in step 5 to "see if the lift supports the weight" depends on static friction and has nothing to do with the force you're applying to the master syringe. In theory you could let go of the master syringe entirely and the slave will still "support" the weight if there's enough friction, and the amount the slave can actually lift is limited only by how hard you can push on the master syringe or when things will start to break (e.g. the tubing pops off). The "Analyzing the Data" section does have you calculate the force you applied to the master piston, but you can't give a sensible answer to "Which piston had the most force applied to it? Which had the least? How does this relate to the principle of force multiplication in hydraulics?" based on the experiment.

All that being said - you've already corrected for that with the experimental setup you've described with the scale. An alternative would be to just have both syringes facing upright with weights stacked on top of them and find a "balance" point. Since you've already invested in all the materials, the best approach I can think of is to try and decrease the magnitude of the frictional forces relative to the forces applied to the pistons, so they have a smaller effect. I can think of two ways to do that:

- Attempt to decrease friction by lubricating the plungers. Maybe pull them out entirely, coat them with oil, then re-insert? Not sure what impact, if any, oil/water mixing in the tube would have on performance, so maybe better to switch over to oil as the hydraulic fluid?
- Significantly increase the mass of the weights you're using. If the strength of the setup becomes an issue, maybe try using tiny ring clamps on the tubing to keep it attached to the syringes.

Of course - if your son is doing this for a science fair, there's nothing wrong with an experiment "not working." If he can analyze WHY it didn't work as planned and explain that to the judges, along with what he would do to fix the problem (given more time and resources, which you might not have right now), that may be impressive enough on its own. Or, you could twist this into another experiment - you already mentioned a few possible variables in your post - "And these numbers vary wildly depending upon how high the master cylinder's plunger is, and how long the plungers have been static--the longer they sit, the stickier they are (to a point)." Can he devise an experiment to systematically measure how those variables affect static friction? This would be a great "real-world" example of a problem, since the physics-textbook style hydraulics diagrams like Figures 1 and 2 in this project usually ignore friction.

Hope that helps, let us know how it goes.

-Ben

*One additional thought - another variable you could measure, that isn't mentioned in the current project, is the distance each piston moves. i.e. how far does the slave piston move if you fully depress the master piston through its full range. You can also use this to analyze mechanical advantage since ideally energy (force x distance) is conserved, although as you've discovered friction may be a big factor.