Friction. Let s Find Out: A Sole ful Experiment

Transcription

1 Friction Let s talk about another ever present force on this Earth, friction. Friction is the resistance that one object encounters when it is rubbing against another object. Friction is what makes things slow down. Without friction things would just keep moving unless they hit something else. Without friction, you would not be able to walk. Your feet would have nothing to push against, and they would just slide backward all the time as if you were doing the moon walk. Friction is a very complicated interaction between pressure and the type of materials that are touching one another. Let s do a couple of activities to get the hang of what friction is. Let s Find Out: A Sole ful Experiment You Need: About 5 different shoes (they do not need to be stinky) A board, or a tray, or a large book at least 15 inches long and no more than 2 feet long. A ruler Paper Pencil A partner First we ll do a practice run with one of your shoes: 1. Put the board, or whatever you re using, on the table. Mechanics 24

2 2. Put the shoe on the board with the back of the shoe touching the back edge of the board. 3. Have a partner hold the ruler upright at the back of the board so that the 12 inches end is up and the 1 inch end is on the table. 4. Slowly lift the back of the board, leaving the front of the board on the table. (You re making a ramp with the board.) Eventually the shoe will begin to slide. 5. Stop moving the board when the shoe slides and measure the height of the board. The greater the friction, the higher the end of the ramp has to be lifted. 6. Before you test any more shoes, analyze all of the shoes. Are the bottoms made out of the same type of material? Are they light or heavy? How much of the bottom will actually be touching the board? Is there anything else about them that you think might have an effect on how much friction they have? Form a hypothesis that explains why you think each shoe will have more or less friction than the other shoes. 7. Using your newly formed hypothesis, look at the 5 shoes you chose and make a guess at which shoe will have the most friction. On a scale from 1 to 5 (or however many shoes you re using) rate the shoes you picked. 1 is lowest friction and 5 is highest friction. Write the guess next to a description of each shoe on a piece of paper. Test all of the shoes. Did you have any surprises with which shoe had the most or least friction? Compare the shoe with the most friction to the shoe with the least friction. Do you notice anything? Usually the shoe that has the most friction has more shoe surface touching the board than most of the other shoes. Often the shoe with the least friction has the least amount of surface touching the board. Since friction is all about two things rubbing together, the more surface that s rubbing, the more friction you get. A tire on you car should have treads, but some race car tires will be absolutely smooth with no treads at all. Why? The race car driver doesn t have to worry about rain or wetness so he or she wants every single bit of the tire to be touching the surface of the track; that way there is as much friction as possible between the tire and the track. The tires on your car have treads that cut through mud and water to Mechanics 25

3 get to the nice firm road underneath. The treads actually give you less friction on a flat dry road! Some of you might have used a skateboard shoe for this activity. Notice that the skateboard shoe has quite a flat bottom compared to most other shoes. This is because a skateboarder wants as much of his or her shoe as possible to touch the skateboard at all times. Let s Find Out: What a Drag You need: A 6 inch long piece of 2 x 4 wood, or a heavy book A string A spring scale or a rubber band and a ruler Paper Pen 5 or so different surfaces, table tops, carpet, chairs, etc. 1. Write a list of the different surfaces that you chose on a piece of paper. 2. Observe the different qualities of the surfaces you chose. Are they smooth, rough, clean, dirty, wet or dry? Is there anything else about them that you think might affect how much friction they will have? Make a hypothesis that explains why you think each surface will have more or less friction than the other surfaces. On a scale from 1 to 5 (or however many surfaces you chose) rate the surfaces you picked. 1 is the lowest level of friction and 5 would be the highest level of friction. Write your ranking next to the surfaces on the paper. 3. Take your block and attach a string to it. (To keep things simple I will assume that you are using a block throughout this experiment. If you are using a book, just substitute book where ever I say block.) 4. Place your block on the surface to be tested. 5. If you have a spring scale, attach it to the string and carefully pull on your block until it just starts to move. What you will probably see is Mechanics 26

4 that you will keep pulling and pulling until suddenly your block moves. Try to record the number that the scale said just before the block moved. It takes a little bit of practice to read that number so keep trying. 6. If you don t have a spring scale, tie a rubber band to the string that is attached to your block. Now put a ruler with the first inch mark at the end of the rubber band farthest from the block and the 12 inch mark closest to you. Now pull on the rubber band holding it next to the ruler. When the block moves, record the number on the ruler where the end of the rubber band was. In other words, you are measuring how far the rubber band stretches before the block moves. 7. Remember this takes some getting used to with both the scale and the rubber band so keep playing with it until you get it. 8. Write down your results next to your hypothesis and rankings chart. 9. The higher the number, the more friction there is between your block and the surface the block is on. In other words the harder you had to pull to get the block moving, the more friction there was between the block and the surface. 10.Now analyze your data and see how it matches your hypothesis. Which surface really had the most friction and which had the least. Write numbers 1 to 5 (or however many surfaces you chose) next to the results. How did the data correlate with your hypothesis? Any surprises? You ve probably noticed with this activity and the last one that the kind of surfaces rubbing together make a huge difference. Flat, hard, smooth surfaces will have much less friction than a rubbery, soft, or rough surface. Muddy, wet or icy surfaces will often have even less friction. So, if you remember what we talked about with shoes and tires, the job of the tread on a shoe and a tire is to cut through the lower friction water or mud and get down to the higher friction road or dry ground. Static Versus Kinetic Friction Something else I d like you to notice is that friction acts differently depending on whether something is moving or at rest. If you have ever Mechanics 27

