# FRICTION Part 1 – Physics Class 8th CBSE #4

This video introduces, Friction. Friction in solids is also called dry friction. Car racing is so exciting, so much of action
and slipping and sliding and winning and loosing. But while all this action is happening, did
friction ever come to your mind? Think about friction within the engine parts
when they move, friction in the bearings of the wheel and most importantly, friction between
the wheels and the surface of the road. No matter if billions are spent on the technology
of the engine and aerodynamics of the car, some races in history were lost simply because
and you see it all around you in vehicles, walking, cycling, playing things like carrom,
and every time a ball is rolling and comes to a halt, its everywhere. So lets get to the basics and understand what
is friction. Frictional force or friction is the resistance
to the motion of one object moving relative to the other object, or surface while both
are in contact. Frictional force always acts in the direction
opposite to the motion of the object. Force of Friction. If you push a book on the table, what happens?
it moves and then it stops! What if you do it from the other side? Same result. So you are applying force to move the book,
but then it stops the moment you remove the force, ideally it should keep moving forever
but there must be a force that is opposing this motion that makes it stop. THAT force is called friction. The force of friction always opposes the applied
force. So when i push to the left then it acted in
the right direction so that it could stop the book. And when i push to the right, it acted in
the left direction and once again stopped the book. Now the book and table are made out of certain
materials. What happens if the material changes. What happens if the weight of the book changes? Factors affecting Friction. For this, i will do an interesting set of
experiments using one structure we built, and it can show all the aspects of friction
that we will cover in this lesson. And at the end of the lesson, i will also
how to make a simpler one as a DIY project as well. So i have tied the cap of the sketch pen to
a thread and have put the thread in the middle of the thick book to lock it in place. The other end of the thread is taken over
a pulley and looped to hook up to a small bowl for putting weights in. Now we start adding weights, Note that the
book is resting on the surface of the board and there is friction between them. We are going to keep adding weights to see
how much force it will take to overcome this friction. That will give us an exact measure of the
amount of frictional force that is there between the book and the table. There, that 700 grams. Since Force is mass into acceleration, and
mass is in Kgs and the acceleration due to gravity is 9.8 meters per second, the frictional
force in this case was 0.700 x 9.8 thats 6.86 Newtons. Note that there are three things to take in
consideration here. One, force applied by the book owing to its
weight. Two, force required to overcome the static
friction. And three, friction between the two surfaces. This is called the coefficient of friction
and written as mu and the value is pre-calculated between different materials. For a book and platform surface like this,
the value of mu is 0.43 In our experiment the frictional force showed
as the force being applied by the weights to move the book and it was 6.86 Newton. But if you didn’t have this experiment to
actually see how much force was required to move the book, you could also calculate this
frictional force using the formula Ff=μ x N
0.43 x 15.63=6.72 Newton and this is very close to the result we got @ 6.86 Newton. What do you think will happen if i increased
the weight of the book? I’ll do that by putting a 1kg weight on
it and now lets see if we need if the frictional forces have changed. Totalling 1.05 kgs. So the frictional force increased to 1.05
x 9.8=10.29 Newtons. That means increase in weight between two
objects increases friction. Now lets change the material of the board
and see if the nature of material affects friction. We have 3 sheets prepared for this. We have a styrofoam sheet, a synthetic flooring
carpet and a thick rubber sheet. Let’s start with the rubber sheet by pinning
it on the board. The rest of the setup with the book and weights
remains the same. On rubber sheet, it took 1kg to get the book
moving, thats frictional force of 9.8 N, and that’s much more than 6.8 N which it was
when the book was directly on the board. We can see that the the styrofoam sheet offered
7.84 Newtons of frictional force, less than the rubber sheet, but more than the board. lastly the carpet offered the least friction,
5.88 Newton. Now i’ll add another twist to the experiment. Under the book i will put a small tray we
made so that the book fits tight inside the tray, and under this tray i will stick the
same materials that the book is going to slide on. You will be surprised to see what happens. Remember that the carpet offered the least
friction on the book when we tried earlier. Rubber on rubber, friction increased to 16.17
N Styrofoam on styrofoam, now you can see friction
increased to 11.76 Newton. Carpet on carpet, now for the surprise, and
the winner by a huge margin. The friction increased so much that all our
weights, that’s 2.5 Kg total, could not get the book to move. So we got our electronic spring balance to
see how much force is required to move it. So you can see that it took almost 2.2kgs
