FRICTION Part 2 – Physics Class 8th CBSE #5

FRICTION Part 2 – Physics Class 8th CBSE #5


This lesson explores if friction is good or
bad, and how do we increase or decrease it to suit our needs better. It seems that almost everything we do has
an element of friction, from walking, running, cycling, driving, pulling, pushing, picking
up things to putting them down, do you think you could do all this if there was no friction? What would happen if you changed the amount
of friction in various things? Come, lets explore all these and have some
more fun, in the name of science. Let’s look at some advantages of friction
using something that you will definitely come across in classroom. When you write with a pencil or a ball-pen,
or if the teacher is writing on the board with a chalk, all these things are possible
because of friction. As we saw in the previous episode, the surfaces
of all things even if they look smooth, are actually rough. And your pencil or chalk is rubbing off and
leaving a trail of particles. And thats why pencils and chalks keep becoming
shorter till they disappear. Lets see what happens if we change the surface
of the paper in the case of the pencil. Here i will take a soft but thick lead pencil,
this is an 8b pencil. “B” stands for black and as the number
goes up, the lead is thicker and the softness is more. What you use in school is HB pencil and they
are optimum in hardness and not very thick. And by the way, paper sizes have their standard
sizes, and this one 29.7cm x 21cm, and this size is called A4, and A3 is double of A4,
while A5 is half of A4 and there are many more sizes…but this much information is
good enough for now. Now let’s make a line on 3 different surfaces
of paper. One is a butter paper, one is a regular paper
that you write on and one is a handmade sheet from a drawing book. all with different levels of smoothness. Lets make a line on all these sheets in one
go so that the pressure is the same. Let me show you how i loaded the camera on
the pencil itself to get a perfect shot When we walk or run, we need friction to hold
our foot in place, otherwise we will slip. What do you think a banana peel does if we
step on it? It reduces friction and our foot slips on
it. How does it reduce friction? The peel is soft but thick and its layers
keep sliding on the ground like ball bearings as it slips away. In fact most of its layers get left behind
on the ground after it has done the job of making you fall. The car moves forward because of friction,
stops because of friction and manages to turn because of friction, and as we saw in the
last video, a few can turn the car successfully even by deliberately reducing friction. But then thats different. The brakes of cycles, bikes, cars etcetera
are all designed in a way that they add a lot of pressure on the moving wheel and as
you learnt in the previous lesson, to increase friction, one can increase the force with
which the two surfaces are pressed together. In fact the brakes always act from both sides
of the wheel or the disc so that this huge pressure applied not he brakes is balanced
from both sides. Here we will take 2 nails. one is a rough nail and the other is clean
and coated with grease. Now we’ll hammer both into a piece of wood
and there are small wires around them to attach the hook of the spring balance. And now lets use the spring balance to see
how much force is required to pull it out. The rough one showed 9.6kgs, while the greased
one showed just one kg, hence proving our point. Another example of friction is when someone
lights a match, see if you try to light it on the smooth surface, then theres no friction
so its impossible, but one strike on the rough surface, and the match lights up. Even a saw cuts wood or metal using extreme
friction…look at the teeth of this pocket saw…its designed to cut wood cleanly, as
well as metals like aluminium. What would happen to a moving car if there
was no friction? The answer is, It would never manage to start
moving in the first place, and if it was already moving when friction disappeared, then it
would not stop, even if it ran out of fuel or if it ran out of battery if it was an electric
car. So friction is good and bad, good because
without it you couldn’t move, and bad because if it wasn’t there, you could move for free,
but never stop. Lets look at some disadvantages of friction: Best examples are the soles of your shoes. you would never have to buy new ones if there
wasn’t any friction. Same thing happens to the rubber of the tires
of cars, and thats surely a disadvantage because firstly they become dangerous once they become
“bald”. then they are expensive to replace. but the worst is that the world is filling
up with thrown away tires. There are more than a billion tyres thrown
away in the world every year. We saw that friction in brakes helps us stop
the vehicle, but this same friction also wears out the brakes and they too need to be replaced
