Physics issue (levers)

[treefrog]

Hello,

I am an 8th grade physical science teacher and we are in the middle of simple machines. I am doing a lever llab and I keep getting data that results in the output work done by the lever (at certain fulcrum points) being greater than the input work. This is theoretically impossible since at least some work done is lost to friction....you always need to use a bit more energy on the machine (input) than the resulting work (output) done by the machine.



It's killing me...I have the students measure the input distance by the tip of the fulcrum to the end of the effort arm (strip of plywood). The measure the output distance by measure the other part of the board (from the fulcrum tip to the end of the Load part of the lever arm). They are lifting metal weights (500g and 200g). To determine the output force, the students measure the weight each weight in newtons (by using a spring scale). They measure the input force by pulling down on the Effort end (an eye hook inserted in the plywood) with a spring scale. The take these measurements at different fulcrum points to determine which fulcrum point yields the greatest mechanical advantage (output force/input force). They will also calculate mechanical efficiency (output work/input work).

Having done the lab before the students, at the two points farthest from the load, I am getting data that results in more output work done than input work (leading to a mechanical efficiency of greater than 100%). This defies the law of conservation of energy.

Help!! :mad:

[K^2]

Do you make sure that the lever is balanced on the fulcrum before starting measurements?

Because if not, the answer is quite trivial. The longer arm is heavier, and therefore, provides extra torque. You can view it as an extra force pulling on the longer arm that you are not accounting for. That will certainly yield an observed efficiency > 100%.

To get rid of this error, simply balance the strip of plywood on the fulcrum by adding extra weights to the shorter arm. Then add the 200g or 500g weight to the shorter arm, and measure the force needed to balance it at the longer arm.

Since you really have no work lost to friction in this setup, after balancing the arms, you should get the efficiency to be 100% ± Error. Error should be normally distributed and quite small.