PASCO SE-8747 Kinesthetics Cart KINESTHESIA-1 - Manual (Page 18)

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Kinesthetics Cart 012-05787C
Experiment 9: Center of Mass
Conservation of Momentum
➤ NOTE: EXTREME CAUTION IS REQUIRED WHILE PERFORM-
ING THIS EXPERIMENT! This is the only experiment where a
student walks on the plank. In all other experiments students
sit on the cart and hold on to the handles or the cart itself with
at least one hand. To keep the student from injury:
• line both sides of the cart’s path with foam mats
• place spotters along the cart’s path to support the student’s
ascent and descent.
If we align the upper and lower cart and separate them by a distance equal to their length we
can readily clamp 2”x12”x8’ plank to both carts and use it for a number of momentum conserva-
tion experiments and center of mass illustrations. A student running from one end of the plank to
the other moves the plank (that is in effect supported by ball bearings) in the opposite direction.
The total momentum of the system is (almost) conserved. The center of mass (almost) stays in its
place.
➤ HINT: If the person walks extremely slowly and very carefully from left to right then the
force exerted on the plank and thus the force exerted on the two carts is too small to overcome
friction (roller blade wheels/ floor). In this case it is possible to walk from left to right without
the plank and carts moving at all. This seems to violate momentum conservation. Of course,
momentum is still conserved, but this time in the larger system containing the floor of the class-
room (or the earth). This can be presented to the students in form of a puzzle or extra credit
homework.
Fig. 9-1: As the student moves to the right the plank and the two
Kinesthetics Carts move to the left. This nicely illustrates the conserva-
tion of linear momentum in the closed system consisting of the student,
the plank and the two carts.