regularly. Friction wears out even hard surfaces like
stone, you can see that in the steps of old monuments and even some not so old buildings. Friction wears out machine and engine parts. and for example, in the engine of a car, one
of the part that moves would be a something like this, but this is an old one, THIS is
how it looked when it went into the engine, what caused all this worn out state, friction. Friction causes heat. this is something you can try immediately
by rubbing your hands hard against each other, at the start the temperature of my hands was
33 degree celsius, and within 10 seconds it became 38 degrees. The roughness of the skin and the pressure
of rubbing the hands increases friction and therefore the temperature. This time lets take the temperature of the
disc of the front brakes of the scooter. its 36 degrees. Anyway I’m fast forwarding the video to save
time. In fact, in formula 1 racing, many times the
disc brakes would be glowing red because they get so hot, that they burn out. And now after just 4 or 5 instances of applying
the brakes the temperature has increased to 42 degrees. And lets do the same thing for the engine. First lets see the temperature of the cold
engine, and then we start the car. Once again the video is fast forwarded to
save time and we kept the engine running for 3 minutes, and now you can see the temperature
rising. In this case i have just driven the car to
the office after a 10 minute drive, and you can see the temperature is much higher, at
more than 80 degrees. The same thing happens with any machine, mixer,
grinder. The initial temperature is about 30 degrees,
this part is attached to the main shaft of the motor so it gets heated by it. I kept it on for about 1 minute, and then
the temperature reached 35 degrees. You already know that friction slows down
motion, you saw it in the earlier episodes as well with the rolling ball and rolling
pen. so the same thing it does to all the parts
of any machine, or any engine, and thats why we are mostly trying to reduce friction. But then there are times when we want to increase
friction. Increasing friction. Friction can be increased by adding grooves
into the surfaces like the grooves and patterns in shoes that are different for different
purposes. It can be increased by adding treads as there
are in tyres, the treads are different for regular road driving and different for any
other surfaces. Athletes use spikes and that adds a lot of
friction for a sprinter because they dig into the ground for maximum friction. You can increase friction by adding weight
or pressure like i did when i added weight to the sliding book in the last episode. Gymnasts also put chalk powder to increase
friction and absorb the sweat as well so that they don’t slip,
But one of the best way is in using a combination of the right materials and shape. In fact a simple but brilliant example is
the belt that moves a pulley wheel. The material of the belt is such that it is
rough for gripping the metal. This is a cut belt to show the section to
see how it sits on the pulley wheel. And the v shape adds to the surface area in
contact to increase the friction, plus the design of the v shape is a genius idea, because
as the belt gets more load, it pulls deeper into the wheel, increasing the pressure and
therefore the friction. Now i will do a series of interesting experiments
and it shows how friction changes with different surfaces. We’ll use 3 tracks of different surfaces,
A is rough & soft. B is rough & hard, and C is smooth & hard. We’ll pull identical weights over these
tracks and we will keep changing the surface below the boxes to see how friction changes
with different surfaces meet each other. First we start with plain rubber sheet under
the boxes. This is how they are stuck, so that we can
change them to put the next type after each set of trials. We’ll pull with a spring balance and note
the force, currently it’s set to pounds and not kgs otherwise the number was always
below 1 kg and that was not fun. 1 kg is 2.2 pounds. The box moves at a reading of 1.4 pounds on
A. Now it moves at a reading of 3 pounds on B.
Here the readings is 1.2 pounds on C. Now we’ll use the surface with wide grooves
like in shoes On A the box moves at a reading of 1.8 pounds. On B the reading is 2.9 pounds. On C the reading is 1.7 pounds. Now let’s use the surface with a tread like
in tyres On A the reading is 2 pounds. On B the reading is 2.8 pounds. On C the reading is 1.7 pounds. Now we’ll use the surface with thin and
sharper grooves. On A the reading is 2.3 pounds. On B the reading is 3 pounds. On C the reading is 1.9 pounds. Now we’ll use the surface with spikes, using
thumb pins and we’ll reverse the order. On C the reading is 0.9 pounds. On B it is 3 pounds. On A it is 3.5-4.5 pounds. Before we move on, lets try one more thing. Here we are using the first option of plain
rubber and it still moves at 1.4 pounds on C.