to start moving the book but then once it was moving, it settled down to about 1.8 kgs. Remember this fact, that it required a higher
amount to start moving….because later on in this lesson, we will also learn why that
happens. Before we move on, let me do a fun test and
tilt the platform in a few of these situations. Here’s the book sliding on the board. The angle is shown on screen. Sliding with a little weight already pulling
on in. Sliding on rubber. Sliding with carpet on carpet. As the angle reaches 90 degrees, the effective
weight of the book against the platform becomes closer to 0 because all the weight is acting
downwards and not against the platform. Therefore the frictional force also becomes
close to 0 and the book starts slipping easily. Spring balance is a device used for measuring
the force actin on an object and it does that using a spring. The spring is already checked and marked after
hanging different weights from it because it stretches according to the weights hung
on it. For this episode i made one, and the DIY for
this can be seen on the channel. The links in the description. You can very easily make it instead of buying
it and this way you can learn exactly how it works also. So you saw how different materials affected
friction. Let’s look closely at these same materials
through our makeshift microscope camera. The surface of the book seems smooth. The board is almost smooth. The rubber is less smooth as you can see. The styrofoam has large air pockets. The carpet is very rough, like a jungle. No wonder it increased the friction so much
that we ran out of weights to pull it. All this shows how different materials in
contact with each other changed the forces of friction acting on the object. And you saw how the increase in weight or
the gravitational force on the object also increased the amount of friction the object
faced. This means that the harder two objects are
pressed together, the force of friction will increase. Do you think there is frictional forces acting
on a rolling object Lets use a simple inclined plane. Do you know what an inclined plane is. It is simply a surface kept at an angle. Thats all it is. Now we will stick a marker to it, you can
do this experiment with pencils too, but I’m using a bigger a marker so that you can see
that better. Now we will roll a marker down from this reference
marker so that it’s starting position is always the same. Now lets roll on the marble floor. Now lets see on the cement paver blocks. On rough ground. On grass. And let’s see on sand. See, on every surface it was different. Now what if we covered the marker with glue,
and roll it in sand and then dried it in the sun. Now we have a marker with the texture of sand. which means we made it rough. Now lets try all of them again. Now with a split screen we can see the difference
between the marker with the sand and the smooth marker. This shows that an object with a smooth surface
will face less friction than an object with a rough surface if both are rolling on the
same surfaces. Now we’ll look at 3 types of friction. Static, Sliding and Rolling friction. Static friction is the resistance force that
keeps an object at rest on another object or surface. Now we know that friction is caused by the
nature of the two surfaces in contact with each other, and thats because of the irregularities
on their surface. Even those surfaces which appeared smooth
did have a large number of minute irregularities when we saw them very closely. These Irregular shapes lock into one another
when the object are pressed onto each other, by gravity or any other force. When we try to move any surface, we need to
actually do two things. First we need to apply enough force to overcome
this interlocking, and only then we can keep moving it as long as we keep the force applied. This brings us to a new question. What is easier? To move the object from its resting position,
or to move it when it is already in motion? Answer seems logical. if its already in motion, i has to be easier
to keep it going. And thats true. This means you need to apply more force to
start moving the object, because you need to break the “locking” caused by the surfaces. The force required to overcome friction at
the instant an object starts moving from rest is a measure of static friction. Once it starts moving, lesser force is required
to keep it moving because the irregularities in the surfaces do not get a chance to lock
as well as they do when the object is at rest. Sliding friction is the resistance force between
an object sliding against another object or surface. The force required to keep the object moving
with the same speed is a measure of sliding friction. Sliding friction is always slightly less than
static friction. And there you can see that when we pulled
the book, first the spring balance showed a higher reading of around 500 grams and then
as the book starts moving it settles down to 400 grams. What will happen if we increased the weight
by 500 grams? It obviously shows a higher reading, now the
reading on the spring balance shows between 500 and 750 grams. Rolling friction is the force resisting the
motion when an object rolls on another object or surface. Now lets introduce rolling friction to the
same set up. You can see that the force required is reduced
to just about 25 grams, and the book is still moving. I hope you realise that this is why we move
everything on wheels. I showed you those race cars in the beginning
