Overview of Experiments with Friction
by Ron Kurtus (revised 16 November 2012)
Friction is a force that resists motion. It is usually caused when two materials rub against each other. Besides the types of material, there are a number of other factors that affect the friction. Friction experiments usually concern measuring these factors, determining the relationship between factors, and optimizing the values to either increase or decrease friction. Sometimes unique or new methods can be invented.
(See Understanding Friction for a list of friction lessons.)
Questions you may have include:
- What are friction factors?
- How are they measured?
- What experiments can determine relationships or optimal values?
This lesson will answer those questions.
There are three major types of friction: static, kinetic (sliding), and rolling.
Possible factors involved in friction experiments are:
- Force applied to objects
- Materials of each surface
- Surface hardness (or softness)
- Elasticity of surfaces
- Roughness of surfaces
- Flatness of surfaces
- Radius of wheel, in the case of rolling friction
- Contact surface area
- Force between surfaces
- Temperature of materials
- Speed of motion
- Treads, if applicable
- Dampness of surfaces
- "Stickiness" of one or both surfaces
These factors are also called parameters or variables.
Experiments require measurements or data-taking. Not only do you need to measure the characteristics of the parameters or factors you are varying, but you also need to measure the applied force on an object and the resistive force of friction.
Coming up with clever ways to measure things is one of the greatest challenges in performing experiments.
Forces involved in friction are the force moving the objects, the force pushing object surfaces together—which is often the force of gravity—and the resistive force of friction.
Often a scale, such as a spring scale, is used to measure those forces.
Other parameters such as surface hardness, elasticity, speed and temperature must be measured, if they are part of the experiment. Some parameters are difficult to measure, and you may need to depend on charts and measurements done by others.
Keeping things constant
Even if you can't get exact measurements for a parameter, you want to make sure you keep things constant, so that you can effectively compare your results.
The purpose or objective of your experiment may be to find a relationship between factors or to optimize one or more factors to find the best value.
You may want to determine the relationship between the force of friction and one or more factors or parameters through your experiment.
For example, suppose the purpose of your experiment is to find the relationship between kinetic friction (Fr) and the force pushing two objects together (N). By keeping everything else constant and just varying Fr and N, your experiments should show that the relationship is:
Fr = μN
- Fr is the force of friction causing motion
- μ is the kinetic coefficient of friction, determined in the experiment
- N is the normal or perpendicular force pushing the objects together
- μN is the product of μ times N
You could also see what the relationship is between the surface area of two objects and the force of friction, keeping everything else constant.
Typically, you can pick one factor and vary it, while keeping all other factors constant. In this way, you can determine an optimal value.
For example, you could examine the relationship of surface roughness of one material to the kinetic friction. In this way, you might select an optimal surface roughness to use for either more or less friction.
Friction experiments concern determining the important factors involved in causing friction, measuring these factors, and determining the relationship between factors or optimizing the values to either increase or decrease friction. Sometimes unique or new methods can be invented.
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Overview of Experiments with Friction