§ Am I right about VanderWaals forces and friction?
§ Static friction is greater than sliding friction because when objects sit still, the molecules on their surfaces have more time to interact. The molecules settle in between each other and VanderWaals forces help hold them together.
§ I mentioned the proportionality of force and friction, and the independence of area and friction. Do we really want these included?Version / Date / Who / Revisions
04 / 2003/12/02 / dk / § Added revision table
§ Added static and sliding friction
§ Added proportionality of force and friction
§ Added independence of area and friction
§ Students should understand that friction is a force.
§ Students should understand that friction affects motion.
§ Students should understand that you can’t get away from friction.
§ Students should understand that static friction is greater than sliding friction.
§ Students should understand that sliding friction is constant, regardless of speed.
Concepts & Skills IntroducedArea / Concept
physics / friction
physics / static friction
physics / sliding friction
What idea was Aristotle missing in his thoughts on motion that kept him from seeing inertia? Introduce the warm-up question with a short dissertation on Aristotle and his thoughts on natural motion.
In the inertia activity, you should have gotten to the point that an object doesn’t change its speed or direction unless something makes it change. The thing that makes it change is force, at this point defined as just a push or a pull. We measure force in Newtons.
Friction resists motion. Friction is everywhere; we can’t get away from it. Even in deep space, there are particles, about one per cubic meter, so a rocket does run into things and slow down.
If you increase the pulling force on an object at a constant rate, the friction force increases at the sam rate until the object begins to slip. At that point, the pulling force is greater than the friction force, so there is a net force on the object and it will begin to accelerate. Once accelerating, the friction drops off even more because the molecules lose contact. That is the transition between static and sliding friction. Then the sliding friction becomes constant.
Here's a schematic graph of the force of friction when you pull on an object. At first, static friction increases. Then it drops off as the object starts to move. Then it becomes constant as the object slides.
If sliding friction were greater than static friction, you could never get the object moving. No matter how hard you pulled while the object was not moving, the friction of sliding would be more. Even if they were equal, how would the object know whether to stay in place or slide?
Friction resists motion.
Can you measure the friction on something? Is friction constant?
1 Spring scale
1 school backpack
1. Work in groups of two.
2. Record your observations.
3. Place a backpack on a table. Hook the spring scale onto the backpack and slowly increase your pull horizontally (sideways) on it.
- Note the readings on the scale before the backpack moves.
- Pull the backpack at a constant speed and note the readings on the scale.
4. Put some more weight on the backpack and repeat.
1. When you pull on the backpack, does the scale always read the same, even when the object is sitting still?
2. What is the force keeping the backpack from moving? Is that force constant or does it change? Describe what happens.
3. When something is moving, is there still a force resisting its motion? Is that force constant, or does it change? Describe what happens.
Galileo came up with the idea of inertia, which was a huge jump from the ideas of Aristotle. Aristotle was very logical and did a systematic study of nature. He thought that the earth didn't move. That makes common sense. He thought that rocks fell down because they're made of earth, and smoke went up because it was made of air. That seems reasonable, too. He believed in natural motion – by their nature, objects were at rest, and they only moved if they were pushed or pulled by a force. Throw a rock. It'll always stop. Again, common sense. His ideas were reasonable enough to last for 2,000 years.
Aristotle had the idea of force, but he missed something that Galileo found: friction. Friction is a force that resists motion. When Galileo understood that, he was able to separate friction from the ideal principle of inertia.
Friction always acts opposite to the direction of force or motion. If there is no force in a direction, there is no friction in the opposite direction. There are two main kinds of friction, static and sliding. Both increase if the force between the two objects increases. For example, a heavier object will have more friction than a lighter one of the same kind.
Have you ever pulled on something that won't move, and then suddenly it starts to move and it's easier to pull? The first part is static friction and the second part is sliding friction.
Static friction is the friction between two objects that aren't moving relative to each other. There is only as much static friction as there is force trying to make the objects move. For example, if a hockey puck is sitting on flat, level ice, it won't start moving because the force of gravity is straight down, so there's no sideways force. It doesn't matter if there's friction or not. Put the puck on a table that's flat and level, and there's still no sideways friction. But tip the table, or try to slide the puck, and friction appears, exactly equal and opposite to the force trying to make the puck move. Static friction grows until the object starts to move, then drops slightly and becomes sliding friction.
Sliding friction (also called kinetic or dynamic friction) stays the same regardless of the relative speed of the two objects (unless they lose contact by bouncing), and it is less than static friction. That is why anti-lock brakes are safer than older types. Once tires start sliding (skidding), the friction is less, so the car takes longer to stop. If the tires keep rolling, they have static friction with the road and the car can stop in less distance. Also, once the tires start sliding, you lose steering control.
Friction depends on the type of material in the objects, and friction is proportional to the force between the two objects. For example, if you double the weight of something, the sliding friction will double. Also, friction is nearly independent of area. A brick will have about the same amount of friction whether it is on its end, side, or flat part.
1. If Galileo was right, and things should keep moving forever because of their inertia, why do they stop?
2. Why do surfers wax their surfboards?
3. Why do skiers wax their skis?
4. Why do we put oil in car engines?
5. Name something that operates without friction.
6. If a box is sitting on a level floor, is there friction between the box and the floor? How much? How would you measure it? Can you think of a way to measure the friction without pushing or pulling the box?
7. If a box is sitting on the floor and you pull on it but it doesn’t move, is there friction between the box and the floor? How much?
8. Imagine yourself standing on an ice-covered pond with absolutely no friction. How would you get off the pond?
9. If you're trying to slide a couch across the living room and your brother climbs on it, will it be harder, easier, or just as hard to move it? If the weight of the couch is doubled, will the force needed to move it be double, less than double, or the same? Why?
10. Will wide or narrow tires on your car help you stop more quickly? Why?
1. Devise and run an experiment to check the proportionality of weight and friction.
2. Devise and run and experiment to check the independence of area and friction.
Newton - the measure of force. The symbol for Newtons is N.
Friction – a force which resists motion between two objects in contact with each other.
Static friction - friction between two objects that are not moving relative to each other.
Sliding friction - friction between two objects that are moving relative to each other.
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