## Dual-Range Force Sensor

Experiment
7
Static and Kinetic Friction If you try to slide a heavy box resting on the floor, you may find it difficult to get the box moving. Static friction is the force that is acting against the box. If you apply a light horizontal push that does not move the box, the static friction force is also small and directly opposite to your push. If you push harder, the friction force increases to match the magnitude of your push. There is a limit to the magnitude of static friction, so eventually you may be able to apply a force larger than the maximum static force, and the box will move. The maximum static friction force is sometimes referred to as starting friction. We model static friction, Fstatic, with the inequality Fstatic s N where s is the coefficient of static friction and N the normal force exerted by a surface on the object. The normal force is defined as the perpendicular component of the force exerted by the surface. In this case, the normal force is equal to the weight of the object.
Once the box starts to slide, you must continue to exert a force to keep the object moving, or friction will slow it to a stop. The friction acting on the box while it is moving is called kinetic friction. In order to slide the box with a constant velocity, a force equivalent to the force of kinetic friction must be applied. Kinetic friction is sometimes referred to as sliding friction. Both static and kinetic friction depend on the surfaces of the box and the floor, and on how hard the box and floor are pressed together. We model kinetic friction with Fkinetic = k N, where k is the coefficient of kinetic friction.
In this experiment, you will use a Force Sensor to study static friction and kinetic friction on a wooden block. A Motion Detector will also be used to analyze the kinetic friction acting on a sliding block.
OBJECTIVES Use a Force Sensor to measure the force of static friction. Determine the relationship between force of static friction and the weight of an object. Measure the coefficients of static and kinetic friction for a particular block and track. Use a Motion Detector to independently measure the coefficient of kinetic friction and compare it to the previously measured value.
Determine if the coefficient of kinetic friction depends on weight.
MATERIALS Power Macintosh or Windows PC Sensor Graphical Analysis or graph paper LabPro or Universal Laboratory Interface string Logger Pro block of wood with hook Vernier Motion Detector balance or scale Vernier Force mass set
Experiment 5
PRELIMINARY QUESTIONS 1. In pushing a heavy box across the floor, is the force you need to apply to start the box moving
greater than, less than, or the same as the force needed to keep the box moving? On what are you basing your choice?
2. How do you think the force of friction is related to the weight of the box? Explain.
PROCEDURE Part I Starting Friction
1. Measure the mass of the block and record it in the data table.
2. Connect the Force Sensor to Channel 1 (Student Force Sensor and Dual-Range Force Sensor) of the LabPro or Universal Lab Interface or to the PORT 1 input (ULI Force Probe) of the Universal Lab Interface.
3. Open the Experiment 12 folder from Physics with Computers. Then open the experiment file that matches the force sensor you are using. Set the range switch on the Force Sensor to 50 N. One graph will appear on the screen. The vertical axis will have force scaled from 0 to 20 Newtons. The horizontal has time scaled from 0 to 5 seconds.
4. Tie one end of a string to the hook on the Force Sensor and the other end to the hook on the wooden block. Place a total of 1 kg mass on top of the block, fastened so the masses cannot shift. Practice pulling the block and masses with the Force Sensor using this straight-line motion: Slowly and gently pull horizontally with a small force. Very gradually, taking one full second, increase the force until the block starts to slide, and then keep the block moving at a constant speed for another second.
5. Sketch a graph of force vs. time for the force you felt on your hand. Label the portion of the graph corresponding to the block at rest, the time when the block just started to move, and the time when the block was moving at constant speed.
6. Hold the Force Sensor in position, ready to pull the block, but with no tension in the string. Click at the top of the graph to set the Force Sensor to zero.
7. Click to begin collecting data. Pull the block as before, taking care to increase the force gradually. Repeat the process as needed until you have a graph that reflects the desired motion, including pulling the block at constant speed once it begins moving. Print or copy the graph for later reference. Choose Store Latest Run from the Data Menu to store the run as Run 1 for later analysis.
Part II Peak Static Friction and Kinetic Friction
In this section, you will measure the peak static friction force and the kinetic friction force as a function of the normal force on the block. In each run, you will pull the block as before, but by changing the masses on the block, you will vary the normal force on the block.
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