I had class with Professor Peter. Great teacher...so calm....He surrounds physics from every point. I like professors like that ! Very good explanation how static friction can be responsible for creating an alpha on the wheel rotation.
He is writing it forwards, the camera records it backwards, and software (or video equipment) flips the video. See lightboard.info to see how this was done.
I thought that too, then I realized he was doing all of this left-handed. Not that left-handed people can't do that, but there are far fewer left-handed people than right-handed people, so I realized he might have just flipped the video and wore a symmetrical shirt. You can tell since the buttons are normally on the other side. Good way to do it though.
thanks for such a good explanation since everywhere it was given static friction always come in pure rolling but thats not the case since in normal horizontal surface the velocity of the contact point and the contact surface is same son no friction comes
Isn't the wheel's contact point with the ground the point about which the wheel is rotating ? If so static friction should not produce any torque on the body right ? What's wrong in my thinking ?
The point at which the wheel is rotating about is at a distance of the radius off the ground (the wheel's center) while the contact point is on the ground.
Sorry guys, I'm new to this. If we replaced the circle with a square, and applied the same angular force in this example to the square, why does the square not move as freely as the circle?
Hey man, I hope you got your question answered 4 years ago, but if the torque is applied to a square, it would not be the same because it would be slipping. When rolling, the system is not slipping, that’s why it’s static friction exerting a torque. When it’s a square, the force exerted by friction is kenetic, and thus the square would be slipping, no angular acceleration, and everything would be linear and not angular
@@edwardmalpartida5894 Nope! If the wheel was not rolling, then it was will never roll while it is slipping unless there is torque on it. Mainly by static friction. However if it was put in rolling from the beginning it will keep rolling and translating (slipping) independently of each other, meaning that: Vc.m. not necessarily equal to Rw of the wheel. You need friction to start rolling but after that you don't need it. Just like you need some force to start moving but once you move, you can keep so (Newton's first law)
If static friction is acting on a inclined plane ... Then a torque is acting which is inc angular vel... I get it as it is giving angual acc...but for the translational motion isnt the friction retarding it....if the friction is retarding it as a condition v=r(omega) then v is dec but omega is inc isnt that skitting again??
Yes, you're right, but that's the answer to how it works. Think of it like this: friction is trying to bring that body into pure rolling. Hence, when friction is acting in the opposite direction, it is both impeding the linear velocity, as well as increasing the angular velocity due to the torque, in such a way that v decreases, omega increases and v= (omega) *r , basically setting it into pure rolling.
In the inclined case, we have an external force due to gravity constantly giving it a linear acceleration to the front. Hence, friction is constantly required to oppose this forward skidding motion and keep it in pure rolling. Hence, inclined planes are a special case where friction enables pure rolling. On a level surface, it is different. There is no external force as such in the forward or backward direction, and hence static friction is zero in pure rolling. However, as the wheel shall accelerate or decelerate, friction does again come into play for the same reasons.
if friction causes torque in case of inclined, then which force causes torque in linear case? Is the the person who throws? because as far as my senses, I have seen people giving initial linear push (force) to objects, no body produces torque before throw at bowling. Please correct me. I am so confused.
It is still friction which causes the torque in the linear case. What he discussed in the linear case is that of pure rolling, where the static friction is zero due to the point of contact being at rest. But when you throw a bowling ball, obviously with linear force, and it lands on the ground, the point of contact is not at rest, but has a linear velocity forward. Hence, there is friction in the opposite direction, causing a torque that rotates it forward.
What we need to understand primarily about the role of friction here is that it only comes into play when there is a relative motion between the wheel and the floor at the point of contact, and that causes it to bring about a torque that resists the slipping motion and try to bring it to pure rolling.
What about dynamic friction, along which direction it will act and how it will affect the motion of the rolling object. Please answer me Sir, keeping in view the direction of instantaneous impact of the rolling disc on the surface.
@@idkconflict in the previous video he went over as to why at the point of contact the point P is at rest therefore it cannot of kinetic friction, since you would need non-zero velocity to have kinetic friction. Which is why he uses static friction, since the contact point the point of contact is at rest. Hope this helps!
Wheel will rotate still as it is moving with uniform velocity. Static friction will be there when there is an external force trying to rotate the wheel at rest. Though the point of contact is stationary, there is no external force and so there is no static friction.
then it depends on where the COM is located, and where the force is being applied on the disc so that it could roll due to the angular velocity and acceleration produced by the torque produced by friction once a force is applied collinearly to the COM. If the force is applied on some other part of the disc which does not coincide with the COM, then the body will roll even faster due to both the force and friction
If we take the center of the wheel as a reference point, so relative to this point the contact point between the wheel and the inclined is in motion (not at rest). My question what is the expression of the work done by the friction between the wheel and incline relative to the center of the wheel?
