MINI NOTES:

Friction, friend or foe ? Well, the same thing can be asked of fire, friend or foe. To keep warm, it is definitely a friend, when your house in on fire, it is a foe. The same thing can be said about friction, you could not write on a piece of paper with a pencil or a pen, if it were not for friction. There are times when you want lots of friction, between the soles of your shoes and the surface that you stand on, and there are times when you want as little as possible. When you are running a race, you want as much traction as possible, on the other hand if you were on skates you would want least amount of friction. You gain your speed by pushing with your blades at an angle to the ice, to brake, you change your feet so that they are at an angle against the ice, and you will see the ice scrape up into the air as you stop sideways. Actually when you are skating, the pressure of your blade produces a layer of water between the ice and the blade, so you are actually gliding on a layer of water, the reason why you don't see the water is because it refreezes, just as soon as your blade leaves the ice ( move ). When you get a chance see if you can compare a pair of regular ice skates and a pair of ice skates for hockey players.

Friction between solids, can be classified into two classes, sliding friction and rolling friction. Now, it would seem that the more you polish the surface of an object, ( sliding friction) the less friction there would be, however that is not what happens. If the surface is to polished, there seems to be more friction between the surfaces. Take for an example the smooth surface of one plate glass on top of another plate glass, for a given weight and surface contact, you will find the friction is actually increased. So there must be other forces coming into play here, like electrostatic forces and possible other forces as well.

Just a quick review of Mass and weight. We know that the mass of a body is simply referring to the quantity of matter, and remains constant. There is just so much of you, and your mass does not change, however your weight may change, because weight is the measure of the force that is pulling you down to the center of the earth. You become lighter, when you go up in a elevator say to the 20 th floor. You can easily prove this by using a sensitive bathroom scale, and you become heavier when you go down several floors. When you move away from the center of the earth, earth's pull (gravity) decreases, and your bathroom scale would indicate this. Therefore weight depends upon two factors, the mass of the object and the distance from the center of the earth.

Quite often, you will hear that a certain person weighs 150 pounds, or say a child weighs 50 pounds. We refer to this kind of statement as a pound-weight, this means that a person who has a mass of 150 pounds also has a weight of a 150 pounds, and is three times heavier then the child of 50 pounds. Such that if the person and the child are in one place ( usually at sea level ) the weights are proportional to the their masses. Remember that if you move from a certain location, say to the mountains, you no longer have the same weight, but you still have the same mass. To avoid any confusion between mass and weight in science, they have adapted an absolute system for mass and weight, in the metric system we have the MKS system referring to meter-kilogram-second and the CGS system referring to the centimter-gram-second, such that the kiogram and gram referr only to the mass of an object and a new term a newton, a unit used for measuring force or weight. The smaller unit for the metric system is the dyne. Such that one kilogram mass has a weight of 9.80 newtons, usually at sea level, however this value will change with location, or with repect to the distance from the center of the earth. And to further clearify this confusion in the English system, a new unit called a slug used for the measure of mass, and one slug mass weighs about 32.2 pounds. Because in any one place on the earth all objects are proportional to their masses, we can avoid using the Absolute scale system for the purpose of this course, and continue using kilogram-weight and gram-weight .

Force can be defined as a push or a pull.

Friction is a force that resists the motion of two objects that are in contact with each other. Since coefficient of friction can be expressed as a number, this is another quantitave propertiy of matter.

Coefficient of friction can be determined by finding the weight of the object, divided by the effort that is needed to move the object.

FORCE ( FRICTION )

PROBLEM:

To find the coefficient of friction.

MATERIALS:

* CAUTION *

This experiment has the potential of eye injury !! Be sure to wear your Safety Goggles at all times.

Safety Goggles, (because you will be using the rubber band scale), wooden block, (a one foot, of a 2 by 4 or something similar, or a small board with a text book on top, be sure to weigh your board and book ), a 3 foot board, for sliding the wooden block on, string ( or twine), spring scale (rubber band).

PROCEDURE:

1. Weigh the wooden block with the Rubber band scale, record the weight of your block ________. If you are using a board and a text book, be sure to get the total weight.

2. Stand the wooden block on its narrow side ( the 2 inch side ) attach a piece of string or twine to the block ( tie the twine, so that it is firm around the block, and make a knot and bow, like when you tie your shoe laces. The trick of doing by yourself, with out someone holding their finger on the tie, before you make the bow, the key is to loop it TWICE when you make your tie, this way it will stay firm before you make your bow.) If you have plenty of twine, you may want to just make a not, with out the bow, again if you want to do by yourself, you will have to loop the tie twice before you make your knot. You then can attach a paper clip to the twine that is flush against the block.

3. You should be using an unfinished block and board, you may have to have someone holding the board while you drag the wooden block, or put some thing heavy on the end of the board, to prevent it from moving.

4. Take a calibrated " rubber band scale " and attach the bottom paper clip to the paper clip attached to the block.

5. Place your block as close to the back end of the board as possible, begin to apply a pulling force on the Rubber band scale. Be sure to wear your goggles, and observe how many grams of force is needed to start the block to move. Record you answer in the table provided. You may have to repeat it twice.

6. Now, as you are moving the block, observe how many grams are indicated on the Rubber band scale. Record your answer in the table.

7. Now, repeat the above steps, but this time obtain about 5 pencils, or round pens, or anything that is round. If you are using a one foot block, place your pencils about every five inches apart, such that only two pencils are needed to support the block when you start moving the block. Record how many grams are needed to start the block moving, and record how many grams are needed to keep the block moving.

8. Now, untie or cut you twine from the block, and lay the block on its wide side ( 4 inch side ) ( Have you noticed that a 2 by 4 is really a 1 and 1/2 inch by 3 and 1/2 inch, but it is still referred to as a 2 by 4). Again attach your twine and repeat steps # 5 and # 6 above and record your findings in the table.

9. Now, repeat steps # 5 and # 6, but this time using your round pencils, and record your results in the table.

OBSERVATION:

QUESTIONS:

1. Define friction -

2. How many kinds of friction are there? _____ what are they?

3. Can friction be helpful? ______ Explain.

4. Can friction be harmful ? _______Explain.

5. By looking at your table what can you say about how much force is needed to start a object moving and the amount of force needed to keep an object moving?

6. What is inertia ?

7. Define coefficient of friction.

8. Write the formula for coefficient of friction and explain each letter in the formula.

9. What is the coefficient of friction between the board and wooden block?

10. Name five places where friction is needed.

11. Name five places where friction is harmful.

12. How can you reduce friction?

13. How can you increase friction?

14. How is heat related to friction?

15. Is there a significant difference between narrow side of the of the block and the board and the wide side of the block and the board.

16. How is friction implied in the following verse? Proverbs 27:22 ¶ Though thou shouldest bray a fool in a mortar among wheat with a pestle, yet will not his foolishness depart from him.

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