# Important Forces: Friction

by Jared Rovny

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00:01 Now that we've introduced three of these five important forces, that we're going to discuss, gravity, the normal force and tension and seeing how they work in some problems.

00:09 We're going to move to friction.

00:11 So the friction force arises from small interactions between two surfaces, if we really zoomed in, what we would see is that the surface of this blue box as I've drawn it and the surface that it's resting on are not entirely smooth in fact that they have all sorts of ridges in them.

00:27 What happens is if one object tries to move over the object, these ridges will catch with each other and push each other either to the left or to the right depending on the motion of the object.

00:37 The force of friction if we wrote it down would be exactly what you would expect if you were trying to derive this on your own.

00:44 Friction basically depends on two things, it depends on what I'm calling mu here, it's a Greek letter mu that looks like a funny u.

00:50 This is telling me how rough a surface is so it certainly depends on how rough the surface is.

00:56 So our force of friction which I'm calling F-sub-F force of friction depends on the roughness of the surface which is given by this Greek letter mu but it also depends on F-sub-n, the normal force.

01:08 In other words, how hard are these things pushing against each other? The normal force is exactly the normal force that we've introduced already in this lecture so you can solve for this normal force from a problem and then find the frictional force based on that normal force.

01:24 The mu that I just mentioned which describes how rough or smooth the surface is, is bigger for very rough surfaces like the one on the left here and is smoother for small, or sorry it's a smaller for smooth surfaces like ice or something where those ridges don't really have catches on each other.

01:41 Finally, you can see that this force air that I've drawn is pointing upwards because anytime we're talking about forces, we're considering forces on a particular object and the object in question when considering the normal force is the box, not the ground.

01:55 And so the force of the ground on the box, will be in the upwards direction and that's our normal force but this does not mean that the force of friction is pointing upwards because the normal force is just use as a magnitude in this equation for friction.

02:09 Finally, the direction of friction since it's certainly not upwards out of the surface, is given by the velocity of your object. If your object is moving to the right, as I have shown here, the force of friction will act always to oppose that motion.

02:21 If the velocity is to the left then the force will be acting to the right to oppose that motion.

02:26 So to find the direction of friction always remember that friction opposes motion all the time.

02:31 There's one caveat to the equation that I gave you which is that the equation for friction that I gave you, force of friction is equal to mu-sub-k the kinetic friction times the force, the normal force is only exactly correct in the case of kinetic friction, what we mean by kinetic friction is that an object is actually moving along the surface.

02:53 So if the object is moving, and you know that the object has a non-zero velocity, it's actually moving in your problem, then you can always use this equation that the force of friction is equal to the coefficient of friction mu times the normal force.

03:07 I put a little k under the mu to say that this is the coefficient of kinetic friction because it turns out that coefficient of friction, how rough surfaces are and how much the frictional force depends on the roughness of the surface actually depends on whether the object is moving or not.

03:22 So an object will have a different coefficient of friction if it's moving then it will have if it's sitting still and so that brings us to the static case.

03:30 Suppose an object is sitting still, like anything you have in your table, that's just not moving and suppose you tried to move it, the frictional force will not necessarily be equal to the coefficient of friction times the normal force but it will be less than or equal to the coefficient of friction times the normal force.

03:48 This coefficient of friction we now call the coefficient of static friction or mu-sub-s and the reason it's less than or equal to is this.

03:56 Take an object that you have on your table and give it a tiny push that it doesn't move.

04:00 Since the object isn't moving, even though you're pushing on it very slightly, you know that there must be some opposing force, because otherwise it would be moving in the direction you're pushing it.

04:10 That opposing force is friction because the object doesn't wanna move even though you're pushing it.

04:15 The thing is, if you slightly increase the force, so that the object still isn't moving, the frictional force will react again and it's still not moving.

04:23 So you can see that the frictional force is changing and adapting to how much you push it.

04:27 If you keep pushing, the frictional force gets stronger and stronger eventually you'll reach a point, a threshold at which the frictional force can no longer combat the amount that you're pushing that object like your book or something and eventually, the object will move.

04:41 It will move when your force that you're applying is greater than mu-sub-s the coefficient of static friction times the normal force of your object and that's what we mean when we say that the force of friction for an object that is not moving is less than or equal to mu-sub-s times f-sub-n because it could be much smaller than that threshold if you're not pushing very hard because it doesn't really need to have the full value.

05:04 It is just going to act to try to get the object to stay still.

05:07 One last very important thing is that the coefficient of kinetic friction is less than the coefficient of static friction.

05:15 In other words, kinetic friction is weaker than static friction.

05:18 This is borne out by experience. You know that if you're pushing something and it's hard to get it moving, once you've got them moving and beating friction, it easier to keep it moving.

05:27 And that's because once it's moving, the frictional force is less because the roughness of the surface will matter less if the object is already moving.

The lecture Important Forces: Friction by Jared Rovny is from the course Force.

### Included Quiz Questions

1. Friction always acts in a direction to oppose the motion.
2. Friction always acts in the same direction as the normal force.
3. Friction is a directionless force and just produces heat.
4. Friction always acts with the direction of an object’s motion.
5. Friction only acts directionally for motionless objects.
1. The static frictional force is greater than the kinetic friction.
2. The static frictional force is less than the kinetic friction.
3. The static and kinetic frictional forces must always be equal.
4. Static and kinetic frictional forces must be equal and opposite.
5. Kinetic friction will always act simultaneously to the static friction.
1. The static frictional force will act to oppose other forces, even if they are small until a certain threshold is reached.
2. The static frictional force must be less than the kinetic friction.
3. The static friction must be less than the coefficient of static friction.
4. The static friction will act to oppose other normal forces, but will always be less than the normal force it is opposing.
5. The static friction will act to oppose the kinetic and normal forces until they are too great.
1. It quantifies the roughness of the surface.
2. It quantifies the strength of the normal force.
3. It quantifies the coefficient of the normal force.
4. It provides a ratio of kinetic and static forces.
5. It is the force, in Newtons, that friction causes on an object.

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