We discussed previously that when an object rests on a horizontal surface, there is a normal force supporting it equal in magnitude to its weight. The coefficient of friction is unitless and is a number usually between 0 and 1.0. 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. The importance of the work-energy theorem, and the further generalizations to which it leads, is that it makes some types of calculations much simpler to accomplish than they would be by trying to solve Newton’s second law.Review the concept of friction. If you leave out any forces that act on an object, or if you include any forces that don’t act on it, you will get a wrong result. When calculating the net work, you must include all the forces that act on an object. If an object speeds up, the net work done on it is positive.
(credit: “Jassen”/ Flickr)Īccording to this theorem, when an object slows down, its final kinetic energy is less than its initial kinetic energy, the change in its kinetic energy is negative, and so is the net work done on it.
The work done by the horses pulling on the load results in a change in kinetic energy of the load, ultimately going faster. Let’s start by looking at the net work done on a particle as it moves over an infinitesimal displacement, which is the dot product of the net force and the displacement:įor the mathematical functions describing the motion of a physical particle, we can rearrange the differentials dt, etc., as algebraic quantities in this expression, that is,įigure 7.11 Horse pulls are common events at state fairs. Therefore, we should consider the work done by all the forces acting on a particle, or the net work, to see what effect it has on the particle’s motion.
Workdone on a mass moving on rough surface how to#
We have discussed how to find the work done on a particle by the forces that act on it, but how is that work manifested in the motion of the particle? According to Newton’s second law of motion, the sum of all the forces acting on a particle, or the net force, determines the rate of change in the momentum of the particle, or its motion. Use the work-energy theorem to find information about the forces acting on a particle, given information about its motion.Apply the work-energy theorem to find information about the motion of a particle, given the forces acting on it.By the end of this section, you will be able to: