IIT JEE preparation tips for Friction Chapter in Physics

IIT JEE preparation tips

In this post you will be reading about IIT JEE preparation tips for Friction.

Friction forces

This is the story of a mysterious, shady, hidden interaction. When it was first discovered it revolutionized all sciences and set the bases for growth, with Galileo taking the lead. Let us introduce this topic with an experiment.

Suppose we have a box laying on the ground and we push it with lateral force… but, despite our intent, the box does not move:

IIT JEE preparation tips

In the picture on the left we see the forces that are acting on the box. The weight of the box, P, and the force that the floor applies to the box, N, are the forces that are in the vertical or Y axis. And in the direction parallel to the floor we have the force that we apply on the box while pushing it, F.

We do not see anyone else interacting with the box … but it does not move, therefore, it’s acceleration is zero. And, since we trust Newton and his second law, ΣFx = m ax, we have no doubt that there must be another force, also horizontal, of equal module as F and opposite direction in action. We shall call it the mysterious force, Fm.

Since we are strong willed and we are determined on moving the box, we can make an even stronger force, F‘; but if the box still does not budge we will have to conclude that the mysterious force is also capable of growing, and now it has a value equal to Fm.

If we continue to increase the force we make, we will find that the mysterious force continues to increase with it… but, luckily, there is always a value of push force, or traction force, that the mysterious force is unable to match, and from there on the equilibrium is broken and we are finally able to move the box.

Does the mysterious force disappears when the box is moving? The experiment tells us otherwise. If I stop pushing, the box stops again. And, to keep it moving – let’s say, at a constant rate – we must continue pushing but, fortunately, with a force that is not as strong as the one we had to make to put the box in motion.

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An important key of the mysterious force is provided by the next experiment: is similar to the last experiment, but it is made with two boxes, one on top of the other.


IIT JEE preparation tipsAn the result is qualitatively identical to the last experiment… but the maximum force we have to defeat to put the boxes in motion is greater than before… and the force to keep constant velocity once we got the boxes in motion is also greater.

And the only thing that changed between one experiment and the other one is the net value of the contact force N between the box and the floor.

Every shred of doubt disappears once we repeat the experiment again and again but changing the support surfaces, for example, on a wooden floor, tile, ice, and so on. The smother the surfaces, the lighter the force we have to make is. The rougher the surfaces, the bigger the force.

The mysterious force is the Friction between the surfaces that want to move, or that are already in motion. And the result of the experiments can be synthesized in this three very short expressions:

FricS= Tract.

FricSMax= μs. N

Fricdin= μd. N

The first equation says that the static friction force, FricS, is equal to the traction force (in our experiment, our push) that tries to put the bodies in motion.

The second equation says that the maximum static friction force, FricSMax ,is the one that is acting just before the bodies start moving and it is equal to the static friction coefficient, μs ,which represents the roughness of the opposed surfaces, by the contact force between the floor and the box, meaning it is the force by which the bodies that are functioning are joined, N.

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The third equation says that the dynamic friction force, Fricdin, is the one that acts while the bodies are already in motion and it is equal to the dynamic friction coefficient, μd, which represents the roughness between the sliding facing surfaces, by the contact force between the floor and the box, N.

Let us look at the graphic to visualize this experiments:


IIT JEE preparation tipsThe static and dynamic friction coefficients, are usually tabulated for those surfaces pairs that are relevant for industry. If for some reason you can not find them, they are really easy to calculate. They are adimensional, they have no units, they are just a number that represents the roughness between a couple of surfaces when they are facing without moving,
μs, and when they are sliding on one another, μd.

In general, the coefficient values are between 0 and 1… but you can find some that are higher. Also, generally, the static coefficient is greater than the dynamic coefficient: μs> μd

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As you should soon see in exercises, the static friction force is not always opposed to the movement of the body. Even more, in general, the movement is achieved thanks to the friction force. Remember this, the difference is important:

It is a LIE that
THE FRICTION FORCE OPPOSES MOTION

And also:

It is TRUE that
THE FRICTION FORCE IS OPPOSED TO THE SLIDING

The difference is subtle, but extremely important.

Notes:

  • In every motion (except in vacuum and in subatomic scales) friction is inevitable. The air tables used to play air hockey manage to reduce friction considerably, but it is still there.

  • Aristotelian physicist saw that, for a carriage to remain in motion, it needed to be constantly pulled by a horse. Such error came from their lack in understanding that the friction force stopped the carriage once the pull force disappeared. Aristotelian mechanics were funded on this mistake. Imagine the chaos that ensued when Galileo came along and said that, in the absence of forces, the carriage should continue moving at a constant rate!

If you want learn more about friction you can buy this book.

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