A car’s speedometer, for example, shows the magnitude (but not the direction) of the instantaneous velocity of the car. Over such an interval, the average velocity becomes the instantaneous velocity or the velocity at a specific instant. When we carry this process to its logical conclusion, we are left with an infinitesimally small interval. The smaller the time intervals considered in a motion, the more detailed the information. ( See mechanics.) The Editors of Encyclopaedia Britannica This article was most recently revised and updated by Adam Zeidan.\): A more detailed record of an airplane passenger heading toward the back of the plane, showing smaller segments of his trip. This advantage was realized historically with the transition from sledge to wheel. Coefficients of sliding friction are generally 100 to 1,000 times greater than coefficients of rolling friction for corresponding materials. The internal losses in the two substances are similar to those that keep a ball from bouncing back to the level from which it is dropped. The elastic deformation or compression produced at the leading section of the area in contact is a hindrance to motion that is not fully compensated as the substances spring back to normal shape at the trailing section. If a hard ball is rolling on a level surface, the ball is somewhat flattened and the level surface somewhat indented in the regions in contact. The main source of friction in rolling appears to be dissipation of energy involved in deformation of the objects. Rolling friction occurs when a wheel, ball, or cylinder rolls freely over a surface, as in ball and roller bearings. This smallest force required to start motion, or to overcome static friction, is always greater than the force required to continue the motion, or to overcome kinetic friction. The value of static friction varies between zero and the smallest force needed to start motion. Static friction, in contrast, acts between surfaces at rest with respect to each other. Because the friction thus far described arises between surfaces in relative motion, it is called kinetic friction. The frictional force itself is directed oppositely to the motion of the object. The value of the coefficient of friction for a case of one or more bricks sliding on a clean wooden table is about 0.5, which implies that a force equal to half the weight of the bricks is required just to overcome friction in keeping the bricks moving along at a constant speed. Because both friction and load are measured in units of force (such as pounds or newtons), the coefficient of friction is dimensionless. This constant ratio is called the coefficient of friction and is usually symbolized by the Greek letter mu ( μ). Thus, the ratio of friction F to load L is constant. If a pile of three bricks is pulled along a table, the friction is three times greater than if one brick is pulled. Second, friction is proportional to the load or weight that presses the surfaces together. If a brick is pulled along a table, the frictional force is the same whether the brick is lying flat or standing on end. First, the amount of friction is nearly independent of the area of contact. Two simple experimental facts characterize the friction of sliding solids. SpaceNext50 Britannica presents SpaceNext50, From the race to the Moon to space stewardship, we explore a wide range of subjects that feed our curiosity about space!.Learn about the major environmental problems facing our planet and what can be done about them! Saving Earth Britannica Presents Earth’s To-Do List for the 21st Century.100 Women Britannica celebrates the centennial of the Nineteenth Amendment, highlighting suffragists and history-making politicians.COVID-19 Portal While this global health crisis continues to evolve, it can be useful to look to past pandemics to better understand how to respond today.Student Portal Britannica is the ultimate student resource for key school subjects like history, government, literature, and more.
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