Ever Wondered About Cycles?
Now let’s try and use that knowledge and our picture of formation of bonds and breaking of bonds to a very real life problem. Have you ever wondered when you’re riding your cycle in which direction is friction acting on your wheels? The road looks really rough right? So there must be some friction acting. Then if it is, is it acting forwards or is it acting backwards? Is it the same for the front wheel and for the back wheel? There are simple questions like this to ask right? I want you to think about them. The answer for these is what we will see next.
To find the answer to this question let’s do something really mischievous. Just when somebody is about to start their cycle, right, we bring in a giant who just lifts the cycle up with the handlebars and the seats and let’s them pedal. Now they are pedaling the cycle but then the cycle is not touching the ground, what will be happening? One wheel will rotate for sure, the back wheel, because that’s very much what you’re doing with your legs right, you’re rotating that wheel. But what will happen to the front wheel? It will not rotate at all. It will be at rest. Now what the giant does is he slowly begins to lower the bicycle but not before making the road super smooth and slippery and putting a paper on it. Yeah. So this means the paper can just move either ways really, really easily. Then he tilts the bicycle little bit and puts it on the road such that the front wheel touches. But now is that anything meaningful, is that anything eventful? Nothing right? The wheel is not rotating. It’s touching now so it will remain that way. It gets interesting when he does this, when he puts the back wheel as well and the moment the back wheel touches the piece of paper something interesting is going to happen, right? Because when initially the front wheel alone was touching, there was contact but there was bonds was forming but nothing much happening, but now when the back wheel is also touched and the bonds are forming between the back wheel and the piece of paper and the back wheel is rotating right? So it’s going to move and along with it push the piece of paper in that direction and it’s going to keep doing this. New parts of the wheel are going to touch the paper, form a bond, push it and go back. This continuously begins to happen and the paper is going to move that way but while that’s happening, what has to happen is that this paper that has formed bonds with the front wheel will start pulling that front wheel right? So here the back wheel is pushing the paper that way but that’s leading to the paper pulling the front wheel this way. So after, if you just look at it now, it will look like both wheels are rotating but one of them is the cause of this paper to move and the paper in turn creates the front wheel to rotate in the forward direction.
Now coming to the question of what’s the frictional force in these cases? Let’s look at the back wheel first. The back wheel came in contact with the paper, forms some bonds with it and through those bonds pushed the paper backwards. Parallel to the surface right? This is kind of what we call, or this is exactly what we call friction. So the back wheel applied a frictional force on the paper backwards. By Newton’s third law, the paper must have applied a frictional force on the back wheel forwards. So the frictional force on the back wheel is forwards. Now that’s interesting. What about the front wheel? Is it going to be the same? If you look at the front wheel, it did form some bonds with the paper already and then the paper began to move that way thereby pulling the, the front wheel that way. Now this front wheel is feeling a pull from the paper in a direction parallel because of the contact. That’s exactly friction. So that force that we mark there is the friction on the front wheel and that’s backwards. So when a cycle is being pedaled forwards on a piece of paper right, so it’s still going to remain here, the friction on the back wheel is forward, the friction on the front wheel is backwards. But what’s this got to do with the cycle going on the road? Now we are looking at the cycle being stationary with its wheels rotating on a piece of paper that is moving backwards, but what if I now attach my point of view or my frame of reference to the piece of paper so that I am moving along with the piece of paper in that direction. Then, what will this whole scene look like? The paper will come to a halt and the bicycle will start moving in the opposite direction. Now again the wheels are rotating. Everything is same except that I have changed my point of view. Can this change or can this do anything to the direction of frictions that we marked? No right? Changing my point of view can’t change anything physically over there. Then, I can also ask the question, now I have a stationary piece of paper with a cycle going on it. Is, does it make any difference to the direction of frictions if I replace this stationary piece of paper with a stationary road? Yeah. If it doesn’t then we can do that, and that will give us the answer to our original question. If a cycle is being pedaled on a stationary road, then what will be the direction of friction on both the wheels? Now we know the answer is that the back wheel has a friction in the forward direction and the front wheel in the backward direction. Note that that’s when you’re pedaling the cycle.