By: Harshita Singh Chauhan
Friction is a mysterious force. No, I’m not talking about the friction in the China-US relations, but rather, the physics concept that is truly miraculous but at the same time, causes inconvenience at times. In the English language, friction is given a somewhat negative connotation, for when there is friction between two people, it’s definitely some sort of clash amongst them. However, friction as a physics concept isn’t that bad. Although it definitely has its cons, it has its benefits too! Without friction to cause wear and tear, I won’t ever have to replace my bicycle tyres (imagine the cost savings!). But then again, without friction, I’m not too sure I would be able to cycle in the first place! So maybe, friction isn’t that bad after all...
With such a complex force, comes a lot of misconceptions. And in this article, we will be debunking a few of the main misconceptions that students have and determine whether what we have learnt in school is #Fact or F(r)iction!
#1 Friction is a resistive force, it acts in the opposite direction relative to motion
Friction can be viewed as a resistive force, however, it is important to note that, without it, it's practically impossible to move about (imagine trying to walk on a super slippery floor!) In fact, in a way, it assists motion by resisting motion. Think about walking, for instance - when you walk you exert a backward force from the balls of your feet to the ground, and due to the friction, your foot gets “stuck” at a certain point on the path. Subsequently, an equal and opposite force will be exerted on you and you will be able to move forward. Furthermore, it is worth noting that the direction of the frictional force is, in fact, in the direction of motion! Since we are pushing our feet back to move forward, the frictional force is also pointing forward, as seen in Figure 1.
Figure 1: Forces Acting on a Man as He Walks
Verdict: Half Fact, Half Fiction!
#2: Friction is a kind of Force
Friction is a force, however, there are many different kinds of frictional forces. The two main ones are Static and Sliding Friction. Other types of friction include rolling and fluid friction.
Ever tried pushing a sofa and it just won’t move? But somehow, when it starts moving, it moves with a relatively smaller push. Why does that happen?
This need for a large force to get the sofa moving is due to static friction. Static friction is the frictional force between two stationary surfaces (in this case, the leg of the sofa and the floor), which prevents the object from just sliding away.
The force needed to keep the sofa moving is, you guessed it, sliding friction. Sliding friction is the frictional force between two moving surfaces. It is much weaker than static friction (thankfully, if not it would be so hard to move that sofa!)
As such, the force - displacement graph of the sofa would look something like this:
Fig 2: Force-Displacement Graph for Sofa
#3: Friction is Dependant on Surface area of an Object
It might seem intuitive to reduce the surface area of an object to reduce the friction acting on it, however, that is not the case. Friction, contrary to popular belief, is actually independent of the surface area of an object because even though a large surface can lead to a greater area of contact and increase the friction acting on an object, this is offset by the reduction in pressure from the increase in the area, since Pressure = Force / Area . Due to this, the resultant force is independent of the surface area.
#4: Friction and Drag are the Same Thing!
Drag and friction are used interchangeably colloquially as both of these are resistive forces. However, there are two key differences between the two - drag is dependant on the surface area of the object, whereas friction is not and drag changes with velocity whereas friction stays fairly constant. For example, if you drop both a stone and a piece of paper from a certain height, the friction acting on both objects will be the same but the drag will differ (since the surface area of the paper is larger than that of the stone). The drag will be larger for the paper, which explains why it takes longer for the paper to get to the ground.
GSCE Science (2015). Forces and Motion Retrieved from - http://www.gcsescience.com/pfm33.htm
cK 12 (2012, November 02). Types of Friction. Retrieved from - https://www.ck12.org/physics/types-of-friction/lesson/Types-of-Friction-MS-PS/
PhysLink. Why doesn’t Friction Depend on Surface Area. Retrieved from - https://www.physlink.com/education/askexperts/ae140.cfm