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Basic Physics lesson-11 : Friction

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basic-physics-lesson-11-friction

Introduction

Newton's first law of motion states that if a body is moving it will remain moving and if it is stationery it would remain so till an external force is applied on it. That is all good but my students used to ask me a question that when they roll a ball on the floor why it stops after some time and does not go on moving here and there in accordance with the first law. I usually explained to them that someone might be doing some trick to stop the ball. They would ask as who was doing that mischief. I told them that there is an entity called friction between the ball and the floor and that is creating a force in opposite direction to retard it and finally the ball stops. Hearing this students used to smile and nodded their heads.

So, we have to read the first law in its entirety which says that ' ... ... till some external force is applied on it.' Here, in case of the ball, the frictional force is applied to it in oppisite direction and that is the reason why it stops.

Impact of force on a body and friction

In earlier lessons in this series we learned about ( a ) Acceleration, ( b ) Force, Work done, and Energy etc and now we would examine in great details about the force as how it impacts a body. We would be interested to know as when this force is able to moves the body and when not.

Please note that so far in our earlier chapters on displacement, velocity, and acceleration, we did not talk about friction as we ignored it being a negligible thing but when it comes to exact calculation then we have to take the friction also in cognisance as that also plays an important part there.

Before going to understand the concept of friction, let us revise our knowledge about what happens when we apply force on a body. Let us take an example of a billiard ball and place it on the rough playing ground outside and give a little push by applying force on it with our palm or fingers and roll it on the ground. What happens is that it moves may be 2-3 feet and stops. Now take that ball and put it on a billiard table and push it with almost same push. See, what happens! It moves much more, may be 10-15 feet taking a few rebounds here and there.

There is a lot of difference between the frictional force on a rough surface than that of a smooth one. From this we understand that frictional force depends on the surfaces of both the things that is the ball and the surface on which it moves.

This observation we made with the rolling ball. What happens when we replace the ball with a stone or brick piece. Push it. Does it go far? No. The reason is it is trying to slide and now the friction is much more than what it used to be for a rolling ball. Apparently the sliding friction seems to be bigger than rolling friction.

When we apply a force on a body then it will either slide or roll or even fly in air if one is trying to do that but until the force is sufficient the body would not move. I have a big cot in my house which has storage boxes inbuilt in it and they are filled with household items and it is so heavy that when I alone try to move it by sliding on the floor, nothing happens. It does not budge from its place. It is so heavy that whatever amount of force I apply it does not move. It means that it is beyond my capacity to move it. So, applying force does not mean that body will move. This gives us an idea that if something is heavy friction is going to create more problem in moving it on a surface. So, what I should do. Next time while ordering for such a cot should I ask them to put a few small wheels below it so that we can move it with less force?

Another interesting thing to note in this context is that if we apply a force for some time and then stop it then the body would move for some time and then come to standstill as force is removed. So a continuous force would be required to move a body continuously. The reason is to sustain motion against other external force that is friction.

These are the things that we learn when we think of this entity called friction.

Frictional force

Let us now understand the frictional force with the help of a diagram in Figure-1. We have a body, a rectangular brick or box type of thing, placed on a surface A to B, may be a big table. There is a string attached to it in the front and It can be pulled in a forward direction. The mass of the box is m and we have applied a force F on it. The weight of the box is mg in downward direction towards the surface of the Earth and there will be a reaction for this weight known as R which acts on the body upwards. It is to be understood clearly that R is the force of reaction applied by the table on the box.

In this situation the frictional force on the box is defined by µR where µ is coefficient of friction and the direction of this frictional force is opposite to F. Now when we apply F then if F is less than µR the box will not move.

Now I remember that cot which I could not move. It had big µR there. The source of friction between two bodies touching each other and moving relative to each other comes from the roughness of the two surfaces. Even if they are smooth the microscopic bumps on their surfaces here snd there would make them to grind sgainst each other and that is what creates friction. The coefficient of friction would be different for different surfaces. Sometimes due to presence of liquid or some lubricant between two surfaces, the coefficient of friction is drastically reduced and the body can slide or move on a surface with little force. That is why lubricants are used all over the machines between the moving parts. Sometimes some lubrication is mandatory otherwise the moving parts would worn out and even damaged due to the heat generated due to friction. Before the invention of fire making matches the ancients created fire by scratching one small wood piece with other to create fire by friction only. Some barks of trees were more effective for creating fire and the people living in snow areas used to keep it with them for creating emergency fire while waiting under some natural shelters in isolated places.

