We have claimed that the magnitude of the force of friction on a moving object depends only upon the normal force on the object, n, and the types of material of which the object and the surface are made, which is accounted for with the coefficient of kinetic friction, µk. Specifically we claim
fk = µk n.
In particular, this says that the force of kinetic friction does not depend on other things such as amount of surface area in contact, sliding velocity, or other quantities. This lab will investigate these claims.
To begin set up the force sensor as you did in the Acceleration, Force & Mass Lab. This time you will only need one graph, that of the force vs. time, so drag the graph display icon onto the Force, Ch A. You will want to enlarge this graph window and click on the statistics icon. Tare the force sensor and it should be ready to go. The general procedure will be to use the force sensor to pull a block along the table. The force on the sensor will balance (and hence equal) the force of kinetic friction, fk. You will do this with different amounts of weight on the block and with it moving with the flat side down and with it on its side. By adding weights to the block, you are changing the normal force, n, which is simply the mass of the block and any added weights times g. There are several things to watch for as you do this. First, you need to make sure the table top is free from debris like eraser crude, drink spills, etc. You need to pull with the force sensor level at all times, but making sure it does not drag on the table itself. You also need to pull at a constant rate. This is best accomplished by walking along with the block and sensor as you pull it.
PART I: BLOCK FLAT
Begin by massing the block. Lay the block flat, begin recording data, and steadily pull the block along the table with the force sensor. Once you stop the data run, autoscale your graph. You should see a relatively flat “valley” in your graph. (Remember that a pull on the sensor gives a negative force.) Highlight this relatively flat region and read the mean off the stats window. Remember to take the absolute value of it. Add a 200-g mass to the middle of the block and repeat the procedure. Continue repeating the procedure as you add weights, keeping them as close to the middle of the block as possible. Use weights of 400 g, 600 g, 800 g, 1000 g, 1200 g and 1400 g. For each run, record the frictional force, the added mass, the total mass and the normal force.
PART II: BLOCK ON SIDE
Repeat the procedure you followed in Part I, this time with the block on its side. Start with no mass on the block, then do runs with 200 g, 400 g, 600 g, 800 g, 1000 g, 1200 g and 1400 g, recording the same data you did in Part I.
PART III: DEPENDENCE OF fk ON SPEED
Lay the block flat again and place 400 g of mass on it. Pull the block slowly across the table—as slowly as you can and still keep the motion steady. Record the force of friction as you did in Parts I and II. Now pull the block across the table at a medium speed and then at a rapid speed, recording fk both times.
ANALYSIS
Construct a graph of fk vs. n for Part I (with the block flat). Also construct a graph of fk vs. n for Part II (with the block on its side). Assume each graph is linear and put in a best fit line. Find the slope of this line. Then answer these questions.
1. Do your graphs support the relation fk = µkn? In other words, are graphs linear; do they pass through the origin?
2. What quantity does the slope of your graphs represent?
3. Do you see any significant dependence of fk on the contact area (block flat or on its side)?
4. From Part III is there a significant dependence of fk on speed?
5. Do you think that fk = µkn is a “good” physical law?