Winches & Ramps

In MOVIE 1, Earl anchors his lesson on winches with references to an easier-to-grasp simple machine (levers). Notice how he asks the same question regarding levers and winches to check for student understanding: If I change the lever or winch this way, will more or less Applied Force be required to lift the same Load? Also listen for which of Earl's students don't yet understand the challenge, and which do. What brings them to a better understanding?

In MOVIE 2, science teacher Mike Ryan (now of Georgia Tech) introduces the notion of Trade-Offs and Mechanical Advantage in introducing his students from Alpharetta, GA, to ramps. Students do a short lab where they test the forces required to move the same object up the same height using three ramps of different lengths. NOTE: The key trade-off with MA involves "less force-more distance", or vice versa. Mike's note at the end of MOVIE 2 says that there are greater frictional losses with long ramps and so more work must be done to raise a Load a given height.

The Mechanical Advantage (MA) of a simple machine can be calculated based on the measure of distance traveled by the Load, divided by the distance traveled by the Applied Force. With the winch, you measure the circumference of the winch's crank, and divide that by the distance the Load travels while being raised. (The actual ratio is based on the radius of lever arm divided by the radius of the axle.) The ratio of vertical height traveled, divided by the length of a ramp, is the MA for a ramp. Making a winch with a very long crank arm will increase MA. Putting a spool with a large diameter on the axle would increase the speed by which the Load gets raised, but would also reduce the machine's MA and require more Applied Force to raise a given Load. Lengthening the ramp makes going uphill easier, but with the trade-off that you must travel further to reach a given height.