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Explanation of the resistive force of friction - Succeed in Physical Science. Also refer to Physics, sliding, rolling, fluid, surface roughness, deformation, plowing, molecular attraction, coefficient of friction, normal force, ultra-smooth, stickiness, Ron Kurtus, School for Champions. Copyright © Restrictions Resistive Force of Frictionby Ron Kurtus (revised 15 February 2008) When two objects or materials are in contact with each other and a force is applied to one of them, the force of friction resists the motion caused by the force. There are three different types of friction, which are sliding, rolling and fluid friction. If the object does not move, the friction is considered static. If the force is sufficient to move the object, the friction is called kinetic. The cause of friction is a combination of molecular adhesion, surface roughness, and deformation effects. Questions you may have include:
This lesson will answer those questions. There is a mini-quiz near the end of the lesson. Useful tools: Metric-English Conversion | Scientific Calculator. Sliding frictionWhen two solid objects are in contact and a force is applied to slide one object against the other, sliding friction force resists the motion. If F is the force pushing on an object and Fr is the force of friction, the relationship between F and Fr will determine whether the object will slide or not move at all. Kinetic frictionIf force F is greater than friction Fr (written as F > Fr), then the object will slide or move. The friction is considered kinetic friction, which means moving friction.
Pushing force greater than friction force Static frictionIf F is less than Fr (written as F < Fr), there is no motion and the objects remain static with respect to each other. In this case, the friction is considered static friction, which means it is not moving. Static > KineticWhat is interesting is that the static friction that holds an object in place is greater than the kinetic friction that slows down a moving object. In other words, once you start an object moving, the friction decreases from the static friction holding the object in place. You have seen this in trying to slide a heavy box across the floor. It may be very difficult to move, but once it starts sliding, it is easier to push. Causes of sliding frictionThe causes of sliding friction are molecular attraction or adhesion between the materials, surface roughness of the materials, and deformation resistance in the case of soft materials. Rolling frictionWhen a ball or wheel is in contact with a solid object and a force is applied to the wheel, it will start to roll due to the friction at the point of contact with the other surface. This is the starting friction for a wheel. Once the wheel starts rolling, there is a resistive force that slows the wheel's motion on the other surface. This is called rolling friction. Starts rollingWhen a force is applied to a wheel is not enough to overcome the static force of friction, the wheel will start to roll. If the force is greater than the static resistance, the wheel will slide or spin. It will also roll, but not at the same rate as with static friction. A good example of this is accelerating an automobile on wet pavement. Pushing on the accelerator peddle too hard will cause the wheels to spin, and the car will not move forward as fast as when you push on the gas peddle less. Slows rollingOnce the wheel is rolling, friction at the point of contact with the other surface slows down the motion of the wheel. Typically, rolling friction is much less than sliding friction. A wheel can roll for some distance before slowing down and stopping. But there are situations where rolling friction can be large. Trying to ride a bicycle in loose dirt is an example of friction greatly slowing down the rolling motion. Less than slidingAn advantage of rolling friction is that it is much less than sliding friction. When the Great Pyramids were being build in ancient Egypt, they used logs as rollers under the giant blocks of granite instead of trying to slide the rocks along the ground.
Rollers reduced friction when moving granite block Causes of rolling frictionThe causes of rolling friction are similar to that of sliding friction. They are molecular attraction or adhesion between the materials, surface roughness of the materials, and deformation resistance in the case of soft materials. Included in surface roughness is the existence of treads on the wheel or tire. Fluid frictionWhen a solid object is in contact with a fluid, such as a liquid or gas, and a force is applied to either the object or the fluid, there is a friction force that resists the motion. Examples where fluid friction occurs are water flowing through a hose, an airplane flying through the atmosphere, and oil lubricating moving parts. Static and kineticIf the viscosity or thickness of the fluid is great, there may be no movement due to static friction. One example is trying to move heavy grease through a hose. You need to apply a great pressure to finally break the static friction and start the grease moving. Once a fluid moves through a hose or an object is moving along a fluid, the resistance is considered kinetic friction. The grease will still move much slower than a fluid with low viscosity, like water. Note that it is also possible to have fluid friction with moving one fluid in contact with another fluid. That subject is usually classified as part of Fluid Dynamics and is not within the scope of our lessons. Causes of fluid frictionCauses of fluid friction are turbulence effects from surface roughness and deformities, molecular attraction or adhesion between the materials, and deformation resistance of the fluid. SummaryFriction is a force that resist the motion of the object that is in contact with another object or material. The different types friction are sliding, rolling and fluid friction. When the objects don't move, the friction is called static. When they do move, the friction is called kinetic. The cause of friction is a combination of molecular adhesion, surface roughness, and deformation effects. Don't let the resistive forces of others slow you down ResourcesThe following resources provide information on this subject: WebsitesFriction Concepts - HyperPhysics Friction - Wolfram Research Science World BooksThe following books are available from Amazon.com. Top-rated
books on Friction for Children Top-rated
books on Friction Physics Top-rated
books on Friction Experiments BasicComplete Idiot's Guide To Physics by Johnnie T. Dennis; Alpha (2003) $18.95 What Is Friction? (Ages 4-8) by Lisa Trumbauer; Children's Press (CT) (2004) $4.95 AdvancedFriction Science and Technology (Mechanical Engineering Series) by Peter J. Blau; Marcel Dekker Pub. (1995) $89.95 Physics of Sliding Friction (NATO Science Series E:) by B.N. Persson, E. Tosatti; Springer Pub. (1996) $358.00 MiscellaneousMini-quiz to check your understanding1. Why is it harder to start to slide a heavy box across the floor than to keep it sliding? 2. What is an advantage of using rollers instead of sliding an object? 3. Why is it easier to move water through a hose than grease? If you got all three correct, you are on your way to becoming a Champion in Physical Science. If you had problems, you had better look over the material again. What do you think?Do you have any questions, comments, or opinions on this subject? If so, send an email with your feedback. We will try to get back to you as soon as possible. Share linkFeel free to establish a link from your website to pages in this site. 
Or use our form to send this link to yourself or a friend. Students and researchersThe Web address of this page is Please include it as a reference in your report, document, or thesis. Where can you go from here?
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