Eugene P. Mosca
Physics Department United States Naval Academy
Annapolis, MD 21402
(410) 293-6659 Office
(410) 293-3729 Fax
Advanced Laboratory or Lecture Demonstration
To demonstrate kinetic friction, a wooden block is placed on a horizontal surface that is moving at constant speed. The block is held stationary by a horizontal string connecting it to a spring scale. Additional mass is placed on the block, and the increased force required to preventing it from sliding is immediately observed.
Approximate size: 3 ft wide by 2 ft high by 1 ft deep.
Does this apparatus require Electrical Power? yes
Will you be present to set up your apparatus? yes
Other support needed for the proper operation of this apparatus: None
To demonstrate kinetic friction, a wooden block is placed on a horizontal cotton denim belt that is moving at constant speed. The block is prevented from moving by a string connecting it to a stationary spring scale as shown in Figure 1. As additional mass is placed on the block, the increased force required to prevent it from moving is observed. This apparatus demonstrates that an increase in the normal force results in a corresponding increase in the kinetic frictional force.
Figure 1. The moving belt pulls the block to the left via a kinetic frictional force. The block is prevented from moving by the string attaching it to the spring scale.
Typically, the relation between the normal force and the kinetic frictional force is demonstrated by pulling a block along a horizontal stationary surface with a horizontal string. If the block moves at constant speed, it follows from Newton's second law that the tension in the string equals the frictional force. Additional mass is then added to the block and the measurement is repeated.
If the student is a Newtonian thinker, this demonstration can effectively reveal the correct empirical relation between the frictional and normal forces. However, we know that many of our students are not yet Newtonian thinkers. These students are likely to interpret the observations as follows: "Increasing the mass of the block requires an increase in the tension in the string needed to maintain motion at constant speed because to maintain a larger mass at constant speed requires an larger net force." To these students, the demonstration does not reveal the correct empirical relation between the frictional and normal forces.
However, by performing the demonstration with the block stationary and the surface moving, the correct empirical relation between the frictional and normal forces is revealed. This is so even for those students with non-Newtonian beliefs. Interestingly, these students (who believe that a net force is required to maintain motion at constant speed) believe that no net force is required for the block to remain at rest. That is, they believe that the tension in the string is balanced by the kinetic frictional force.
A plot of actual data gives
where Fk is the frictional force and FN is the normal force.
Demonstrating the empirical relation between the kinetic-frictional and the normal forces by keeping the block stationary and the surface moving is superior because:
• Students will interpret the demonstration correctly. Students who incorrectly believe that a net force is required to maintain motion at constant speed will still be able to understand the empirical relation between the frictional and normal forces.
• The feedback occurs in real time. As soon as additional mass is placed on the friction block, the scale-reading changes. Real-time feedback is very difficult with the moving block/stationary surface demonstration because when additional mass is added to the block it is difficult to maintain the block's speed, and it is cumbersome to add mass to a moving block.
• The velocity can be varied in a controlled manner. By using a variable-speed belt sander, effects due to the relative speed between the block and the belt can be demonstrated.
Construction and Assembly
A wooden friction block was fabricated from a scrap piece of lumber (a 2 by 4). A depression was milled into the top of the blocks so weights could be placed in the depression.
A 3" X 21" belt was fabricated using the Stitch Witchery and a steam iron to make the seam. If the belt is either too loose or too tight, reapplying the steam allows one to modify its length. The seam was cut at an angle rather than straight across
We needed is a 5-N spring scale that can be read from the back of a classroom. Unable to locate one, we produced one by combining parts from two Ohaus scales -- a 5-N Dial Spring Scale and a 20-N Demo Spring Scale. The 20-N scale has a stiffer spring and a larger face. Other than that the two spring scales are identical. The faces of the two scales were exchanged, resulting in a 5-N spring scale that could be read from the back of a classroom.
The sander is clamped upside down on a horizontal lecture table using the clamps purchased for that purpose. The denim belt is put on the sander and the wooden friction block is placed on the belt. The spring scale is mounted so it's hook is the same height as the hook eye on the sanding block, and a string connects the sanding block to the spring scale.
|1||3" X 21" variable-speed belt sander||RYOBI
BE321 Type II
|1||Pair Clamps||RYOBI #6071273||Home Depot||Special
|1||5-N Dial Spring Scale||OHAUS||Central Sci. Co. (CENCO)||05511N||12.00|
|1||20-N Demo Spring Scale||OHAUS||Central Sci. Co. (CENCO)||05150N||21.80|
|1||Iron-on seam binder||1"-wide Stitch Witchery||Local fabric store||2.50|
|1||30" X 3" cotton denim||Local fabric store||2.50|