This is kinda weird to me b/c he's facing me but his writing is legible which makes me feel like this is a mirror reflection which makes me feel like he's actually where I am but I'm where I am so he must be me and now I'm having an identity crisis.
yes, friction depends on the friction coefficient for every body(surface type or material) and it is a backward pull which opposes the direction of linear velocity of every body and supports angular velocity if the body can roll
Can someone explain me this? It says that the slipping wheel has not static friction, well that's right. But still has kinetic friction, right? And the other question is: in the case of the rolling wheel where there is static friction. If the wheel is rotating with zero linear acceleration, does it still has static friction? Or not and it only stops in theory because of air resistance?
your first ques. During pure rolling there is no relative motion between the surfaces of sphere and the floor or ground.So only static friction acts.But during Rolling WITH slipping kinectic friction acts uptill pure rolling doesn't start. 2nd ques.yes if it has no linear accn it still suffers static friction(Remember Friction is the cause of rolling without it the topic doesn't exist because it produces the necessary torque for the body to roll).Hope it helps:)
@@daksheshprasad9941 Thanks. What I still don't understand is: I supposed that static friction only occurs when there is an acting force (e.g., in the case of gravity in inclined plane). Yet if I imagine an scenario where there is no linear acceleration taking place, what force is the one that generates the static friction then? Or is this "static friction" different from the static friction that takes place in the usual situations in non rolling bodies, and occurs in absence of active forces?
@@lea1822 No motion can start spontaneously in the absence of external force.A wheel or ball on a surface is held there by static friction. In order to start the rolling motion, a force or torque must be applied to the wheel. The force of the static sliding friction prevents the wheel from sliding and thus initiates the rolling motion.
Doubt: (For pure rolling motion on a horizontal surface) if static friction does not act the net torque will also be zero , then how can the body continue the rolling motion ?
Static friction is resistance to the body at rest against external force trying to move it. Here in the first case, the the body doesn't subjected to any external force and hence no static friction.
Friction does not depend on area, but it does depend on the contact material and the plane inclination . You can see how the inclination dependency comes to play by drawing a force schematics on a problem similar to the one shown in the video , for some angle alpha . The contacy material affects friction since every surface has different friction coefficint (mu) , and static friction follows f
jagmohan gautam in reality when an object rolls there is also a normal reaction acting on it. All real objects a compressible, therefore this normal reaction does not act exactly through the center of mass. Instead it acts through a point slightly to the right or left to the center of mass according to the direction of rolling. Torque produced due to this decelerates the object and brings it to rest
Hi, it would be very helpful if someone could help me with this question: Let's say we have a wheel, we give it a starting speed (Ucm let's call it) and it rolls on a surface with friction. Now, everyday life observation says the wheel must stop at some point, right? Well, how does it stop? I can't seem to find out a way where the friction's torque does not accelerate one movement and decelerate the other...
I'm late, also a real wheel isn't perfectly circular and the contact point isn't a point, but rather an ellipse. They both cause the wheel to slow down.
its because nothing in real life is perfectly circular. And every point on the wheel is irregular with respect to the surface. Hence, friction is irregular on each part causing different linear accelerations which oppose and support each other.
@@awwadsharaf the fs is a force that is opposite to slipping, hence it should be pointing down the incline, not upwards. The rolling movement will tend to slip relatively to the plane...the wheel surface point of contact will not slip backwards and therefore, rolls, because fs prevents that from happening...or does it?
That's the whole point no, the bottom point that is in contact with the surface is at instantaneous rest! If that portion is not moving at all then how would you apply Kinetic friction?!
The peculiarity of pure rolling is in the fact that the contact point has velocity equal to zero, and for this reason the force exchanged between the wheel and the plane is of static friction.
@@antoniomangiapane8922 So you are saying when it's rolling with sliding we should use rolling friction and when it's pure rolling static friction should be used?
He says the wheel has an initial velocity (Vcm) for the horizontal case (in a vacuum). Therefore, it will continue to roll if no other forces are acting on the wheel. I hope this clears that up a little bit. Good luck!
