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pg 76 - 76 Chapter 5 Using Newton’s Laws We ve said this...

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Unformatted text preview: 76 Chapter 5 Using Newton’s Laws We ve said this before, but it’s worth noting again: Force doesn’t cause motion but rather change in motion. The direction of an object’s motion need not be the direction of the force on the object. That‘s true in Example 5.7, where the car is moving horizontal— ly at the top of the loop while subject to a downward force. What is in the same direc- tion as the force is the change in motion, here embodied in the center—directed acceleration of circular motion. WWW GOT IT? 5.3 Do the riders in the roller coaster of Example 5.7 need seatbelts to keep them from falling out of the car? Explain. Friction results from fllese regions where surfaces adhere. 5.4 Friction Your everyday experience of motion seems inconsistent with Newton’s first law. Slide a book across the table, and it stops. Take your foot off the gas, and your car coasts to a stop. But Newton’s law is right, so these examples show that some force must be acting. That force is friction, a force that opposes the relative motion of two surfaces in contact. On Earth, we can rarely ignore friction. Some 20% of the gasoline burned in your car is used to overcome friction inside the engine. Friction causes wear and tear on machinery and clothing. But friction is also useful; without it, you couldn‘t drive or walk. The Nature of Friction Friction is ultimately an electrical force between molecules in different surfaces. When two surfaces are in contact, microscopic irregularities adhere, as shown in Fig. 5.17a. At the macroscopic level, the result is a force that opposes any relative movement of the surfaces. Experiments show that the magnitude of the frictional force depends on the normal force between surfaces in contact. Figure 5.l7b shows why this makes sense: As the nor- Hfilllilill FriCtiO” originates in the ma] forces push the surfaces together, the actual contact area increases. There’s more ad» contact between two surfaces. herence, and this increases the frictional force. At the microscopic level, friction is complicated. The simple equations we’ll develop here provide approximate descriptions of frictional forces. Friction is important in every— day life, but it’s not one of the fundamental physical interactions. With increased normal force, there’s more contact area and hence greater friction. (I!) As the applied force This is the maximum increases, so does _frictional force. Frictional Forces the frictional force. _,:' . . The net force remains 5" Now me ap lied force Try pushing a heavy trunk across the floor. You start puslung, and at first nothing happens. :9“: 3:11;“? 0'39“ exceeds mg“ and Push harder; still nothing. Finally, as you push even harder, the trunk starts to slideeand o n e. . . . . , . . c .- the obiect accelerates. you may notice that once 1t gets gomg, you don t have to push quite so hard. Why is that? ': 1:53:20“! “me With the trunk at rest, microscopic contacts between trunk and floor solidify into rela— : ' tively strong bonds. As you start pushing, you distort those bonds without breaking them; g Accelerating they respond with a force that opposes your applied force. This is the force of static i; W1 a _ friction, f5. As you increase the applied force, static friction increases equally, 'as shown in ‘5: Fig. 5.18, and the trunk remains at rest. Experimentally, we find that the maximum static- ???” \i\§\\\\\§\®\§§&§m&m{&mkememahmme‘eeweememirates.‘sxximemfine fs S ugn (static friction) (5.2) Time—a Here the proportionality constant as (lowercase Greek mu, pronounced “myfi,” with the 01"“ agf‘i“ “mo“ bai‘fnccs subscript s for “static”) is the coefficient of static friction, a quantity that depends on the {Eififflfgfiiffgflf‘fii‘e two surfaces. The 5 sign indicates that the force of static friction ranges from zero up to object moves with constant the maximum value on the right—hand side. Speed. Eventually you push hard enough to break the bonds between trunk and floor, and the W Behavior of frictional forces. trunk begins to move; this is the point in Fig. 5.18 where the force suddenly drops. Now ...
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