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#### /0

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##### Inclines and friction
« on: March 09, 2008, 04:24:18 pm »
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A loaded penguin sled weighing $80 \ \mbox{N}$ rests on a plane inclined at angle $\theta = 20^{\circ}$ to the horizontal. Between the sled and the plane, the coefficient of static friction is $0.25$, and the coefficient of kinetic friction is $0.15$.
(a) What is the least magnitude of the force $\overrightarrow{F}$, parallel to the plane, that will prevent the sled from slippig down the plane?
(b) What is the minimum magnitude $F$ that will start the sled moving up the plane?
(c) What value of F is required to move the sled up the plane at constant velocity?

Thankyou... I'm having a hard time grasping the concept of static & kinetic friction.

#### Collin Li

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##### Re: Inclines and friction
« Reply #1 on: March 09, 2008, 04:41:41 pm »
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It's not on the physics course.

I believe "static friction" refers to the "coefficient of friction" that Specialist Maths often refers to. These two terms are not on the course of either Physics or Specialist Maths, so I'm not sure what "kinetic friction" is, but it sounds like it could be the scalar multiple of the negative velocity vector (the air resistance force vector is proportional to the negative of the velocity vector, i.e.: it opposes the direction of motion)
« Last Edit: March 09, 2008, 04:43:31 pm by coblin »

#### Mao

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##### Re: Inclines and friction
« Reply #2 on: March 09, 2008, 05:06:39 pm »
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divideby0 you will not encounter these problems in physics, (the worst will be "this object experiences 30N of friction....)

but if you need to know:

friction is the retardation force when two objects rubs together. It represents the imperfection in the surface of the material. The friction force is directly proportional to the normal force, that is, the harder it is pulled down towards the surface, the harder it is to move along it. the constant "k" in this relationship is your friction coefficients

The static coefficient represents the friction when the object is not moving, and allowing even some of the imperfections on the atomic level to lock together. The kinetic coefficient is lower, as movement does not allow the smaller imperfections to "lock" together, however, larger imperfections still do.

These coefficients are averages, as imperfection is not spread evenly, but rather randomly.

and there can be frictionless surfaces, these are either impossibly hard to produce (nanomachines with perfect surfaces), or are found under extreme gravitational forces (the surface of a neutron-star)

the general equation for friction is Ffriction=k Fnormal, where k is the appropriate coefficient for the given case (where velocity>0 use kinetic otherwise use static, unless of course you are dealing with rolling friction which is different yet again)