Free tutorial in Physics - Understanding Friction
Whenever a body comes in contact with another body and there is a relative movement between them, then friction comes into the picture. This article gives a detailed understanding of the physical entity called friction.
Introduction
Whenever something moves or slides over another thing then there is a contact between them. During this contact an unseen force acts between them which tries to retard the motion of the moving object. For example, when we push a ball on the floor it moves ahead and after some time it stops. Why does the ball stop? Why it does not move ahead continuously? That is where the concept of friction comes in place. Whenever there is contact between the different bodies and there is a relative movement then friction plays a very crucial role and that is to produce a frictional force in the opposite direction to the force that is moving the object.
Let us recollect the first law of motion in Physics which states that - If a body is moving it will remain moving and if it is stationary it will remain so till an external force is applied to it. In the above example of a moving ball, the ball stops because the frictional force between it and the floor which is acting in the opposite direction stops it. That is the external force on the ball.The frictional force
When we roll a ball on a billiard table it moves ahead and after striking the sides of the table it again moves and stops after a few rebounds but the same ball if we roll it on the rough ground outside our house it would stop after a small distance. What does it mean? It simply means that the friction between the ball and the billiard table is much less than that of the rough ground outside. From this observation, we infer that friction depends upon the physical condition of the surface on which an object is moving.
In the common household game of carom, we sprinkle a lot of powder on the board and then the striker moves smoothly making multiple rebounds. The powder fills the small microgrooves on the surface of the board making it smooth and reducing the friction.
Let us replace the ball with a rough stone. If we push it on a surface then it moves slightly and comes to a stop quickly. Because of its shape, it cannot roll and further, the frictional force between the stone and the ground is much more.
From this observation, one more thing is also clear if we roll an object on a surface then it moves ahead easily but the same object does not move ahead so easily if we simply slide it. So rolling friction is less than sliding friction.
It is to be understood that when we apply a force (say a push) on a body if it is sufficient then the body moves forward. After some time as force is not continuous, the body stops. This deceleration is happening because of the friction. So it is clear that a moving body will eventually stop if the applied force on it is not continuous and the reason that it stops is nothing but frictional force.
When a body is moving on a surface then the frictional force is trying to stop it. That means that frictional force is acting in the reverse direction. Let us understand it in mathematical terms. Let a body of mass 'm' be resting on a table. The weight of the body which is represented as mg is acting downwards where g is the gravitational acceleration (acceleration due to gravity). Now as the body is resting on the table it has a reaction force on it in the opposite direction that is upwards. We call this reaction as R. In the equilibrium position we have -
R = mg
The students should note that mg is the weight of the body acting on it in a downward direction while R (reaction) is the reaction force by the table on the body in an upward direction.
Now, if we want to move the body by pushing it with some force then frictional force will oppose it and once the applied force is sufficient to nullify the frictional force then increasing it further will move the body. It is also obvious that if the body has having small weight then we will have to apply a small force to move it and at the same time if the body has more weight then we will require more force to move it. From this, we can now understand that frictional force which is opposing the movement of the body is also related to the weight of the body as well as the type of surface on which it is resting. This relationship is the most important one in understanding friction and the frictional force (F) is written equal to µR where µ is the coefficient of friction and R is the reaction of the surface on the body.
All the calculations of friction and understanding it depend on the above two relations which can be shown by the following equations -
R = mg
F = µRSource of friction
The source of friction between two bodies or a body moving or sliding on a surface comes from the roughness of the surface and roughness as felt at the point of contact. The surface or body from outside might appear smooth but if we see them from near or using a microscope then we will find some amount of roughness there. And it is this roughness, whatever little it is, causing friction when there is a relative movement.
When a body slides or moves on a surface a grinding action occurs and friction is created which impedes the movement of the body. There are standard practical methods to reduce this friction like using lubricant liquids at the contact point. The coefficient of friction can be significantly reduced by the use of lubricants. That is the reason why we use lubricants in a machine where moving parts are there. Another benefit of using lubricants is to reduce the heat generated by friction. Examples of friction
In ancient times match sticks were not yet invented and primitive people used to rub the tree bark pieces together to create fire. Rubbing the bark pieces with each other created heat and then the fire was created.
Another example of friction is the common matchbox used in households. There is a rough layer of special chemical material on which the matchstick is to be rubbed. Due to friction the flammable material on the tip of the matchstick gets heated up and a flame is created.
Another example is the friction in the atmosphere. When wandering bodies in space (meteorites) enter into Earth's atmosphere then due to their high speed and the friction presented by the atmospheric air, they get heated up and burn and turn into ashes. So, our atmosphere protects us from the meteorites which fall on Earth from space.Typical values of coefficient of friction
Coefficient of friction varies from material to material, its surface finish, and the type of surface on which it is moving. When we push a body to move on a surface then we have to apply some force against friction to move it and then continuously apply some force to keep it moving. Generally, the force required to keep the body moving is less than the initial force required to move it and hence the initial friction known as static friction is slightly more than the friction during the moving stage. This latter friction is known as kinetic friction.
Some representative kinetic friction values (coefficient of friction) for some materials are as under -
Steel on steel - 0.57
Copper on steel - 0.36
Wood on wood - 0.20
If we put some lubricant in between the moving parts in a machine then the coefficient of friction reduces significantly and comes down to the level of 0.06 to 0.08 only.Conclusion
Understanding friction helps us to learn various physical activities going around, especially in machines and mechanical gadgets. In industrial machines, friction is minimized to protect the moving parts from damage and getting worn out.
Frequently Asked Questions
Can friction be made zero?
No. Friction can be reduced but cannot be made zero. There will be some little friction at the point of contact between two bodies whenever there is relative motion.What is the unit of friction?
It has no unit. It is a dimensionless quantity.If a body is moving in a vacuum space will it experience any friction?
No. In vacuum there is nothing with which the body will have contact so there will be no friction.Why rough tiles or rough flooring are used in public places?
Rough surfaces provide a good amount of friction and there is no danger of slipping on them. That is the reason why these are used in public places.
The physics behind the frictional forces is well explained by the author in this article. Friction will be high between two hard surfaces but it will be less between two soft surfaces. That is why in many industries people use PVC and rubber bolts and nuts which will not cause very high friction. Friction will create heat and that heat may initiate some chemical reactions. That is why when we are handling some chemicals, we should study the Material Safety Data Sheets of that chemical to know how friction sensitive the chemical is and based on that we have to design the handling systems of those chemicals.
There are some high-energy materials which are very friction sensitive and heat because of friction will initiate the reaction and may end up in an explosion also. Lead Azide is a high-energy material which is very sensitive to friction. That is why in the factories where this chemical is used, many precautions will be taken to avoid even minimum friction during processing and handling. That chemical is so sensitive that even a small amount of heat generated will cause an explosion.