PASCO SE-8747 Kinesthetics Cart KINESTHESIA-1 - Manual (Page 17)

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012-05787C Kinesthetics Cart
Experiment 8: Newton’s Third Law
The results of physics education research show that the deceivingly simple Third Law is the hard-
est one for our students to comprehend and accept. Physics education researchers estimate, that by
the time a student manages the action-reaction problems (s)he is 90% converted into a Newtonian
thinker. There is the denial of the reaction: inanimate objects cannot apply forces (e.g., the wall
cannot push against me!), and the familiar student-conception that the stronger, more massive or
more active agent always wins. We can probably agree, that our students grasp the Third Law
fairly rapidly in static situations. However, if, for example, a small vehicle pushes with its bumper
against the rear bumper of a much more massive truck with disengaged engine so that both ve-
hicles move in the forward direction, then the majority of the students will ascribe a larger force to
the passenger car. Their argument: “If the forces between the two bumpers were equally large,
they would cancel and the truck could not start to move in the forward direction”.
Similarly, in the case of the fast moving bus hitting the mosquito, almost all students disallow
the possibility that the mosquito exerts the same force on the windshield of the bus as the wind-
shield exerts on the mosquito.
Kinesthesia-1 again affords a variety of possibilities to illustrate Newton’s Third Law in static
as well as dynamic situations. The “landing gear” of the upper cart and the wheels of the lower
cart are the same kind of roller blade wheels. Thus two carts with identical rolling properties are
available. We equip the students that ride the two carts with spring scales, bathroom scales or
force probes. In many of the possible scenarios we can easily demonstrate the equality of the ac-
tion and reaction force with a spring scale or bathroom scale. The familiar examples like two stu-
dents pushing against each other, or one student “simply holding” and the other one pushing, or
the first one pulling and the other one “just holding,” or pushing against an inert object such as a
wall, fall into this category.
However, in dynamic situations such as collisions, where the interaction happens on a milli-
second time scale, it is impossible to get a reading from a spring scale or bathroom scale. We offer
here a demonstration that has become feasible in a microcomputer based laboratory (MBL) using
computer interfaced force probes. Priscilla W. Laws has recently introduced a new student activity
that involves the collision of two force probes. Two kinematics carts, both equipped with force
probes facing in the forward direction, approach each other on a track so that the two force probes
collide. The output of both force probes is displayed simultaneously on a computer screen.
This demonstration can easily be modified to incorporate two students into the experiment
using the upper and lower Kinesthetics cart and two heavy duty force probes. This affords both
students a kinesthetic experience combined with the graphical display on a computer monitor in
front of them.
For additional new, impressive illustration of Newton’s Third Law, see Experiments 13
through 16 of this manual.