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Teaching Physics Blog

Physics on the Farm

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I've often felt that growing up on a farm gave me an advantage when it came to understanding physics. The next statement is a bit "tongue in check", but I think you could make a case for allowing farm students to be exempted from the AP requirement that AP Physics have a lab component.

Below is a picture of a John Deere tractor, the actual one that I used to work with as a kid growing up in Iowa. I'm not the one on the tractor, I'm the one behind the camera taking the picture. Dad restored this tractor. It didn't look this good when I used it.


The John Deere Tractor Physics Lab.

Each year during cultivating season you had to slide the rear tires out to widen the spacing between the rear tires for cultivating the corn. The tires had to be spaced so that as you drove through the field the tires would fall between the corn rows, and not trample the corn plants. In the picture of the tractor you can see an extended axle shaft that allows for widening the spacing between the rear tires. You could slide the rear tire in and out along this shaft.

The tractor weighed about 4,000 lbs, to adjust the wheel spacing you had to lift that wheel completely up off the ground. Below is a picture of what was affectionately know as "The John Deere Jack". I expect it must have been sold right along with the tractor. It was a very ingenious yet simple device.


Just looking at the background in the picture, you can tell this machine shed is really a "lab".

The lifting handle on this jack was about two feet long, when you pumped it up and down the distance the tip of the handle would travel going from the up position to the down position was also about two feet, and that movement from the up position to the down position is what raised the upright by one notch, I'll call that distance the "throw". The item labeled "lifter" in the figure is a U shaped latch that would catch on one of "notches" on the upright in the jack. You can see this lifter is a pretty heavy piece of metal, it had to stand a force of 4,000 lbs. Well, not exactly 4,000 lbs because you only lifted one wheel off the ground at a time. So if the weight was distributed evenly between the front tires and the two rear tires, it had to lift 1,333 lbs. The spacing on the notches was approximately 1 inch.

The free response question to answer on this lab is, "How can the farm kid lift the rear wheel off the ground using less that 100 lbs of force?"

Answer: The mechanical advantage of this jack is how far the handle moves (the throw) divided by how much the upright in the jack moves. That would give you 24 inches divided by 1 inch for a mechanical advantage of 24. For every 1 lb of pressure you applied to the handle, you could lift 24 lbs of tractor. Ten pounds of pressure on the handle could lift 240 lbs of tractor. Sixty pounds of pressure on the handle could lift 1,440 lbs of tractor, that would do it.

In junior high a farm kid would weigh 60 lbs, and sitting on the handle was allowed. By the time you were in high school, you could even do the algebra, x * 24 = 1,333, so x = 56 lbs.

Perhaps the farm students are the ones that most enjoyed the lab component of the physics course, and they would refuse to be exempted from that component of the class.

Content related to this blog posting can be found at HippoCampus under Work.

How to Make a Playlist on HippoCampus:
A four-minute tutorial video
HippoCampus Physics Study Group on
NIST Reference on Constants, Units, and Uncertainty:
AP Physics B Site from Dolores Gende, AP Central's content advisor for physics since 2004:
College Board's AP Physics C: Electricity and Magnetism Course Home Page:
College Board's AP Physics C: Mechanics Course Home Page:
Table of Information and Equation Tables for the AP Physics Exams:

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