For example, let's assume a chair here: It can be "slid" across the force if we use minimal upwards force, but not enough to actually "lift" the chair. Why should it move?

Here's a better example: I have seen some strong people lift and slide the back of cars, however, the back tires of the car never elevate completely off the ground. With regards to friction, though, how is it possible to actually "move" something without lifting it completely? How does this work is a better question. This image should illustrate the car example:

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Is there enough space within it to allow the angular force to make it "slid" a pinch, or what else would explain its ability to move without being completely suspended in the air that would, say, differ from this with a human?

  • $\begingroup$ Would your question still apply to a block on a table (with friction) without trying to lift it? Let's say you just push the block horizontally when it's at rest on a table. If you push hard enough, it will move, and you don't even have to lift it up. Is your question, then, why is it easier to get it to move when lifting it up? $\endgroup$ – BMS Nov 18 '13 at 20:44
  • $\begingroup$ @BMS Yes, and yes that's what I wanted to know as well, but for my example it's unlikely you can push a car with (human) force and get it to move off of stationary position(unless in neutral). $\endgroup$ – IWishForGreatAnswers Nov 18 '13 at 20:45
  • $\begingroup$ Basically, why is it easier to move it when pulling it at the same time without actually lifting it entirely? It still counts as "moving" the car, and it takes a lot of force, but doesn't 100% count as "lifting" it. How does this work? $\endgroup$ – IWishForGreatAnswers Nov 18 '13 at 20:47

The answer has to do with the form of static friction.

There are two types of friction, static and kinetic. Static friction is the type of friction that occurs when the bit of object that's touching the ground doesn't move (relative to the ground). Kinetic is the type that occurs when there is relative motion. Let's focus on static friction.

Consider a block sitting on a level, rough table. There will not be a frictional force. Why? Well, frictional forces are parallel to the surface (i.e., horizontal with the block on a rough table scenario). If there were a such horizontal force with nothing else acting on it horizontally, the block would be accelerating on its own. But we don't see this.

Now, magnitude of static friction forces can vary; it can range from zero up to some maximum value. The maximum value is $f_\text{max}=\mu_s N$, where $\mu_s$ is just a number that depends on the two materials in contact (e.g., wood block on plastic table), and $N$ is the upward normal force from the table. In order to start to move an object from rest, you have to exceed the magnitude of the static frictional force $\mu_s N$. When this starts to happen, the object will begin to move and kinetic friction will take over.

When you try to gently lift the block upward, but you don't actually let it leave contact with the table, the normal force $N$ decreases. This is because your upward force plus the normal force from the table has to counteract the downward gravitational force. If you apply a large upward force, the normal force doesn't have to be as large.

With this decrease in $N$ comes a decrease in $\mu_s N$, and hence a decrease in the maximum static frictional force. This means the force you need to apply in order to overcome $f_\text{max}$ is now less if you apply a small upward force to the block.

That's about it! Note that this is a very "macro" explanation that hinges on the form $f_\text{max}=\mu_s N$, which is just a very good experimental relationship. There are probably more microscopic explanations that could be more appealing to you.

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  • $\begingroup$ Nice answer. Do you end up using a greater magnitude of force overall when lifting and pushing, compared to just pushing? $\endgroup$ – Kenshin Nov 19 '13 at 7:45
  • $\begingroup$ @Chirs Do you mean is the sum of the magnitude of your upward force and new (smaller) horizontal force greater than just the single required horizontal force without lifting? If so, you can do a bit of addition to figure it out. You'll have to know that in the case without lifting, $N=mg$. $\endgroup$ – BMS Nov 19 '13 at 16:30
  • $\begingroup$ @Chris Come to think of it, it's probably better to add the horizontal and vertical vectors like vectors, which is made simpler since they are orthogonal to each other. $\endgroup$ – BMS Nov 19 '13 at 16:41

So, your question is how one can move something sideways without complete elimination of friction by lifting it into air. On a molecular level objects do not really "touch" each other but keep a minimal distance due to the repulsive interaction of electromagnetic forces. So when somebody stands on the ground she is actually "hovering" a little above the ground. Now, when an object lies on a plane then friction is the resistance to forces which act horizontally on the object. The molecular source of this friction is the "roughness" of the plane and the object: no surface is completely smooth but there are little molecular "bumps" which act as an obstacle to move sideways (see this Wiki article for example). Now there is a point where the horizontal forces are big enough to overcome this resistance and move over those bumps: this is the minimal amount of force needed to move an object horizontally. The strong man lifting cars partially only reduce the friction enough to move it sideways, because the friction is proportional to the weight that is in contact with the surface.

So this is a very rough explanation of friction. It has to be taken with a grain of salt because objects do not just "hover" above the plane due to electromagnetic repulsion but it is also possible to enhance friction by use of van der Waals forces. For example geckos and spiders use such an effect.

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  • $\begingroup$ Could you explain further? I don't mean simpler, but in more detail. The Wiki link just points to the atomic level ... I want to know what digresses evenness. And that is vague because some people can actually lift a car (they do not need to be "strong man" to do this either) completely in the air, but others are less strong, however, can still "slid" the car over a few inches, or possibly feet. $\endgroup$ – IWishForGreatAnswers Nov 18 '13 at 20:53
  • $\begingroup$ No, sorry, I don't know a more detailed explanation. $\endgroup$ – hauntergeist Nov 18 '13 at 21:28

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