Kinetic Friction by Acceleration

Objectives

• Use a Motion Detector to determine the coefficient of kinetic friction
• Determine if the coefficient of kinetic friction depends on weight.
• Graphs & Tables: manually recording graphs, tabulating data values, graphing results

Introduction

With a box sliding, friction will act to slow it to a stop. The friction acting on the box while it is moving is called kinetic friction. Kinetic friction is sometimes referred to as sliding friction. Friction depend on the surfaces of the box and the floor, and on how hard the box and floor are pressed together. Kinetic friction is modelled with F_kinetic = µ_kN  where µ_k  is the coefficient of kinetic friction.

In this experiment, you will use a Motion Detector to analyze the kinetic friction acting on a sliding block.

This experiment will be done without the resource of a computer printer. Students will be expected to sketch graphs obtained into their notes along with recording relevant values. Tables for data and analysis are also to be setup manually in students notes.

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Preliminary Questions

1. Show a free body diagram for a block sliding on a level surface with friction.

2. Describe how normal force is different than weight for a block sliding on an incline

3. Describe how acceleration is used in this experiment to obtain the coefficient of kinetic friction.

Procedure

The coefficient of kinetic friction will be determined from acceleration. Using a Motion Detector, you can measure the acceleration of the block as it slides to a stop. This acceleration can be determined from the velocity vs. time graph. While sliding, the only force acting on the block in the horizontal direction is that of friction. From the mass of the block and its acceleration, you can find the frictional force and finally, the coefficient of kinetic friction. Overview

1. Setup the experiment with a Motion Sensor clamped to the end of your bench. Load the relevant experiment from a link under web browser favorites. The software will show two graphs: distance/time and velocity/time.

2. Test sliding the block toward the Motion Detector so that the block leaves your hand and slides to a stop. Minimize the rotation of the block. After it leaves your hand, the block should slide about meter before it stops and should not come any closer to the Motion Detector than half meter.

3. Test collecting data by clicking software "Collect" button and give the block a push so that it slides toward the Motion Detector. The velocity graph should have a portion with a linearly decreasing section corresponding to the freely sliding motion of the block.

4. Test analyzing the linear region of your velocity/time graph. Select a region that shows the linear decreasing speed of the block (dragging the mouse over this section). The slope of this section of the velocity graph is the acceleration. Determine the slope by clicking the Linear Regression button.

5. Record in your notes an example sketch of the graph and this value of acceleration.

Data Collection

1. Setup a data table for recording acceleration of three trials with different masses. (For each mass, do three trials.) 

2. Conduct the experiment  and collect data for a range of masses (at least  five different masses)

Analysis

1. Setup a results table with average acceleration (of the three trials), average friction force (calculated from Newton's second law F = ma)  and total normal force of the sliding block.

2. Plot a graph of average acceleration and normal force .

•  Does the acceleration depend on the weight of the block?

3. Calculate the mean (average) value of your accelerations from all trials.  

4. Calculate the mean absolute deviation, which is a calculated value for the uncertainty of a mean value.  The equation for the mean absolute deviation can be found here

5. From the mean value of the acceleration determine the coefficient of kinetic friction.  Use a value of g = 9.81 m/s^2, as an exact value for your calculation.