Winches & Ramps
In MOVIE 1, Earl anchors his lesson on winches with
references to an easiertograsp 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 TradeOffs 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 tradeoff with MA involves "less forcemore
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 tradeoff
that you must travel further to reach a given height.
