Teaching outline and Presentation suggestions:
(These are the suggestions from people who have never worked with young students. We hope these suggestions aren't totally out of line but, with your feedback, we may be able to revise them soon into something that really works.)
Pulling: Since we are constantly experiencing forces, it's hard to get out of ourselves and look at them analytically. First, forces always come in pairs. It's impossible to have only one force. One way to experience this might be to stretch a rubber band. The usual way people stretch a rubber band is by pulling outward on either end of the band using two hands.
Now suggest that the students try to stretch the rubber band by pulling on it in with only one hand and at only one place on the band. If a student's solution is to pull on one place with the rubber band looped over another object, see if you can convince the puller that this is "not fair!" Pulling on the rubber band at only one place means it will be attached to something at only one place. Hopefully this activity will lead to the conclusion that in order to stretch the rubber band, it will have to be pulled in at least two places. Have one student pull on one end of the rubber band and another student pull on the other end. (Is it realistic for us to assume that this activity won't lead to rubber band fights?) Is it possible to have one student pull harder than the other? Is it possible for one student to pull on either end of the rubber band with different forces? We hope this exercise will lead to the conclusion that the pulling forces on the rubber band must be equal and opposite. (This of course is Newton's famous third law: For every action force, there is always an equal and opposite reaction force.)
Most people find it almost impossible to believe that when they pull on something, it always pulls back with an equal and opposite force. Find some inanimate object that one end of the rubber band can be attached to and which allows the student to pull on the other end of the band and stretch it without moving the object. (Perhaps a bent paper clip attached to a chair or a nail attached to a desk or a wall.) While the student pulls on one end of the rubber band, ask what the object at the other end of the rubber band must be doing.
Replace the inanimate object with the fingers of another student. When one student pulls on one end of the rubber band, what must the others student always do? (This exercise might convince someone that the inanimate object, such as the nail in the illustration, must also pull just as did the student.)
Pushing: This activity is essentially the same as the pulling experiment with the rubber band only we will be pushing on the "ears of a binder paper clip" We hope the illustration makes clear what should be done.
First have the students push inward with two thumbs to open the clip. Then ask them to try it by pushing only on one of the ears. Then perhaps have two students experiment with pushing on the ears of the clip to open it and see if it is possible to have one student push with one force and the other student push with a different force. Then have one student take the clip up to a wall, press one the the ears against the wall and open the clip by pushing on the other ear.
Ask: "Is the wall pushing?" Hopefully students will lead one another to the conclusion that pushing forces come in pairs even if only a wall pushing back provides one of the forces. (We suspect that clips will be flying around the room--are they dangerous? Also, these clips are designed for adult strength--can young children use them successfully? Finally, it's very difficult even for adults to appreciate that if you push on a wall, it must push back on you with an equal and opposite force. For every action force there is always an equal and opposite reaction force is a very profound idea that everyone up to the time of Newton seemed to have missed.)
Measuring the force of gravity or weight: (This might be better understood if the activity on mass, weight and density is done first. Refer to "Physics Table of Contents.") We use the pound to measure force but the metric unit of force is the newton. Since weight is the force of gravity acting on an object, you are determining the force of gravity acting on you every time you step on a bathroom scale. However, it is very rare to find a bathroom scale calibrated in newtons. (For reasons we don't understand, even the people who invented the metric system are confused over this issue and insist upon calibrating bathroom scales in "kilograms", the unit of mass rather than "newtons", the correct unit of weight. We assure you that these "kilogram scales" will not work properly on the moon! ) We think it would be instructive for young people to weigh themselves in newtons. You could make it an arithmetic exercise for them to convert pounds to newtons or you could post a pounds to newtons conversion table near the scale so they could look up the correct value after they measure their weight in pounds. Help them to see that this is the force that the earth pulls down on them. If you really feel up to an advanced discussion, ask them: "what is the scale doing to the bottoms of your feet while you stand on it?" Hopefully they will come to realize that the scale must be pushing upward on the bottoms of their feet just as hard as gravity is pulling downward on their body.
The major idea to be stressed by this activity is that applying a force may take effort but you really don't get tired (and winded) unless you apply a force and move at the same time. Ideally we would like to have the students push on a car and stand still and then push on the car just as hard while the car moves. Anyone who has done this knows that you only really get winded as the car moves faster and faster. In fact, it is really difficult to keep pushing with the same force when the car is moving fast as you did when the car was not moving.We hope someone comes up with a better idea than the following but the following, at least, is is fairly easy to make.
If you didn't read the construction details earlier, click here to get more information on the construction and use of the bunjy cord force measuring device.