Lets add some carrom powder to the surface. Now the box moves at a reading just around
0.5 pounds. Methods of reducing friction. In some situations, however, friction is undesirable
and we would want to reduce it. One way to reduce friction is to make the
surface as smooth as possible so that the surfaces of the object have less possibilities
for interlocking with each other at the microscopic level. The other way to reduce friction is to add
something between the two surfaces that will not allow the locking that happens between
the surfaces. Thats why we sprinkle powder on a carrom board
and you saw in the lesson for force, how that makes a difference. This also works with a regular talcum powder
but there are special powders for carrom that one uses. The most used material to reduce friction
is oil. When a oily substance is used to reduce friction,
its also called a lubricant. The other day i saw a big fishing boat being
brought on land. these boats are very heavy and you can see
the number of people required to push it. But they use science. They use a lubricant on wooden logs to slide
the boat on, without the logs they will not manage to move the boat an inch on sand and
without the lubricant they would require many more people to push it. The lubricants go between the moving and sliding
parts and doesn’t let the locking of the surfaces to happen. And thats what oil does inside of an engine
as well. you must have seen that your car or bike has
to go for servicing regularly, and one of the most important things they do is replace
this oil, so that all the worn out “metal dust” gets out of the engine with the oil. And its replaced with new oil. See the difference here between the oil that
came out of the engine and the new oil. The one that came out of the engine is full
of fine metal dust that is from the worn out engine. One simple thing you can do with oil is to
put a drop of it on any door hinge that is making a creaking sound. soon the oil goes between the moving parts
and stop the sound. To reduce friction, one of the best results
comes with air. Things like hovercrafts blow air under themselves
so that they are floating on an air cushion. This reduces the friction greatly. And once its floating, even a human could
push it in any direction. You can make this simple hovercraft, the DIY
project can be seen in the link in the description. Wheels are used a lot to reduce Friction,
in cars, suitcases, chairs, trolleys, trains all around us are wheels of some sort. In fact the wheel is said to be the greatest
invention of mankind. And to show you how much different it makes,
here’s the big box with our bodybuilder once again. In fact in this one experiment you will see
one again, static, sliding and then the ease of rolling friction. When he starts pulling the box, the force
reaches more than 60kgs worth to overcome static friction. Then it settles down to around 45kgs worth,
to overcome sliding friction. Now lets bering some rolling friction into
action by using 3 pipes. 3 is the minimum number of pipes you need
for this. The Egyptians moved their heavy rocks using
this technique when they made the pyramids. You can see that the rolling friction has
dropped the force required to just about 5 kgs worth on the spring balance. And after seeing this experiment you will
agree that its the best way. Since the rolling friction is smaller than
the sliding friction as we saw, sliding is replaced in most machines by introducing ball
bearings. Once again, almost all parts in any machine
or engine, wherever constant rotation is required, ball bearings are used. The TIY project that we do later in the lesson,
will show how much difference the balls in the ball bearings make to reduce friction. One thing for sure no matter how much we try,
we cannot create zero friction on earth. Friction can never be entirely eliminated. No surface is perfectly smooth. and no material
has zero friction, including water and air, but that we’ll see in the next lesson,
But for now, lets continue and watch more experiments. Here are two people, one is a bodybuilder,
the other is a young boy. Now both will participate in a game of tug
of war. Let’s use our knowledge of friction and
show how much difference it can make. Before they start, we’ll change the situation
a bit. All we did was add some grease to the board. So that the bodybuilders feet have very little
friction. In fact he was finding it very hard to even
remain standing, forget trying to pull the boy. On your marks. Get set. GO. There! Who won? Science won! Now lets do a quick recap on what we learnt
in this lesson and then we will move on to the TIY. Advantages of Friction. It enables us to walk, run, jog etc. It enables a car to move, turn or stop. It enables us to apply brakes. It enables us to write on paper or blackboard. To pick up or hold things or pull things. Nails remain stuck because of friction. A matchstick lights up because of friction. It enables us to cut wood with a saw. It enables us to cut metal with a metal cutter. Disadvantages of Friction. It wears out shoes, slippers etc. It wears out tyres. It wears out machine parts that move or rotate. It wears out brake pads, clutch pads or anything
that works with high pressure between surfaces. It wears out even materials like stone and
concrete in buildings if 1000s of people use it. It produces heat and that can also lead to
damage of parts. It reduces efficiency of machines as some
power is lost overcoming friction. It constantly slows down motion. Methods of increasing Friction. Grooves and patterns increase friction in
shoes. In tyres, the treads increase friction. Sprinters wear spikes and footballers wear
studs to increase friction on soft ground. Gymnasts and weightlifters put chalk powder
to increase friction and reduce sweat. Certain materials are designed to increase
friction. Certain shapes can increase friction. Methods of reducing Friction. Making the surfaces smooth by polishing can
reduce friction. It can be reduced by applying lubricants (like
oil or grease) to the surfaces that are in contact. It can be reduced by using wheels to move
objects. It can be reduced by using ball bearings between
any moving parts of a machine. Heres one look at all of these. On the channel we have many TIY projects on
the topic of friction and their links can be found in the description below. For the TIY we will do now is a simple one
that shows how ball bearings make a difference in rolling friction. First, this is what we need to make and these
are the main parts of the structure, all are PVC plumbing materials. We need one pipe 50 cm.
