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Frictional forces, kinetic and static—Figures 5.17 and 5.18 |
· Friction can keep an object from moving or slow its motion from what we last calculated on an ideal, frictionless surface.
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· Microscopic imperfections cause nonideal motion.
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Frictional forces, kinetic and static |
A frictional force (摩擦力) is exerted on the body by the surface. It is parallel to the surface and directed so as to oppose the sliding. If the body does not slide, the frictional force is a static frictional force  (静摩擦力). If there is sliding , the frictional force is a kinetic frictional force  (动摩擦力). |
If the body does not move, then the static frictional force  and the component of F that is parallel to the surface are equal in magnitude and opposite in direction. |
The magnitude of  has a maximum value  , where  is the coefficient of static friction (静摩擦系数) and N is the magnitude of the normal force. |
If the body begins to slide along the surface, the magnitude of the frictional force rapidly decreases to a constant value  , where  is the coefficient of kinetic friction (动摩擦系数).
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Applied force is proportional until the object moves—Figure 5.19 |
· Notice the transition between static and kinetic friction.? |
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Coefficients of friction |
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Notice the effect of friction in horizontal motion—Figure 5.20 |
· Moving a 500 N crate has two parts: getting the motion to begin and then the effect of friction on constant velocity motion.
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The angle at which tension is applied matters |
· As one varies the angle at which tension is applied, force spent overcoming friction and force lifting the object are interplayed. These components affect each other.
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