My 8th grade daughter had her final (distance learning) science quiz this week on work, or as it is called in her class, the scientific definition of work. I usually have no idea what she does in her science class since she rarely talks to me about school but she so happened to mention this one tidbit because she was proud that she didn’t get fooled by what she thought was a trick question. I’ve always believed that work, as in force times displacement (not the one where you produce economic value), is one of the most useless concepts in physics and should not be taught to anyone until they reach graduate school, if then. It is a concept that has long outlived its usefulness and all it does now is to convince students that science is just a bunch of concepts invented to confuse you. The problem with science education in general is that it is taught as a set of facts and definitions when the only thing that kids need to learn is that science is about trying to show something is true using empirical evidence. My daughter’s experience is evidence that science education in the US has room for improvement.
Work, as defined in science class, is just another form of energy, and the only physics that should be taught to middle school kids is that there are these quantities in the universe called energy and momentum and they are conserved. Work is just the change in energy of a system due to a force moving something. For example, the work required to lift a mass against gravity is the distance the mass was lifted multiplied by the force used to move it. This is where it starts to get a little confusing because there are actually two reasons you need force to move something. The first is because of Newton’s First Law of inertia – things at rest like to stay at rest and things in motion like to stay in motion. In order to move something from rest you need to accelerate it, which requires a force and from Newton’s second law, Force equals mass times acceleration, or F = ma. However, if you move something upwards against the force of gravity then even to move at a constant velocity you need to use a force that is equal to the gravitational force pulling the thing downwards, which from Newton’s law of gravitation is given by , where is the universal gravitational constant, is the mass of the earth, is the mass of the object and is the distance between the objects. By a very deep property of the universe, the mass in Newton’s law of gravitation is the exact same mass as that in Newton’s second law, called inertial mass. So that means if we let , then we get , and is the gravitational acceleration constant if we set be the radius of the earth, which is much bigger than the height of things we usually deal with in our daily lives. All things dropped near the earth will accelerate to the ground at . If gravitational mass and inertial mass were not the same, then objects of different masses would not fall with the same acceleration. Many people know that Galileo showed this fact in his famous experiment where he dropped a big and small object from the Leaning Tower of Pisa. However, many probably also cannot explain why including my grade 7 (or was it 8) science teacher who thought it was because the earth’s mass was much bigger than the two objects so the difference was not noticeable. The equivalence of gravitational and inertial mass was what led Einstein to his General Theory of Relativity.
In the first part of my daughter’s quiz, she was asked to calculate the energy consumed by several appliances in her house for one week. She had to look up how much power was consumed by the refrigerator, computer, television and so forth on the internet. Power is energy per unit time so she computed the amount of energy used by multiplying the power used by the total time the device is on per week. In the second part of the quiz she was asked to calculate how far she must move to power those devices. This is actually a question about conservation of energy and to answer the question she had to equate the energy used with the work definition of force times distance traveled. The question told her to use gravitational force, which implies she had to be moving upwards against the force of gravity, or accelerating at g if moving horizontally, although this was not specifically mentioned. So, my daughter took the energy used to power all her appliances and divided it by the force, i.e. her mass times g, and got a distance. The next question was, and I don’t recall exactly how it was phrased but something to the effect of: “Did you do scientifically defined work when you moved?”
Now, in her class, she probably spent a lot of time examining situations to distinguish work from non-work. Lifting a weight is work, a cat riding a Roomba is not work. She learned that you did no work when you walked because the force was perpendicular to your direction of motion. I find these types of gotcha exercises to be useless at best and in my daughter’s case completely detrimental. If you were to walk by gliding along completely horizontally with absolutely no vertical motion at a constant speed then yes you are technically not doing mechanical work. But your muscles are contracting and expanding and you are consuming energy. It’s not your weight times the distance you moved but some very complicated combination of metabolic rate, muscle biochemistry, energy losses in your shoes, etc. Instead of looking at examples and identifying which are work and which are not, it would be so much more informative if they were asked to deduce how much energy would be consumed in doing these things. The cat on the Roomba is not doing work but the Roomba is using energy to turn an electric motor that has to turn the wheel to move the cat. It has to accelerate from standing still and also gets warm, which means some of the energy is wasted to heat. A microwave oven uses energy because it must generate radio waves. Boiling water takes energy because you need to impart random kinetic energy to the water molecules. A computer uses energy because it needs to send electrons through transistors. Refrigerators work by using work energy to pump the heat energy from the inside to the outside. You can’t cool a room by leaving the refrigerator door open because you will just pump heat around in a circle and some of the energy will be wasted as extra heat.
My daughter’s answer to the question of was work done was that no work was done because she interpreted movement to be walking horizontally and she knew from all the gotcha examples that walking was not work. She read to me her very legalistically parsed paragraph explaining her reasoning, which made me think that while science may not be in her future, law might be. I tried to convince her that in order for the appliances to run, energy had to come from somewhere so she must have done some work at some point in her travels but she would have no part of it. She said it must be a trick question so the answer has to not make sense. She proudly submitted the quiz convinced more then ever that her so-called scientist Dad is a complete and utter idiot.