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Experiment P-
Friction Force
Objectives
To learn about the relationship between friction force, normal force and friction coefficient. To observe changes in the friction force within different surfaces and different masses. To calculate the friction coefficients (static and kinetic) in these different conditions.
Modules and Sensors
PC + NeuLog application
USB-200 module
NUL-211 Force logger sensor
Equipment and Accessories
Rough and smooth wooden block with hook 20 cm of thread 1000 g mass 500 g mass
The items above are included in the NeuLog Mechanics kit, MEC-KIT.
Introduction
Friction force is created when two surfaces move or try to move against each other. The direction of the friction force is always opposite to the direction of motion (or motion attempt) and parallel to the surface, therefore it resists the relative motion of the surfaces. Friction force depends on the roughness of the surfaces, and the "normal force" which presses the surfaces together. It can be described by the following equation:
When the object is on a horizontal surface and there are not any other force components acting on it perpendicular to the surface, then the normal force equals the object's weight, thus:
The coefficient of friction is the ratio between the force of friction and the normal force that presses the surfaces together. Because it is a force divided by a force, does not have any units. Friction force depends on the object's mass (part of the normal force), however the friction coefficient is not a function of mass; it depends only on the properties of the surfaces.
When an object is at rest, the amount of force required to move it is usually greater than the force required to keep it moving. Therefore, typically there is a difference between the coefficient of static friction (before motion starts) and the kinetic friction (in motion). The static friction coefficient is calculated at the threshold of motion while the kinetic friction coefficient is calculated during motion (friction force is constant).
In this experiment we will use a force sensor to study the frictional force of different surfaces and different weights.
Sensor setup
- Connect the USB module to the PC.
- Connect the force sensor to the USB-200 module.
Note:
The following application functions are explained in short. It is recommended to practice the NeuLog application functions (as described in the user manual) beforehand.
- Run the NeuLog application and check that the force sensor is identified.
Settings
- Click on the On-line Experiment icon in the NeuLog main icon bar.
- Click on the Sensor's Module box.
- Click on the Push=Negative button to get positive values when pulling the hook.
Testing and measurements
Note:
The orientation of the sensor is very important. Before each measurement, hold the sensor in the same position you will use for the specific experiment and zero it.
- Sensor Zeroing: hold the sensor in its proper position (on the table) and press the button in the sensor’s box for about 3 seconds. Alternatively, you can click on the Reset button in the Module Setup box menu.
- Click on the Run experiment icon to start the measurement.
- Hold the force sensor on the table and slowly pull the block horizontally while increasing the force (the direction should be in a straight line); when you reach a certain force the block will start to move, keep the block moving at a about the same velocity until the end of the measurement. The part in which you increase the applied force should be done in around one second. Practice this until you see a relatively smooth motion in your graph.
- If needed, click on the Zoom fit icon (or click on the
Zoom icon and locate the mouse cursor at a point above the graph and press its left button; keep it pressed and create a rectangle that includes the whole graph).
- Your graph should be similar to the following:
- Save your graph.
- The sensor measures the resistance of the block to motion i.e. the friction force. At the beginning of the graph, the friction force is zero because the block does not move and there isn't any applied force on it.
As we apply force on the block by pulling it, the block resists and do not move, at this part, the friction force equals the applied force and is considered static friction.
The highest possible static friction value is called the transition point. We can use this value in order to calculate the static coefficient:
Before applying force on the bock
Transition point
Static friction Kinetic friction
The friction force at motion is 4.11 N. In order to calculate the kinetic friction coefficient, we should place this value in the following equation:
In our case the total mass is 1.24 kg, therefore mg = 1.24 kg x 9.8 m/s^2 (g, gravitational acceleration) = 12.15 kg x m/s^2 = 12.15 N.
To find the friction coefficients we divide each force value by mg (total weight).
Kinetic friction coefficie nt
Kinetic friction force [N]
Static friction coefficie nt
Max. static friction force [N]
Total mass [kg]
Mass on the block [kg]
Block's mass [kg]
Surface
4.1 1 N / 12.15 N = 0.
5.4 2 N / 12.15 N= 0. Rough 0.24 1 1.24 5.
- Fill the first row in the following table with your results and calculations. If you repeated the measurement, use the average values.
Kinetic friction coefficie nt
Kinetic friction force [N]
Static friction coefficie nt
Max. static friction force [N]
Total mass [kg]
Mass on the block [kg]
Block's mass [kg]
Surface
Rough 1
Smooth 1
Rough 1.
Smooth 1.
- Proceed in this way with the smooth side on the table (will the friction force be different? Will the friction coefficients be different?):
- Fill the second row of the table with your results and calculations.
- Proceed in this way with each side of the block, but now put also the 500 g mass on the block.
- Fill the third and forth rows of the table with your results and calculations.