5 had to push something heavy like a refrigerator, you may have noticed that it was harder to get it to move than it was to keep it moving. There are actually two types of friction: static friction and kinetic friction. Static friction happens when something is resting on or against something else and not moving. Kinetic friction is when one thing is moving on top of or against something else. Static friction is usually greater than kinetic friction. It is harder to get your fridge to start moving than it is to keep it moving. You may have noticed that your block required more force to get it moving than was necessary to keep it moving during the What a Drag experiment. If not, go ahead and play with the measurements some more. When you first got the block to move, your scale had measurements that were much higher than when the block was actually moving. It was harder to get it moving than to keep it moving. What Really Causes Friction? Friction isn t really caused by the roughness or smoothness of an object even though it seems that way. Friction is actually caused by the fundamental force of electromagnetism between two objects. Believe it or not, it is caused by two objects chemically bonding to one another. Scientists call the phenomenon caused by the chemical bonding between two objects stick and slip. Think about it this way. When you pulled the block in "What A Drag" you noticed that the force needed to get the block moving was more than the force needed to keep it moving. The surface underneath your block never got any rougher or smoother. It stayed pretty much the same. So why was it harder to get the block moving than it was to keep it moving? When the block is just sitting there the chemical bonds between the block and the surface it is on can be quite strong. When the block is moving, however, the bonds are much weaker. The next activity should make this clearer. Mechanics 28

6 Let s Find Out: Stick and Slip You need: 2 thin flat magnets (businesses often send these types of magnets in the mail or pass them out as business cards for your fridge) Fingers 1. Take two magnets and stick them together black side to black side. They should be together so that the pictures (or what ever is on the magnets) are on the outside like two pieces of bread on a sandwich. 2. Now grab the sides of the magnets and drag one to the right and the other to the left so that they still are magnetically stuck together as they slide over one another. Did you notice what happened as the magnets slid across one another? They stuck and slipped didn't they? What they did is a bit like how electromagnetism works to cause friction. As two surfaces slide across one another they chemically bond and then break apart, bond and break, bond and break as they slide. The magnets in your experiment magnetically "bonded" together and then broke apart as you slid them across one another. Chemical bonds don't work quite like the magnetic "bonds" in our experiment, but it gives a decent model of what's happening. When g (gravity) and f (friction) are equal and opposite, Phillip stops accelerating. Mechanics 29

7 Conclusion Friction is everywhere! Imagine what the world would be like without friction! Friction is a part of everything you do from catching baseballs to eating hamburgers to putting on shoes. If you take a quick look at friction, it is quite a simple concept. It s just two things rubbing together. However, when you take a closer look at friction, it's really quite complex. What kind of surfaces are rubbing together? How much of the surfaces are touching? And what's the deal with this stick and slip thing anyway? Friction is a concept that many scientists are spending a lot of time studying. Understanding friction is very important in making engines and machines run more efficiently and safely. There are many mysteries and discoveries to be uncovered about friction. Go out and make some! Mechanics 30

8 In a Nutshell Friction is the the resistance that one surface or object encounters when moving over another. The intensity of friction between two objects is dependent on the nature of the materials that are in contact with one another, how much pressure is put on the materials, whether the materials are wet or dry, whether they are hot or cold, and more... In other words, friction is quite complicated! Static friction is the friction between two objects that are not moving. Kinetic friction is the friction between two objects where at least one of them is moving. Friction is caused by the electromagnetic forces between two objects. Friction is not caused by the roughness of objects. Rather it is caused by chemical bonds "sticking and slipping" over one another. Mechanics 31

9 Did You Get It 1. What is friction? 2. Walking would be easier without friction...true or False. 3. Why does a feather fall more slowly than a brick? 4. Put a coin on a piece of paper. Quickly pull the paper out from under the coin. Now pull the paper slowly. What does static friction and kinetic friction have to do with what happens when you change the speed at which you are pulling the paper? 5. What concept did the Stick and Slip activity with the magnets illustrate ( Stick and Slip on page??)? Mechanics 32

10 Answers 1. Friction is the resistance that one surface or object encounters when moving over another. Air resistance, by the way, is the friction of one object rubbing against millions and billions of air molecules. 2. FALSE!!! Walking would be impossible without friction. Your feet couldn t push back against the floor to move you forward. 3. Air resistance (a form of friction) slows the feather down. The feather rubs against many, many, many air molecules as it falls through the air. The feather is light and large enough that the air molecules actually slow it down. 4. If you pull the paper slowly, the static friction between the penny and the paper isn't broken. So the penny rides along with the paper. If you pull it quickly, you can overcome that static friction and the paper will slide along under the penny without moving it. As long as the paper is moving fast enough the kinetic friction between the paper and the penny isn't enough to move the penny. 5. Objects "stick and slip" as they rub against one another because of chemical bonds forming and being broken. Don't forget, that the magnet activity is a good model for how friction works but the magnetic bonds you experienced with the magnets and the chemical bonds experienced in friction are not the same thing. Mechanics 33