Physics Lecture 34-Friction
For most common materials, which is less kinetic or static friction?
For most common materials, kinetic friction is less than static friction. This explains why when you push on a heavy piece of furniture, it resists you, but when it breaks free it accelerates, maybe even causing you to lose your balance.
What is a major way by which Mother Nature extracts her Ethermal 'tax' on any real physical process?
Friction
What is one means by which the 2nd Law of Thermodynamics exerts its inexorable influence in increasing the randomness of the Universe?
Friction
What prevents the existence of perpetual motion machines?
Friction
Is Friction dependent on the velocity with which the surfaces are passing by each other?
Friction is not dependent on the velocity with which the surfaces are passing by each other.
Is friction dependent upon the total surface areas in contact with each other?
Friction is not dependent upon the total surface areas in contact with each other.
What is Friction?
Friction is the force of resistance that two adjacent solid surfaces create while trying to slide past each other.
How many form does friction appears in?
Two
What do low values of μmean?
Low values of μ mean that the surfaces slide easily over each other.
What is another factor causing friction?
Another factor causing friction is attraction between the particles of the two samples that are sliding against each other.
Based on the last assumption listen above what should we see the coefficient of friction is?
Based on the last assumption listed above, you will see that the coefficient of friction should have values between 0 and 1.
What is Drag?
Drag is the resistance an aircraft encounters when it tries to move through the air.
What are examples of high values of μ?
Examples are dry glass in contact with dry glass, or a rubber tire in contact with dry asphalt pavement; in both these situations, μ is about 0.9. When we steer a car through a curve in the road, we are thankful that we have that μ = 0.9 working for us.
What are examples of low values of μ?
Examples are water ice with μ = 0.05, and Teflon® used in cookware with μ = 0.04. The Delrin® plastic wheel bearings on our PASCO carts have a very low coefficient of friction. The air pucks that we used in a previous lab had very low friction due to riding on a cushion of air. Because friction robs us of well-organized useful energy, there is an entire industry devoted to minimizing it by creating special materials or by developing lubricants that help surfaces slide more easily.
What is our math model for Friction?
F friction = (μ) F normal
Fill in the blank: ______ is the reason that you cannot ride a roller coaster for an unlimited time; you must get off at the platform, re-enter the ride, and again be pulled to the top of the first hill by a motorized chain.
Friction is the reason that you cannot ride a roller coaster for an unlimited time; you must get off at the platform, re-enter the ride, and again be pulled to the top of the first hill by a motorized chain.
When does Friction occur?
Friction occurs when the surfaces of two objects are rubbing against each other.
What do High values of μ mean?
High values of μ mean that the surfaces are rather sticky with each other and generate a large frictional force.
What coefficient of friction is it if motion has not yet begun?
If motion has not yet begun, it is the coefficient of static friction.
What coefficient of friction is it if motion is underway?
If motion is underway, then μ is the coefficient of kinetic friction. What should we notice what we get if we solve the equation for μ?= We get a ratio of two forces expressed in newtons, so μ itself is dimensionless (like pi).
What happens if we pull hard enough on the stationary object?
If we pull hard enough on the stationary object, our applied force will exceed the static friction force. The object will break free and start to move or to accelerate, depending on how hard we are pulling.
What will happen if we pull with any force that is less than the maximum static friction on the object?
If we pull with any force that is less than the maximum static friction for the object, then the object will just remain in place. The forces on the object are balanced.
What happens to static friction, if we stop pulling on the object?
If we stop pulling on the object, static friction goes immediately back to zero.
In a lab, what will you and your team study?
In a lab, you and your team will study the difference between static and kinetic friction, the effect of changing the normal force, and the effect of changing the material from which the sliding surface is made.
In practical situations, what will happen to the actual value of μ?
In practical situations, the actual value of μ will vary depending on the condition of the surfaces and the possible presence of other materials. A dry road usually gives us good traction for our tires, but rainwater reduces our traction and patches of ice reduce it even more.
What is called the coefficient of friction?
In the equation, the symbol that looks sort of like the letter 'u' is the Greek lower-case letter 'mu'.
What is one factor causing friction?
One factor causing friction is that most real surfaces are lumpy and uneven, so that when you try to drag them across each other they get hung up on the lumps and create an opposing force.
Fill in the blank: Part of _____ is skin friction which is the kinetic ______ caused by air moving across the surfaces of the aircraft.
Part of drag is skin friction which is the kinetic friction caused by air moving across the surfaces of the aircraft.
How large is static friction?
Static friction is only as large as it needs to be.
What is static friction?
Static friction is the force that tends to keep in place an object at rest on a surface when we pull on it.
What is the effect of friction?
The effect of friction is an energy loss. What do we mean by a loss of energy?= By 'loss' we mean that highly organized energy storage, such as the kinetic energy of moving bodies, becomes degraded into more random energy storage, as thermal (internal) energy within the two objects that are in contact.
What is the first form of friction?
The first form is static friction.
What is the force on the object after it breaks free known as?
The force on the object after it breaks free is known as kinetic friction.
What do the frictional forces on two adjacent objects form?
The frictional forces on two adjacent objects form a Newton's 3rd Law pair.
How large is the maximum friction force compared to the normal force?
The maximum friction force for any given situation can be no larger than the normal force.
If you take our ground school or aeronautical technology course, what phenomenon will you study?
The phenomenon of drag.
What is the second form of friction?
The second form of friction is kinetic friction.
As the great physicist Richard Feynman described in one of his freshman lectures at Cal Tech, what can this factor do?
This factor can actually make it impossible to measure friction in some special cases! Consider two samples of the same pure metal. In the high school physics lab, these samples are likely to be oxidized on their surfaces and also to be contaminated by dust, oils from our skin, etc. So when we measure the friction between the samples, we are not really seeing the behavior of two pure metals. In a more sophisticated lab we might be able to clean and polish the samples, and keep them in an evacuated container. However, under those conditions, when we put the two pure metal samples together they would meld into one instead of sliding past each other. The metal atoms along the surfaces would have no way of 'knowing' which sample they belonged to. The result is that we really have no good data on the frictional behavior of two pure metal samples in contact with each other.
Does Friction always acts to oppose motion (pending or actual)?
Yes
How can you see this lumpy-ness?
You can see this lumpy-ness by looking at 'smooth' surfaces with a microscope, where they suddenly no longer appear to be so smooth.