It seems like Indian students are much more into theoritical education than the Mitians. Though we indians dont get practical education but the depth of questions we reach to is better than americans
What makes you say that !! while some students excel in their careers in STEM,there are lots of students who are failed and commit sucide.Indians only give respect to STEM subjects which has really destroyed a lot of talent in India.
ohh..really ? Tell me any major invention done by indian . facebook - abroad ,google-abroad and amny more . We crave to study into MIT .. so why don't u just stfu !
I had class with Professor Peter. Great teacher...so calm....He surrounds physics from every point. I like professors like that ! Very good explanation how static friction can be responsible for creating an alpha on the wheel rotation.
how tf does he just write backwards that fast
He is writing it forwards, the camera records it backwards, and software (or video equipment) flips the video. See lightboard.info to see how this was done.
I couldn't concentrate because of that...
I thought that too, then I realized he was doing all of this left-handed. Not that left-handed people can't do that, but there are far fewer left-handed people than right-handed people, so I realized he might have just flipped the video and wore a symmetrical shirt. You can tell since the buttons are normally on the other side. Good way to do it though.
@@mitocw Backwards recording camera!
It is same like Writing on Mirror and recording Mirror
😂😂😂
thanks for such a good explanation since everywhere it was given static friction always come in pure rolling but thats not the case since in normal horizontal surface the velocity of the contact point and the contact surface is same son no friction comes
Isn't the wheel's contact point with the ground the point about which the wheel is rotating ? If so static friction should not produce any torque on the body right ? What's wrong in my thinking ?
The point at which the wheel is rotating about is at a distance of the radius off the ground (the wheel's center) while the contact point is on the ground.
Sorry guys, I'm new to this. If we replaced the circle with a square, and applied the same angular force in this example to the square, why does the square not move as freely as the circle?
Hey man, I hope you got your question answered 4 years ago, but if the torque is applied to a square, it would not be the same because it would be slipping. When rolling, the system is not slipping, that’s why it’s static friction exerting a torque. When it’s a square, the force exerted by friction is kenetic, and thus the square would be slipping, no angular acceleration, and everything would be linear and not angular
The surface at contact is too large for effective rolling due to which it rather slips than executing a rolling motion
Sir. If the static friction on a rolling wheel is zero, what makes it roll??
If there is no friction it will not roll, it will simply slide like an ice puck on a surface.
Exactly, there’s something off with that “first case”, that object could not be rolling if there was no friction
@@edwardmalpartida5894
Nope!
If the wheel was not rolling, then it was will never roll while it is slipping unless there is torque on it. Mainly by static friction.
However if it was put in rolling from the beginning it will keep rolling and translating (slipping) independently of each other, meaning that:
Vc.m. not necessarily equal to Rw of the wheel.
You need friction to start rolling but after that you don't need it. Just like you need some force to start moving but once you move, you can keep so (Newton's first law)
the work is null notnthe force porco dio
If static friction is acting on a inclined plane ... Then a torque is acting which is inc angular vel... I get it as it is giving angual acc...but for the translational motion isnt the friction retarding it....if the friction is retarding it as a condition v=r(omega) then v is dec but omega is inc isnt that skitting again??
Yes, you're right, but that's the answer to how it works.
Think of it like this: friction is trying to bring that body into pure rolling. Hence, when friction is acting in the opposite direction, it is both impeding the linear velocity, as well as increasing the angular velocity due to the torque, in such a way that v decreases, omega increases and v= (omega) *r , basically setting it into pure rolling.
In the inclined case, we have an external force due to gravity constantly giving it a linear acceleration to the front. Hence, friction is constantly required to oppose this forward skidding motion and keep it in pure rolling.
Hence, inclined planes are a special case where friction enables pure rolling.
On a level surface, it is different. There is no external force as such in the forward or backward direction, and hence static friction is zero in pure rolling. However, as the wheel shall accelerate or decelerate, friction does again come into play for the same reasons.
if friction causes torque in case of inclined, then which force causes torque in linear case? Is the the person who throws? because as far as my senses, I have seen people giving initial linear push (force) to objects, no body produces torque before throw at bowling. Please correct me. I am so confused.
It is still friction which causes the torque in the linear case.
What he discussed in the linear case is that of pure rolling, where the static friction is zero due to the point of contact being at rest.
But when you throw a bowling ball, obviously with linear force, and it lands on the ground, the point of contact is not at rest, but has a linear velocity forward. Hence, there is friction in the opposite direction, causing a torque that rotates it forward.