of the video. Most of them were doing something called “drifting”. Drifting actually requires not only an understanding
of friction, but a complete mastery over it. These drivers are masters at controlling friction. So how does a car drift? Firstly the car ideally needs to be a powerful,
rear wheel drive. the front wheels do the regular job of maintaining
traction with the ground. traction is the positive connection to the
ground and therefore means that the friction between the tyres and the ground is high. The front wheel of the car remain in the direction
of the motion to maintain traction, and that is actually opposite of what is normally done. For example if the turn is curving to the
right then normally people turn the steering wheel to the right, and go around the bend. But while drifting, in the same situation,
since the rear of the car swings out to the left side, you will have to turn the front
wheels also to the left side to remain curving around the curve. Otherwise if you turn the front wheels to
the right the the car will go out of control. And why do they do it? Its not to show off, Drifting is done to maintain
a high speed while going around corners, because after all, they’re racing each other. Now that I’ve told you this, check out the
cars in the video once again, and notice the front wheels. Before we move on to the DIY project, lets
do a short recap of the lesson. Frictional Forces or Friction. Friction in solids is called “dry friction”. Frictional force resists / opposes motion. Frictional force acts in the opposite direction
of the motion. Friction requires the two objects to be in
contact with each other. Friction is caused by the irregularities on
the surfaces of the objects. A spring balance can be used to measure the
force of friction. Factors affecting friction:
Nature of the surfaces that are in contact with each other. Force with which the surfaces are pressed
together. Therefore, it is also affected by the weight
of an object. Types of friction:
Static friction Sliding or Kinetic friction
and Rolling friction Measuring friction:
Since friction opposes motion, it can be measured as the force required to overcome any of the
3 types of friction. Here’s one look at all of these. For the TIY, i will show you how to make the
same kind of set-up that i used earlier to show the frictional forces. Since it is difficult to cut ply board and
all, i thought lets try and use something that can be helpful to you anyway even once
you are done with the project. So for the board, i thought we could use a
drawing board. And its not very expensive and it will do
the job perfectly for this project and many more perhaps, and remain useful for a long
time to come. Also because it comes with clips that we can
use to clip various surfaces to the board. Now for the pulley we can use one like we
did, this is from a mechanical toy kit. The pulley wheel is much better and later
on you’ll see why. But is you don’t have that, it should not
stop you from trying out the experiment anyway, so in that case you can also just use a butterfly
clip like this, or the simplest is to stick two pins on the board, and slide the thread
over the clip so that it doesn’t spoil the board. Now for adding weight, if you don’t have something
like this then you can also make it by cutting a plastic bottle and tie it with threads,
or you can simply use a plastic bag like this. Now you need to find some surfaces to clip
on to the board to see how the frictional forces get effected, this is how you can clip
the surfaces and make sure they’re absolutely tight and there should be no bunching up or
folding happening when the book slides. Get an extra packet of clips for added support. The thread needs to be light but it should
be very strong. For tying the book you can use the same method
that is used. First tie one end of the thread to the cap
of the pen and then leave about 1 meter thread and tie the other end back to the first knot,
this way you will end up with a long loop. keep in mind that you might have to make this
loop longer or shorter later depending on how far down the weights are hanging. I also had to make some adjustments. You can do all the experiments that i showed
you with my blue board. Hope you enjoyed this lesson on the introduction
of friction, the next lesson is on the topic of Friction too, but there We will explore
how it is evil, but a necessary evil. But keep in mind, if you’re not using a pulley,
then the strings slipping over the clip also adds to the friction, therefore the same weights
that moved the book earlier will not manage to move it now. So if you have access to a pulley wheel, it
will be much better. And you can see that now it shows the same
results. Now for changing the materials under the book,
you could simply look for bags in various materials and put the book in them. Heres i have used a plastic bag. Then i also found a cloth bag, this is a synthetic
bag that is being used in the market now. Thats it, you are ready to do the experiments
that i did in the earlier part of the video so i wont show them again here. Things required for the TIY project. Drawing board. Drawing board clips, 4 or 8. Pulley wheel. Butterfly clips. Hammer. Two thumb pins. Rubber sheet. A Pen cap. Thin strong thread. A heavy book. Weights and a bowl to keep them. A plastic bag. Cloth bag. Synthetic bag and any other bag you can find. So i hope you’ve enjoyed this lesson on the
introduction of friction. Next lesson is also on the topic of friction,
but there we will explore how it is evil, but a necessary evil.