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Chapter 5: Lesson 3 - Friction
Chapter 5: Lesson 3 - Friction
Equations
Equations
We are going to expand our available math equations in this lesson. It will be helpful to have your equation sheet ready. A copy of the equation sheet is under resource materials on this website and on google classroom. It can also be found HERE.
Friction
Friction
In lesson 1, we learned about the different types of contact forces. Friction is a drag force. It slows the motion down and points in the opposite direction of the applied force.
Two different types of friction
Two different types of friction
There are two different types of friction, kinetic friction and static friction.
Kinetic friction can sometimes be called sliding friction. Kinetic friction is exerted on one surface by another when the two surfaces rub against each other because one or both of them are moving.
Static friction is the force exerted by another when there is no motion between the two surfaces.
Static friction is always greater than kinetic friction. If you have ever pushed something really heavy, then you know that it takes more force to start something moving than it does to keep it moving.
Moving with a constant velocity
Moving with a constant velocity
Remember from chapter 4, lesson 3, we learned that if an object is moving at a constant velocity, there is no acceleration because it it not speeding up or slowing down.
Looking at the equation F=ma, when there is no acceleration, there is no net force. If a=0 m/s2 , then mass times 0 will also equal 0. Your net force is 0 N.
If the net force is zero, then the applied force on the object is equal to the frictional force.
If you pull a block along a surface at a constant velocity, according to Newton's Second Law of Motion, the frictional force must be equal and opposite to the force with with you pull.
You can pull a block of known mass along a table at a constant velocity and use a spring scale, as shown in the picture, to measure the force that you exert.
You can then stack additional masses on the block to increase the normal force and repeat the measurement for different normal forces.
Plotting the data from the experiment above on a graph looks like the one shown here. The graph shows kinetic frictional force, found by moving the object at a constant velocity along the table vs the normal force, found by the weight of the object. There is a direct proportion between the kinetic frictional force and the normal force.
The different lines correspond to dragging the block along different surfaces. The steeper the slope, the rougher the surface. The less steep of a slope on the graph, the slippery the two surfaces are.
Coefficient of Friction
Coefficient of Friction
You would expect it to be much harder to pull the black along sandpaper than along a polished table, so the slope must be related to the resulting frictional force.
The slope of this line is called the coefficient of friction. We abbreviate the coefficient of friction with a greek letter, mu (𝛍.) There is a coefficient of static friction, μs and a coefficient of kinetic friction, 𝛍k. There is a coefficient of friction for every two substances. It is a constant that tells us how slippery or sticky those two substances are. The lower the coefficient of friction the slippery the two substances are. The higher the coefficient of friction, the stickier the two substances are.
The static frictional force acts in response to a force trying to cause a stationary object to start moving. If there is no such force acting on an object, the static force is zero.
If there is a force trying to cause motion, the static friction force will increase up to a maximum value before it is overcome and motion starts. Once the motion starts, then you would have kinetic frictional force.
Kinetic Frictional Force
Kinetic Frictional Force
The coefficient of kinetic friction between the two surfaces and relates the frictional force to the normal force. The kinetic friction force is equal to the product of the coefficient of the kinetic friction and the normal force.
Static Frictional Force
Static Frictional Force
In the equation for the maximum static friction force, μs is the coefficient of static friction between the two surfaces, and μsFN is the maximum static friction force that must be overcome before motion can begin.
Sample Coefficients of Friction
Sample Coefficients of Friction
Here is a table with sample coefficients of static and kinetic friction.
Notice that the static coefficient of friction are larger than the kinetic coefficient of friction. It takes more force to get something moving than it does to keep it moving.
Although all the listed coefficients are less than 1.0, this does not mean that they must always be less than 1.0. If a coefficient of friction is greater than one, then the frictional force is greater than the normal force. This means that the two substances are very sticky, but are very light.
Check Your Understanding:
Check Your Understanding:
Click on the down arrow when you have your answer to check to see if you are correct.
Look at the table of coefficients of friction above. What substances are the most slippery when they are not moving?
Steel on steel (with oil). This has the lowest coefficient of static friction.
2. Look at the table of coefficient of friction above. What substances are the most sticky when moving?
Rubber on dry concrete. It has the highest coefficient of kinetic friction.
Math
Math
Click here to see the paper that I used in the video.
As you watch this video, pause it and try the math on your own. When you have your answer, start the video and see if you are right. It is important to know where YOU make your mistakes so you can correct them for next time!
Remember there are more practice math problems under Chapter 5 Resources. Practice the math and see if your answer matches my answer.
Before the Next Class Period:
Before the Next Class Period:
Practice these Math Problems Before Class!
Practice these Math Problems Before Class!
Click on the picture to take you to the practice problems that I will be going over the next time we have class. Remember, you need to try the problems on your own to see where you make your mistakes if you want to learn how to do the math problems on your own.
You have finished Lesson 3!
You have finished Lesson 3!
Be sure to head over to google classroom and fill out the exit pass.
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