When we push an object it starts moving with our efforts but when it starts moving then efforts slightly reduce just to maintain it moving. It shows that in static state friction was more than the moving (kinetic) state. This is an important observation and is useful in calculating frictional forces in these situations.

We have now seen that coefficient of friction which is a dimensionless quantity varies with so many factors. Static friction is more or less an academic matter but kinetic friction is a practical reality wherever relative contact movement is there. To have an idea of range of kinetic friction values, I would quote some typical values as below -

  • Steel on steel - - -> 0.57
  • Copper on steel - - -> 0.36
  • Wood on wood - - -> 0.20

Now put some lubricant in between and it would decrease drastically to somewhere near 0.06 only. So much reduction in friction! That is the reason of using lubricants in machineries.


Figure-1

basic-physics-lesson-11-friction

Friction on a slope

We have just learned with the help of the Figure-1 as above that force due to friction has a retarding tendency on a body and it is equal to μR where μ is coefficient of friction and R is the reaction on the body having weight as mg. Let us now concentrate ourselves in Figure-2 where a body is being pulled up with a force F on a slope A to B having an angle as θ with the horizontal plane C to D. Now, we have to remember that the reaction of the slope surface on the body is in a perpendicular direction to slope that is with respect to the surface on which it is moving.

Weight of the body is mg and it is acting in a downward vertical direction. To find out R we have to find the value of the weight acting in the opposite direction of R. From basic trigonometry, we can find it as mg cosθ. The weight is having another component which is in the direction of the slope and is equal to mg sinθ.

Now from the Figure-2 it is clear that -

R = mg cosθ

If F < (μR+ mg sinθ) then body will not move

If F > (μR + mg sinθ) then body will move in up direction

Figure - 2

basic-physics-lesson-11-friction

Friction in atmosphere

We have seen how a body experiences friction while moving on a surface. We have also understood that a moving object may experience more and less frictional force depending upon how much rough or fine the surface is. Let us see what happens when a body moves through a medium as air like a stone falling in the atmosphere downwards. Does it experience any friction. Yes, it experiences a drag by air on it which is just like a frictional force retarding its speed.

Do you know about the meteorites which fall on the surface of the Earth. When a meteorite from outsides enters Earth's atmosphere then due to its high speed it gets heated up due to the friction between it and the atmospheric air. This is so high that many small meteorites are burnt completely and turned into gaseous and ash form and are absorbed in the atmosphere and scattered around. But the bigger ones partially burn and the remaining part strikes on Earth. If it is big and falls in a populated place, it can create disaster.

What happens on moon? As there is no atmosphere, the meteorites, even the smaller ones, reach the surface of moon without atmospheric burning phenomenon and simply collide with it.

Life without friction

Many people find friction a big nuisance as we have to overcome it to get some work done effectively. It is not exactly so. Without friction life would be much difficult. Though it is hypothetical to think that friction may not exist there but if it is not there then if you push and roll a ball on a surface it would not stop and keep on moving. Someone will have to stop it from opposite direction! That would be a fearful responsibility.

Practical use of friction

Where do we use friction to our advantage? There are many places where we use it. For example the brake drums in a car to stop it create friction between the wheel and the brake drums and arrest the movement of wheels. Another example is that when we walk on a hilly rough terrain, it is the friction of the rough surface on our shoes bottom that protects us from slipping. Have you ever played 'tug of war'? The players are happy if the tug that they are pulling is rough so that it does not slip because of enhanced friction. They also require that field where they are playing should be rough to give them frictional support on their feet from the Earth surface below.

Friction

Vote

Conclusion

Friction is an important entity. It acts opposite to the force applied on a body kept on a surface and until it is overcome, the body would not move. When frictional forces are high it is a matter of concern and technology has to find ways to minimise it with newer designs of machineries. Ball bearings, Railway tracks and many such things were invented to fight the friction and minimise it. If we can minimise friction we can save energy and also minimise the wear and tear of moving parts which are in contact of each other or in contact of some surface in our machines.

