![]() ![]() Textbook content produced by OpenStax is licensed under a Creative Commons Attribution License. We recommend using aĪuthors: Paul Peter Urone, Roger Hinrichs Use the information below to generate a citation. Newton’s third law tells us that if object A applies a force on object B, then object B applies a force equal in magnitude but in the opposite direction back on object A. Then you must include on every digital page view the following attribution: If you are redistributing all or part of this book in a digital format, Then you must include on every physical page the following attribution: If you are redistributing all or part of this book in a print format, ![]() Want to cite, share, or modify this book? This book uses the ) and difficult to predict theoretically, but shear stress is yielding a fundamental understanding of a large-scale phenomenon known since ancient times-friction. At small but nonzero speeds, friction is nearly independent of speed.įigure 5.2 Frictional forces, such as f f size 12 ![]() Once an object is moving, there are fewer points of contact (fewer molecules adhering), so less force is required to keep the object moving. Adhesion varies with substances in contact and is a complicated aspect of surface physics. Part of the friction is due to adhesive forces between the surface molecules of the two objects, which explain the dependence of friction on the nature of the substances. The harder the surfaces are pushed together (such as if another box is placed on the crate), the more force is needed to move them. A considerable force can be resisted by friction with no apparent motion. So when you push to get an object moving (in this case, a crate), you must raise the object until it can skip along with just the tips of the surface hitting, break off the points, or do both. Close-up inspection of these surfaces shows them to be rough. Furthermore, if you oiled the concrete you would find it to be easier to get the crate started and keep it going (as you might expect).įigure 5.2 is a crude pictorial representation of how friction occurs at the interface between two objects. Frictional forces, such as the traction needed to walk without slipping, may be beneficial, but they also present a great measure of opposition to motion. If you add mass to the crate, say by placing a box on top of it, you need to push even harder to get it started and also to keep it moving. friction, force that resists the sliding or rolling of one solid object over another. Once in motion it is easier to keep it in motion than it was to get it started, indicating that the kinetic friction force is less than the static friction force. But if you finally push hard enough, the crate seems to slip suddenly and starts to move. This means that the static friction responds to what you do-it increases to be equal to and in the opposite direction of your push. Imagine, for example, trying to slide a heavy crate across a concrete floor-you may push harder and harder on the crate and not move it at all. If two surfaces are in contact and moving relative to one another, then the friction between them is called kinetic friction. ![]()
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