Students will require a working knowledge of normal force, weight, free body diagrams, and graphical analysis.
Students will be able to graphically determine an expression for kinetic friction and calculate kinetic, static and rolling friction in a variety of situations. Students will also be able to qualitatively and quantitatively compare kinetic, static, and rolling friction.
Students should understand the applet functions that are described in Help and ShowMe. The applet should be open. The step-by-step instructions on this page are to be done in the applet. You may need to toggle back and forth between instructions and applet if your screen space is limited.
The applet will be used to demonstrate the difference between Kinetic and Static Friction. | |
On the applet, stack the five books that are attached to the force scale as shown below.
a. Using the mouse, slowly pull on the open ring of the force scale and observe the reading on the scale just as the stack of books begins to move. What is the maximum force applied before the books start to move? ________ N b. Once the books begin to move, the force required to keep the books moving is reduced.What is the applied force required to keep the books moving? _________ N |
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Before the stack of books begins to move, the force of friction is called Static Friction. After the motion begins, the stack of books is subject to Kinetic Friction. In your own words, describe the relationship between the size of the Static Frictional Force and the Kinetic Frictional Force. |
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Click "FBD" () to display the FBD drawing. In Figure 1, the red vector represents the frictional force (Ff) while the brown vector represents the applied force (F). Note the behavior of the FBD as you pull on the force scale.
Figure 1 a. What happens to the size of the static frictional force as you start
to pull on the force scale? c. Is there a minimum size for the static frictional force? If so,
under what conditions will the minimum static frictional force be observed? |
The force of friction arises from an interaction between surfaces. In the case of the books and rough table surface, the table surface exerts a force on the bottom book. This force is the normal force (FN) and is represented in blue on Figure 1. Opposing the normal force is the object's weight. The weight (W) is represented in green on Figure 1. For the books resting on the table, the normal force is equal in magnitude to the weight. The normal force and weight are "balanced". The magnitude of the normal force can be calculated by equating it to the weight. For example,
FN = W
FN = mg
Even though friction is an extremely complex phenomenon, there is a very simple relationship between the magnitude of the frictional force and the normal force.
a. Using the applet, measure the applied force required to keep the books moving and complete the following table. To calculate the normal force, assume that each book has a mass of 1.00 kg.
b. Complete Graph 1 by plotting the Normal Force on the x-axis (manipulated variable) and the Kinetic Frictional Force on the y-axis (responding variable).
Number of Books | Normal Force (N) FN = mg |
Kinetic Frictional Force (N) |
1 | _________ | _________ |
2 | _________ | _________ |
3 | _________ | _________ |
4 | _________ | _________ |
5 | _________ | _________ |
6 | _________ | _________ |
7 | _________ | _________ |
8 | _________ | _________ |
9 | _________ | _________ |
10 | _________ | _________ |
Kinetic Frictional Force vs.Normal Force
Which of the following best describes Graph 1: Kinetic Frictional Force - Normal Force? Use your answer to write an equation expressing the relationship between Kinetic Frictional Force and the Normal Force.
The graph is constant and of the mathematical form y = b (b is constant).
The graph is linear and of the mathematical form y = mx + b (b is zero and m is the slope).
The graph is a quadratic curve and of the form y = ax2 +
bx + c (a, b and c are coefficients).
Equation: ________________
The equation should have the form y = mx where m is slope of the equation. This means that the equation for the kinetic frictional force is Fkinetic = μFN where,
Fkinetic = | kinetic force of friction (y-axis) |
FN = |
normal force (x-axis) |
μ= |
coefficient of kinetic friction (slope) |
Using Graph 1, calculate the coefficient of kinetic friction (μ) for the
book - table interface. According to the slope calculations, does μ have
any units?
This gives rise to three basic forms of friction:
Rolling Friction |
Kinetic Friction |
Static Friction |
A rolling tire will still experience a small frictional force. |
When an object slides, it experiences a frictional force. |
When an object is at rest a static frictional force resists motion. |
Usually, rolling frictional forces are much smaller than kinetic and static frictional forces. |
Kinetic frictional force always opposes motion. It acts opposite the direction of motion. |
The maximum static frictional forces are larger than the kinetic frictional forces. |
* μrolling, μk and μs are the coefficient of rolling, kinetic, and static friction and FN is the normal force.
1. The coefficient of kinetic friction between a block and the level surface it slides on is 0.45. If the mass of the block is 10.0 kg, what is the minimum force needed to keep the block moving with uniform motion?
The applied force need only balance the kinetic frictional force in order to maintain uniform motion.
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2. A student pulls on a 5.00 kg object and discovers that she needs to exert 30.0 N of force before the object moves. What is the coefficient of static friction between the object and the surface on which it rests?
Since the applied force is 30.0 N just before moving, the frictional force must be equal in size. Therefore,
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3. A 7.30 kg box is at rest on a level table. The coefficient of static friction between the box and table is 1.03. How big is the static frictional force?
There is no static frictional force acting since there is no net force attempting to move the box . The static frictional force could, at most, be equal to ustaticFN. |
Devise a method to determine the coefficient of static friction used in the applet. Explain how you found the coefficient and give its value. |
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A student drew the free body diagram for a box sitting at rest on the floor. Explain what is wrong with this diagram. |
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The coefficient of static friction between a book and the level surface it slides on is 0.65. If the mass of the book is 2.0 kg, what is the minimum applied force required to move the books? An engine provides 300.0 N of force to keep a 1600 kg vehicle moving at a uniform speed (air resistance is negligible). What is the coefficient of rolling friction between the tires and the road surface? A 100.0 kg crate is at rest in the back of a truck. If the coefficient of static friction between the crate and the truck bed is 0.30, what is the maximum acceleration that the truck can have before the crate begins to slide? Draw a Free Body Diagram and explain what force causes the crate to accelerate along with the truck. Anti-lock brakes prevent car tires from locking up and skidding. Explain why a car can stop faster and safer if the tires do not skid. |
Physics 20-30 v1.0
©2004 Alberta Learning (www.learnalberta.ca)
Last Updated: June 16, 2004