Force Probe Operation
Country: United States
Date: June 2008
How can I explain to middle-school students (6th grade)
how a force probe works to measure the mass of a substance. All I
can find are higher level explanations, and I need something more
When doing school science experiments there is often a need to measure
force. A simple method would be to use a spring scale, but that is not very
useful, and there is no easy way to connect a spring scale to a computer.
Instead, school and hobby companies make an inexpensive device that measures
force, sort of like a miniature bathroom scale, that they call a force
probe. They must be reasonably accurate, but not as accurate as expensive
industrial devices. I do not know how the school force probes actually work;
the method would depend on the company that makes them.
A device that measures force, like pushing and pulling, is ordinarily
called a "force sensor." Industrial force sensors often use strain-gauge
load cells, and you can read about that kind of sensor on the Internet.
Other mechanical sensors are torque sensors (for twisting force) and
pressure sensors (for pressures of gas or liquid).
There is something called an "atomic force probe" that measures the forces
necessary to push atoms around, and is part of an atomic force microscope,
which is a device that costs about a hundred thousand dollars, but that is
probably not the force probe that the 6th grade students are using.
Eventually you have to explain the equation, F=M*A. It seems reasonable
to approach this one letter at a time. Fortunately, middle schoolers
already understand this equation in their bones, and all you have to do
is move that understanding from their bones to their brains.
I think I would not start with a force probe. I would start with something
simpler, like a spring. A student can *feel* how a spring works: the further
you stretch it, the more effort you must exert. So the extension of a
spring is a good indicator of the force that is being exerted to stretch
it. This is a good enough notion of force for the purpose of relating
force to mass.
The next notion required is acceleration. I would use the force due to gravity
to help, but this requires that you first show that gravity exerts a force.
I would hang weights from a spring and show how the extension of the spring
depends on the weight. It is not much of a stretch for a student to look at
a spring that she has just stretched by hand, which is now extended by a
weight hanging from it, and to appreciate that the weight is doing the
same thing she just did.
So, gravity exerts a force, and the amount of force gravity exerts on an
object depends on the object. Now, take an object, that stretched the
spring only a little, in your hand, and shake it back and forth. Do the
same with an object that stretched the spring a lot. Notice that it
requires more effort to shake the object that stretched the spring a lot.
This is the essential point. Shaking is not in any obvious way similar
to hanging a weight from a spring, but you can nevertheless know which
objects are going to require great effort to shake, by observing how much
they stretch a spring when they are suspended by it. Shaking an object
is accelerating it, of course, and you will have to explain that acceleration
is just changing the speed -- increasing it, decreasing it, reversing it,
all the same, because they all change the speed.
Students will probably not be floored by this notion. They probably already
'knew' it at some level, and may even regard it as obvious. But this is
a very deep piece of information about the universe. This is what physics
is all about. The property of an object that gravity works on is the very
same property that shaking works on. Gravity behaves exactly like an
acceleration, and the property they both work on is mass. If this does not
give you goose bumps, you have not thought about it long enough.
Once a student understands the concepts, you can use a force probe to get
numbers, but do not let numbers and measurements get in the way of the
understanding they are intended to produce. Understanding is way more
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Update: June 2012