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Re: Newton's Second Law Lab



John,
The problem with physical laws is we can't "prove them." They are our
basic assumptions - any force measuring device uses force as defined in the
second law to measure it. When I look at a force probe - accelerometer
combination, I see it as you do, but that is not what the student sees, and
the fact that you can obtain the mass of the cart from this system is
meaningful to the student.
One of my favorite demos is to collide force probes mounted on two carts
together, plotting the forces on both probes on the same screen. When I
first saw this demo, my reaction was how stupid, it is just Newton's third
law in action - three times. Forgive the pun, but it like the
accelerometer-force probe combination does illustrate the intimate
relationship between force and acceleration with respect to time - it is
always true.
In the case of the third law colliding force probes, the demo does show
both that the third law is true at all times. The force verses time plots
are mirror images, and by changing the mass of one of the carts, they are
always equal. A truck does not exert more force on a small car then the
small car exerts on the truck.

Gary
Gary Karshner

St. Mary's University
San Antonio, Texas
KARSHNER@STMARYTX.EDU