Module 3—Effects of Force on Velocity

Lesson 3 Lab: Friction

The applet used for this lab lets you simulate the force exerted on a spring (force) scale as it pulls a stack of books across a rough table surface. You can learn more about the simulation and how to use it by reading Show Me found at the top of the simulation screen.

Problem 1

What is the difference between kinetic friction and static friction?

Open the Friction simulation; then continue with the procedure. You may be required to login with a username and a password. Contact your teacher for this information.

Procedure

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  Using the simulation, stack 5 books, which are attached to the force scale as shown. Use the  button to add a book.
  
  
  
  

   

   

   

   

   

• Click in the "Show FBD" square () at the top to toggle on the free-body diagram. Note the behaviour of the FBD as you pull on the open ring of the force scale attached to the bottom book. The red vector represents the frictional force (), and the brown vector represents the applied force ().
  
• 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. Notice that once the books begin to move, the force required to keep the books moving reduces.

Observations and Analysis

Module 3: Lesson 3 Assignment

Remember to submit the answers to LAB 1, LAB 2, and LAB 3 as part of your Module 3: Lesson 3 Assignment.

LAB 1. What is the maximum force applied before the books start to move?

LAB 2. What is the applied force required to keep the books moving?

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. Note that kinetic friction is smaller in magnitude than static friction.

LAB 3. Complete the following questions.

1. What happens to the size of the static frictional force as you start to pull on the force scale?
   
   
2. Is there a maximum size for the static frictional force? If so, what happens to the object if the applied force exceeds the maximum static frictional force?
   
   
3. Is there a minimum size for the static frictional force? If so, under what conditions will this force be a minimum?
   
   
4. What type of frictional force acts when the system is at rest?

Problem 2

What is the mathematical expression for kinetic friction?

Re-open the Friction simulation, if necessary. Then continue with the procedure.

Procedure

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. Opposing the normal force is the object's weight. The weight (W) is represented in green. 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.

Observations and Analysis

Module 3: Lesson 3 Assignment

Remember to submit the answers to LAB 4 and LAB 5 to your teacher as part of your Module 3: Lesson 3 Assignment.

LAB 4. Using the applet, measure the applied force required to keep the books moving. Then complete the following table. To calculate the normal force, assume that each book has a mass of 1.00 kg. Use the  button to remove books, and use the “Reset” button to start over.

LAB 5. 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). Label the graph appropriately.

Graph 1: Kinetic Frictional Force vs. Normal Force

LAB 6. Complete the following questions.

1. Which of the following alternatives best describes Graph 1: Kinetic Frictional Force vs. Normal Force?
   
   
   1. The graph is constant and of the mathematical form y = b, where b is constant.
   2. The graph is linear and of the mathematical form y = mx + b, where b is zero and m is the slope.
   3. The graph is a quadratic curve and of the form y = ax2 + bx + c, where a, b, and c are coefficients.
      
      
2. Use your answer to LAB 6.a. to write an equation expressing the relationship between kinetic frictional force and the normal force.

The equation you derived in LAB 6 should have the form y = mx, where m is the slope of the equation. This means that the equation for the kinetic frictional force is , where

• coefficient of kinetic friction: ratio of friction force to normal force once two objects in contact stop moving as one object
  equals the magnitude of the kinetic force of friction (y-axis)
• FN equals the magnitude of the normal force (x-axis)
• equals the coefficient of kinetic friction (slope)

LAB 7. Using Graph 1, calculate the coefficient of kinetic friction () for the book-table interface. According to the slope calculations, does have any units?

Conclusion

Frictional forces are complicated. They depend not only on the two objects involved but also their conditioning at the moment of the interaction of the two objects. For example, roads made of asphalt become very slippery when freezing rain or snow covers the surface, and tires lose their grip when they are worn. Furthermore, even if all these circumstances are fixed, the frictional force also depends on whether an object slides (kinetic) or is stationary (static) with respect to the other surface. This gives rise to two basic forms of friction.

Kinetic Friction

When an object slides, it experiences a frictional force, .

Notice that the formula only involves magnitudes of forces. There is no vector sign above the F. The directions must be determined separately.

Kinetic frictional force always opposes motion. It acts opposite to the direction of motion.

Static Friction

When an object is at rest, a static frictional force resists motion.

The formulas for the two different types of friction are similar. But the constants of friction differ. To find the , the coefficient of kinetic friction  () is used. To find the , the coefficient of static friction () is used. FN is the magnitude of the normal force in both formulas.
 

The maximum static frictional forces are larger than the kinetic frictional forces.

Physics 20 © 2008 Alberta Education