References

  1. http://www.studyphysics.ca/newnotes/20/unit01_kinematicsdynamics/chp05_forces/lesson20.htm#:~:text=Friction%20always%20acts%20in%20the,be%20able%20to%20calculate%20it.
  2. https://www.khanacademy.org/science/physics/forces-newtons-laws/inclined-planes-friction/a/what-is-friction
  3. https://www.physicsclassroom.com/Class/newtlaws/U2L2b.cfm#friction
  4. https://www.teachengineering.org/activities/view/duk_friction_smary_act3
  5. https://www.generationgenius.com/motion-and-friction-lesson-for-kids/

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2020 Umesh Chandra Bhatt

Comments

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on November 03, 2020:

Nanasaheb, Thanks a lot for your comment. Highly appreciate.

Nanasaheb Mohite, Kharghar on November 02, 2020:

It is very important and helpful information. Thank a lot sir.

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on October 19, 2020:

SKMUNSHI, thanks a lot for your elaborate comment. You have given some thought provoking ideas, appreciate much.

Skmunshi on October 19, 2020:

The concept of friction and role of frictional force has been nicely introdued by giving examples from day to day activities related to relative motion between various surfaces.The importance on the effort to reduce friction to save enery and also on system to create friction for safety during relative motion has been also introduced and article will gteatly help beginners in understanding the subject and others in refreshing this important topic.

In fact in addition to relative motion between various types of surfaces and fluids we have to encounter friction in our day to day life such as interpersonal friction , friction between different sections of societies, friction between various states and central and friction between countries. Though this type of friction is not governed by any laws of friction but all efforts are made to reduce this type of friction for peaceful living.

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on September 21, 2020:

Flourish, thanks a lot.

FlourishAnyway from USA on September 21, 2020:

You provided good relatable examples in this refresher course. I recall liking studying this section.

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on September 18, 2020:

Anderson, thanks for your supportive comment.

Dale Anderson from The High Seas on September 18, 2020:

Good article and refreshing too. All too often I see the same kinds of articles over and over again so this was a very nice change. Thank you.

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on September 15, 2020:

Eric, thanks a lot. Appreciate.

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on September 15, 2020:

Bill, thanks for your interest in the hub.

Eric Dierker from Spring Valley, CA. U.S.A. on September 15, 2020:

Went through part of this with my 10 year old. He studied it a bit at around 9 yrs old. Thank you.

Bill Holland from Olympia, WA on September 15, 2020:

I did study this, I do remember parts of it, but I appreciate the refresher course. Thank you!

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on September 14, 2020:

Adreinn, thanks for visiting and feel happy that you liked it.

Adrienne Farricelli on September 14, 2020:

I don't remember studying friction at school, but my memory is not that good. Thanks for writing this as it helped as a refresher.

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on September 14, 2020:

emge, very happy to note that you are also a PG in Physics. Thanks for your comment.

MG Singh emge from Singapore on September 13, 2020:

Nice article reminds me of the time when I was doing my PG in physics and we went into the details of friction.

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on September 13, 2020:

Liz, thanks a lot. Appreciate.

Liz Westwood from UK on September 13, 2020:

This is a well-presented and interesting article.

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on September 12, 2020:

Linda, thanks for your visit and sparing time. Appreciate.

Linda Crampton from British Columbia, Canada on September 12, 2020:

This would be a very useful article for physics students. Thanks for creating it, Umesh. It's a great addition to your series.

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on September 12, 2020:

Kavya, thanks a lot for your visit and knowledgeable remark. God bless you.

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on September 12, 2020:

Eric, 700 WPM is quite a good speed as I am known in my circles as a voracious reader having a WPM hovering around 300-400 which is a matter of astonishment for many. So your 700 might not be mysterious but is remarkable. Thanks for quenching my curiosity and have a good day.

Kavya Jain on September 12, 2020:

A well rounded article, aptly explained. I think, Friction is something that we cannot live with and something we cannot live without. It is a necessary evil.

Eric Dierker from Spring Valley, CA. U.S.A. on September 12, 2020:

Umesh my friend, I cheat. I learned speed reading in 1973. At an easy 700 words per minute my comprehension is better that most. While kids were watching cartoons my mom was taking me to classes. I was the only boy in my typing class. Hihhihi. Anyhow, nothing mysterious.

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on September 12, 2020:

Eric, I am amazed with your reading habits. So many articles in so much details. I see you commenting on them almost as first one like an early bird. And you have to do your elaborate writings as well. It is amazing. Highly appreciate.

Eric Dierker from Spring Valley, CA. U.S.A. on September 12, 2020:

Most excellent. I did some work in the area of coefficient of friction. Automobiles and stopping distances. The weight of the car. The distances and the surface and state of tires is just fascinating. Thank you for this one that I will share with my son as he has been asking about such things.