Download Pasco Friction Expt

Sliding Friction
INTRODUCTION
The purpose of this experiment is to find the coefficient of static friction and the
coefficient of kinetic friction for different surfaces. As it pulls a Friction Tray from rest to
a constant velocity, the Force Sensor can measure both the static friction and the kinetic
friction. A plot of each of these forces versus their respective normal forces yields both
coefficients.
THEORY
When a force is applied to an object resting on a surface, it will not move until the force
applied to it is greater than the maximum force due to static friction. The coefficient of
static friction (µs) is simply the ratio between the maximum static frictional force (Fs) and
the normal force (FN):
To keep the object moving at a constant velocity, a force must be applied to the object
equal to the kinetic frictional force. Hence, the coefficient of kinetic friction (µk) is the
ratio between the kinetic frictional force (Fk) and the normal force (FN):
PROCEDURE
1. Measure the mass of the Friction Tray and each bar mass. Record these values.
2. With no tension on the string, press the "tare" or "zero" button on the force sensor.
3. Place one mass in the Friction Tray. Attach one end of the string to the tray. Attach
the other end to the hook of the Force Sensor.
4. Press the START button.
5. Place the Friction Tray on a rough surface (carpet works well). With the force sensor
tied to the tray, slowly pull the Friction Tray horizontally, from rest, across the lab
station until it reaches a constant velocity. Continue pulling at a constant velocity for
5 seconds.
6. Use the cursor to highlight the region that corresponds to the maximum static friction.
The legend box gives the value of the maximum Static Friction. Record this value
into a STATIC data table. Also, enter the value of the Friction Tray's normal force.
7. Use the cursor to highlight the region where the velocity is constant. The legend box
gives the MEAN or average Kinetic Friction. Enter this value into a corresponding
KINETIC data table. Also, enter the value of the Friction Tray's normal force.
8. Repeat steps 4-8 with two masses on the Friction Tray.
9. Repeat steps 4-8 with three masses on the Friction Tray.
10. Repeat steps 4-8 with four masses on the Friction Tray.
11. Click on the graph labeled "Friction v Normal."
12. Enter the data from your static frictional force trials and create a plot of Fs vs FN. Fit it
to "Linear." Record the slope and vertical intercept.
13. Enter the data from your static frictional force trials and create a plot of Fk vs FN. Fit
it to "Linear." Record the slope and vertical intercept.
14. Repeat the previous steps using the other Friction Trays.