It will take a certain amount of force to stretch the bunjy cord until the nylon string almost touches the bunjy cord. (The amount of force required to accomplish this can be easily adjusted by changing the amount of "droop" the nylon string has when little or no force is applied to either end of the bunjy cord. You should adjust this droop to the needs of your class.) If you are standing still or even if you are moving, it will take the same force to pull the bunjy cord until the nylon string just touches it. First have two students stand still with one student behind the other. Have the student behind hold one end of the bunjy cord in his/her hands and have the student in front pull the bunjy cord over her/his sholder "stevadore style". Then have the student in the back pull the bunjy cord until the nylon cord is almost tight. Have the students stand still for a while and ask if anyone is getting tired. (We suspect that no one will admit to getting tired when standing still.) Now, have the student in front start moving while the student behind pulls back and tries to keep the nylon string just touching the bunjy cord, as before. (It is difficult to do this but it can be done and, it is a lot of fun.) Does anyone get tired now that you are moving fast? (We suspect at least one of the students might admit to getting tired in the moving case and we even predict that it will more than likely be the student in the front.)
If you can get a wagon to use behind and put a student in the wagon dragging his/her feet to keep it moving at constant speed, while another student acts as the horse in front, this should be a little easier to do.( A bicycle, or a tricycle or any wheeled device that a student can ride and brake easily to supply the necessary retarding force should work as well as a wagon. We hesitate to suggest this, but an office chair with wheels works fine. However, should young students be encouraged to use such a chair in this way?) Once again, the idea is to have the "horse" find out that pulling without moving is simply applying a force. Pulling while you move is doing work. Exerting a force only, does not tire you in the same way as, actually doing work.
Measuring a student's power:
When you lift a weight vertically upward, you do work. This is because you are applying an upward force through a distance. Surprisingly, you don't have to lift the weight only upward vertically to do the work. If you lift the weight along a slanted path, or even a complicated path, the work you do will always be the same as if you lifted the same weight the same vertical distance.
If you climb a rope, you do work because you are applying a force (equal to your weight) through a distance (equal to how high you climb.) If you run up a flight of stairs you do work since you are applying an upward force, equal to your weight, through a distance, equal to the vertical height you climb. (Don't be confused by the fact that when you run up a flight of stairs you go a greater distance that you would if you lifted yourself the same height by climbing a rope. It can be shown using trigonometry that when climbing the stairs, the force in the direction of motion is less, by just the right amount, to compensate for the larger distance you move.)
The purpose of this activity is to measure the amount of power a student can develop while climbing a flight of stairs.
You first must have each student weigh him/her self in newtons. We suggest that you have the bathroom scale located near a pounds to newtons conversion table so each student can find her/his weight in newtons. (place second link to pounds to newton table here.) Next you must find a flight of stairs where lots of children running and shouting won't interrupt other classes. Using string and a meter stick or any method you choose, find out the vertical height of the stairs.
(This might be a good project for several students to do the day before the activity. Counting the stairs and measuring the height of each also works. Make sure that the final result is the only vertical height of the stairs from the point where the climbing will start to the point where the climbing will end.) Finally, you must work out a method of crowd control so you can have each student climb the stairs while another student (or you, the teacher) time them while they climb. (The surprising and wonderful thing about this exercise is faster is not necessarily better. Heavier people can go slower and still develop the same amount of power. We suspect that for young children, there will also be little difference between the amount of power girls or boys will develop.) Since power is work per time, the final calculation of each student's power will be accomplished by multiplying his'/her's weight in newtons by the height of the stairs in meters (this gives the work in joules) and then divide this answer by the time to climb the stairs in seconds. The final answer will be in watts.
(If the students want to know their power in "horsepower" the answer in watts can be divided by 746 since there are 746 watts per horsepower.)
We suspect that the students will develop power in the "order of magnitude" of 100 watts. In high school,unusually fast and strong people might approach 1000 watts. We are fairly certain everyone can develop more power than 10 watts. So, depending upon the size of your students, you should expect the power they develop to be roughly 100 watts. After the students have completed the activity, ask them how long they think they could keep running up stairs at the same speed. Try to relate the power they developed to the power required to keep one 100 watt light bulb burning. Remember, as long as the light is on, work is being done at the rate of 100 joules per second to keep the bulb burning. It's one thing to develop 100 watts for a short time, (like the short time required to run up a flight of stairs) but if you are going to generate the power to keep a 100 watt bulb burning, you will have to keep producing the power for as long as the bulb is burning. Power is work per time, but to keep doing work or to keep delivering energy, you must keep producing the power for the length of time required to do the work.
Brief review of the basic concepts presented:
Force is a push or a pull.
Work is done when a force is applied through a distance.
Power is the rate of doing work, or, which means the same thing: power is work per time.
Back to backgound material on Force, Work and Power
Student Activities on Force, Work and Power.
Back to Table of Contents