Force, Work and Power

(Remember, anything we say that is surrounded by parenthesis can be ignored without missing any of the main message of the discussion. Interesting details might appear in parenthesis but sometimes these details might simply confuse the issue.)

(Our approach to the presentation of Physics topics has been to focus on fundamental concepts which are frequently confused. It's our opinion that if we make a real effort to define, clarify and distinguish these "topics of possible confusion" we will help elementary school teachers to teach these confusing topics more effectively. If you just stumbled upon this section of our page, look around, form an opinion of its worth, find the evaluation form and let us know what you think.)

Background Material

I Words: Compared with Biology, Physics is a subject of few words. Unfortunately, many of this small number of words have found their way into common usage without retaining the precise definitions that the physicists originally intended. Force, work and power are words we all use in ordinary conversation but usually without adhering to the definitions used by physicists. In the following we will try to define and differentiate between these closely related concepts.

Force: Whenever two things interact, a pair of forces is always involved. Forces make it possible for you to experience the Universe. Force can be defined as a push or a pull. Because an assortment of stimuli are pushing and pulling on the receptors in your sense organs, you experience the world around you. The metric unit of force is the newton (abbreviated N) and the unit of force we often use is the pound. A newton is about one fifth of a pound. (Weight is the force of gravity so we usually measure our weight in pounds but, any force could also be measured in pounds or newtons.)

Work: If you have ever pushed a car (using your own hands and feet) in an attempt to start the car, you should remember that at first, when you were pushing hard, you weren't winded. However, as you moved faster and faster you really started to breathe hard. You do work when you apply a force and move in the direction of the force while you are applying the force. Work is done when a force is applied through a distance and the force is in the same direction as the distance moved. It is almost impossible to discuss work without discussing energy (another one of those physics words that is often used without understanding.) Work is the way we transfer energy. (You eat food and the energy contained in the food is stored in your body through an assortment of complex biological processes. In order to transfer this stored energy into something useful like pushing a car, you must do work. You can tell you are doing work because you start breathing hard and this hard breathing helps to convert the stored energy in your body into the work you are doing through a complex process called respiration. The main result you notice is that you start breathing hard in order to supply the oxygen to your body so it can undertake this process of respiration. The important point of all of this is, you transfer the energy in your body to the energy of the moving car by doing work.) The unit of work in the metric system is the joule. One joule of work is done when a force of one newton is applied through a distance of one meter.

Power: You can walk up a flight of stairs or you can run up a flight of stairs. Either way you apply the same force through the same distance so you do the same amount of work. However in the running case, you develop more power. Power is work per time. Another way of saying the same thing is to say: Power is the rate of doing work. (Whenever we use the word "rate" we always mean : "per time". Speed is the rate of covering distance, interest rate is the amount of money per year, and work per time could also be called the rate of doing work. That is, power is the rate of doing work.) Let's say a clunkey old car and a powerful sports car happen to weigh the same and they both climb the same hill. Since they weigh the same, they must exert the same force, and since they both go the same distance, they each do the same amount of work. However, the sports car does it much quicker because it has a much more powerful engine. The metric unit of power is the watt. If a joule of work is done every second, the amount of power developed is one watt. (In many parts of the world the power of automobiles is measured in "Horsepower". This was a unit of power established by James Watt so he could describe to others the relative power of his recently developed steam engine. Since the horsepower and the watt are both units of the same thing, power, they can be related to one another. One horsepower equals 746 watts.)

II Purpose of the Activity:

The purpose of the activity is to experience forces, do work and learn how work is different from force. Finally, to measure the amount of power a student can develop.

III Materials required for the activity:

Rubber bands, paper binder clip type things (the ones that have a black spring steel piece that goes around the paper and two shiny metal "ears" that you squeeze to force them apart.), bunjy cord force puller (to be described later), a bathroom scale, a pound to newton conversion chart, a stop watch (many modern wrist watches have a stop watch capability), a flight of stairs (in a location where running up and down the stairs won't be too much of a distraction to others), a meter stick and some string (or a way to measure the height of the stairs in meters.)

IV What the teacher must do in advance of the activity:

Other than gathering together the required materials, making the bunjy cord force puller is the only item that requires special advanced preparation and we aren't sure it's even worth the effort. Be sure you read how we plan to use it before you commit any time to its construction. We hope you have no difficulty in obtaining a meter stick. (If this is a problem, check a store like Ikea that sells products which are designed and produced in metric sizes. They will often have free metric calibrated paper tapes that they will gladly give to schools.) We would like to come up with an easy way to change the dial on a bathroom scale to newtons (not kilograms!) but such an alteration is still in the works. Meanwhile, you can either copy the pounds to newtons conversion table we have provided (link) or, make up your own using the simple relationship: one pound = 4.45 newtons. Work out the conversion table so it fits well into the range of the weights of your students. Many middle and high schools do the stair climbing exercise with great success and we see no reason why younger kids wouldn't have fun and learn a lot with it. (If we are wrong, please tell us why.) However, you must find a place where you won't distract other teachers and classes.

Force, Work and Power: Teaching Suggestions.

Force, Work and Power: Student Activities.