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Name: Gail
Status: Educator
Grade: 6-8
Location: NY
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 kid-friendly.

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.

Robert Erck

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 important.

Tim Mooney

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