Purpose:
The overt purpose of this lab is to explore the basic features of the standard physics model for friction. Physics models are descriptions of how the world appears to behave based on our best understanding of the situation. Sometimes we have clear ideas about how the physics works so the model is driven from core concepts -- think sandbox lab as an example. Sometimes we are much less clear about the root causes and the details of a particular effect and the model is more descriptive. Friction between rigid objects is one of these less well understood settings. This lab will explore some features of that basic physics model.
Procedure:
This lab will be a mix of rough estimates and slightly more careful measurements. In each case it will be important to make a freebody diagram and be sure you understand the net forces on your object before diving in. The Engineer's Handbook has this list of coefficients of friction which will be useful for comparison.
- 1) Take your block (could be wood, metal, or any material really) and slide it across the desktop or countertop. The distance it takes to come to a stop is affected by friction and some other factors. Make sure you identify the other factors. Can you reproduce the stopping distance multiple times? Can you determine/estimate the speed at which you are sliding the object from what you learned in the sandbox lab? Using kinematics can you determine the acceleration of your object as it comes to a stop? Using Newton's 2nd Law determine an estimate for μk - the kinetic coefficient of friction. What is a mathematical expression you could use easily to determine μk for any other surface in the room or the hall?
2) For kinetic friction the physics model is Ff = μk * FN. If you were to plot the frictional force as a function of the normal force what should the plot look like? In this next section you will use a spring scale to drag an object across the desktop or counter at a constant velocity. Make a freebody diagram of the object being dragged and determine the relationship between the spring force and the frictional force. Gather data to test the physics model proposed at the begining of this section. Plot your data and determine μk , the kinetic coefficient of friction, that is consistent with your data. Compare to your more casually estimated result from part 1.
- 3) The physics model for the frictional force between surfaces that are not moving is more complex because static friction is tricky. The simple model is Ff ≤ μs * FN . Using the spring scale how can you verify this model? Test the model for a number of different normal forces. Does your data confirm the general assertion that the coefficient of static friction is greater than the coefficient of kinetic friction?
- 4) Another approach: Make a freebody diagram for an object sitting on a ramp and not moving. From your freebody diagram determine the relationship between μs and the ramp angle. We will use the cart tracks as the ramp since they are easily available. First determine μs using your technique from part 3 and then lift the ramp and determine μs using the ramp angle. Compare your two results.
LAB DELIVERABLES:
- I) Describe your data and method for determining μk using kinematics and the stopping distance. Be sure to present your formula for doing so. If you knew the coefficient of friction could you predict the initial speed of the object? This is a standard technique used in accident reconstruction.
- III) Present your data, and method for determining μs using the spring scale. What is your assessment of the validity of the standard physics model for static friction.
- IV) Present your data, and method for determining μs using the ramp method. Compare your results from part 3 and part 4 and discuss their consistency or lack thereof.
- Friction Rubric
