The coefficient of friction is unitless and is a number usually between 0 and 1.0, but there is no theoretical upper limit to its value. If the floor were lubricated, both coefficients would be much smaller than they would be without lubrication. Would keep it moving at a constant speed. 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.į k = μ k N = ( 0.30 ) ( 980 N) = 290 N f k = μ k N = ( 0.30 ) ( 980 N) = 290 N 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. Magnifying these surfaces shows that they are rough on the microscopic level. If, on the other hand, you oiled the concrete you would find it easier to get the crate started and keep it going.įigure 5.33 shows how friction occurs at the interface between two objects. If you were to add mass to the crate, (for example, by placing a box on top of it) you would need to push even harder to get it started and also to keep it moving. Once in motion, it is easier to keep it in motion than it was to get it started because 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. ![]() You may push harder and harder on the crate and not move it at all. ![]() Imagine, for example, trying to slide a heavy crate across a concrete floor. Look at the table of static and kinetic friction and ask students to guess which other systems would have higher or lower coefficients. ![]() Explain the concept of coefficient of friction and what the number would imply in practical terms. Ask students which one they think would be greater for two given surfaces. Start a discussion about the two kinds of friction: static and kinetic.
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