3 pipes 30cm. and 2 pipes of 24.5 cm length. These pipes are 3 cm in diameter. We need 3 “T” joints as they’re called. One “L” joint and 2 end caps. Now i’ll assemble it without any cuts in
the video but in fast motion so that you can see every step. The actual tool required to hammer materials
like PVC is called a mallet and its made of wood and sometimes has plastic head so that
the pvc will not get damaged. But if you’re careful then a regular hammer
is good enough, like I’m using. PVC pipes are an excellent material to make
things because then you get all the joints that you might need, and the best part is
that you might not have to use glue even, and that means you can re-use the parts to
make something else later on. There, the structure is ready. Now we have to put a cycle rear sprocket on
the pipe. This is why we used pipe of 3 cm diameter. but as you can see, it’s still a bit loose. you can tighten it by putting a layer of paper
tape on it. You can see i did that but its still loose. So i will put another round of tape on it. And now inside ring of the sprocket seems
tight enough. We need 1mt long strong thread and about 60cm
long bicycle chain. First lets work on the thread. We have to make a loop on the thread. the loop has to be a wide one so that it can
go around the pipe. Now put the chain on the sprocket so that
it is equal on both sides and it balances. Take the open end of the thread and put it
through the hole. And take the other end of the thread with
the loop and put it around the leg of the structure. Add the bowl for weighing to the other end
of the chain. Bring the bowl down till the point it is close
to the ground and then tie the open end of the thread to the chain. This will limit the motion of the chain otherwise
it will keep falling off the sprocket. It should also be free to fall down on the
side of the weight bowl. While it should lock if its rotated towards
the thread. Now balance the chain once again, along with
the bowl hanging and at the top point, put a marker. So that we will always know where to start
the experiment from. Here you can see the full set up from the
thread to the chain to the sprocket and the bowl. And then we need some small weights or small
objects to check the force of friction. Our set up is ready, lets start with the experiment. I’m putting a 1gm weight on the finely balanced
system. And thats enough to rotate the sprocket, the
friction is that little with ball bearings. Now we will remove the sprocket, open it up. You will notice that everything in a sprocket
opens clockwise and tightens anticlockwise, this is called reverse threading. And this is opposite to the way you tighten
anything including nuts and bolts. Inside sprocket there is a ratchet system,
don’t worry about the word, it makes the sprocket lock in one direction and free in the other. The small flag like parts that fell out with
their springs were part of that. We don’t need it now. Now assemble the sprocket and put it back
in place, now we will have sliding friction in action. Because we removed the ball-bearings. Lets see if it increase friction. Yes, it increased it to about 25 grams worth
compared to 1 gram that was earlier. Now lets open it up again and put anything
thats i made of rubber. I had a rubber piece from which i cut these
thing strips, you could also use rubber bands. Put the rubber bands on both sides of the
sprocket tighten it again and lets try once more. Remember, rubber here is increasing friction
because of the nature of material as well as the pressure by the side rings of the sprocket
when we tighten it. All our small weights could not bring it down,
and eventually it took 325gms to bring it down. So you saw that friction increased by more
than 325 times as compared to the friction with the ball bearing. Thats why the world uses ball bearings for
everything thats rotating. Things required for the TIY Project. PVC Plumbing Items listed during the experiment. Hammer or mallet. Rear sprocket of a cycle. Paper tape. Thin strong thread. Bicycle chain. A bowl for weights. Weights in a range of 1 to 500 grams. Small dot shaped sticker or bindi. Scissors. Pliers. Rubber strips or rubber bands. Thats all in this lessons, Hope you are enjoying
these lessons as much as i am enjoying making them. Our next lesson is on fluid friction, see
you there.

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