Phy Chapter 3 Friction Caltec S.2

5.4 FRICTION Friction is a force that opposes relative motion between two surfaces in contact with one another. It results from two surfaces rubbing against each other or moving relative to one another. It can hinder the motion of an object or prevent an object from moving at all. The strength of frictional force depends on: The nature of the surfaces that are in contact, The magnitude of the force pushing them together and The weight of the object or objects. In cases involving fluid friction, the force depends upon The shape of the object moving through the fluid, Viscosity and Speed of an object as it moves through the fluid. The Causes of Friction Friction is thought to be caused by: Small irregularities on the rough surfaces becoming interlocked. E.g. rubbing two files together. The attractive force between the surface molecules. This factor seems to play major role than interlocking of the small irregularities. (b) Types of friction There are two types of friction, namely; (i) Static friction and (ii) Dynamic (Sliding or kinetic) friction. 1. Static friction Static friction is the frictional force between two surfaces that are not sliding over each other. (a) Facts about Static Friction Static friction occurs between stationary objects. It prevents an object from moving against a surface. It is the force that keeps an object e.g. a book from sliding off a desk, even when the desk is slightly tilted, and that allows you to pick up an object without the object slipping through your fingers. Static friction depends on: - The coefficient of static friction (µs) between the surfaces of the objects in contact, - The nature of the surface and - The normal reaction (R) of the object. NB: In order to move something, you must first overcome the force …