What we need to understand primarily about the role of friction here is that it only comes into play when there is a relative motion between the wheel and the floor at the point of contact, and that causes it to bring about a torque that resists the slipping motion and try to bring it to pure rolling.
What about dynamic friction, along which direction it will act and how it will affect the motion of the rolling object.
Please answer me Sir, keeping in view the direction of instantaneous impact of the rolling disc on the surface.
The point of contact is in rest...so why would dynamic friction come into picture..?
@@tysonff2627could you elaborate on that
@@idkconflict in the previous video he went over as to why at the point of contact the point P is at rest therefore it cannot of kinetic friction, since you would need non-zero velocity to have kinetic friction. Which is why he uses static friction, since the contact point the point of contact is at rest. Hope this helps!
If no friction is present in the first example how does it roll? Wouldn't it just slide?
I think It has a constant angular velocity.
I agree
This topic is very important if you want to study mobile robots.
Wonderful! Thanks fo r the upload.
if static friction=0 how the wheel rotate?
It will rotate at its position.
@@shaswatsatyam what are you talking about, it wouldn't rotate then. It would just slide down the hill
Wheel will rotate still as it is moving with uniform velocity. Static friction will be there when there is an external force trying to rotate the wheel at rest. Though the point of contact is stationary, there is no external force and so there is no static friction.
What if the center of mass is not located at the center of the disc?
then it depends on where the COM is located, and where the force is being applied on the disc so that it could roll due to the angular velocity and acceleration produced by the torque produced by friction once a force is applied collinearly to the COM. If the force is applied on some other part of the disc which does not coincide with the COM, then the body will roll even faster due to both the force and friction
If we take the center of the wheel as a reference point, so relative to this point the contact point between the wheel and the inclined is in motion (not at rest). My question what is the expression of the work done by the friction between the wheel and incline relative to the center of the wheel?
Work done = Torque times angle of rotation and hence equal to frictional force * radius * angle of rotation
Is static friction drag?
This is kinda weird to me b/c he's facing me but his writing is legible which makes me feel like this is a mirror reflection which makes me feel like he's actually where I am but I'm where I am so he must be me and now I'm having an identity crisis.
Is friction a backward or inward pull against a motion in some v direction and depends on the coefficients of friction of surface material?
yes, friction depends on the friction coefficient for every body(surface type or material) and it is a backward pull which opposes the direction of linear velocity of every body and supports angular velocity if the body can roll
Isn't the maximum value of static friction not "mu N" Here?
Sir can we perform this on inclined plane
Can someone explain me this? It says that the slipping wheel has not static friction, well that's right. But still has kinetic friction, right? And the other question is: in the case of the rolling wheel where there is static friction. If the wheel is rotating with zero linear acceleration, does it still has static friction? Or not and it only stops in theory because of air resistance?
your first ques. During pure rolling there is no relative motion between the surfaces of sphere and the floor or ground.So only static friction acts.But during Rolling WITH slipping kinectic friction acts uptill pure rolling doesn't start.
2nd ques.yes if it has no linear accn it still suffers static friction(Remember Friction is the cause of rolling without it the topic doesn't exist because it produces the necessary torque for the body to roll).Hope it helps:)
@@daksheshprasad9941 Thanks. What I still don't understand is: I supposed that static friction only occurs when there is an acting force (e.g., in the case of gravity in inclined plane). Yet if I imagine an scenario where there is no linear acceleration taking place, what force is the one that generates the static friction then? Or is this "static friction" different from the static friction that takes place in the usual situations in non rolling bodies, and occurs in absence of active forces?
@@lea1822 No motion can start spontaneously in the absence of external force.A wheel or ball on a surface is held there by static friction. In order to start the rolling motion, a force or torque must be applied to the wheel. The force of the static sliding friction prevents the wheel from sliding and thus initiates the rolling motion.
Sir,is friction is necessary to establish pure rolling?
yes(if you are talking about a wheel)
Doubt: (For pure rolling motion on a horizontal surface) if static friction does not act the net torque will also be zero , then how can the body continue the rolling motion ?
i guess it was rolling at inital otherwise it cannot roll without angular accelaration
I miss Walter everytime when I see these lectures
Same here...miss him every time
Then go watch him instead of missing! He was not the only one good teacher
Plz show some respect
dev pandey He was a perv
He did get fired for being a perv search it up
Why is the static friction zero in first case?
Static friction is resistance to the body at rest against external force trying to move it. Here in the first case, the the body doesn't subjected to any external force and hence no static friction.
Will it also depend on material and inclination angle and also the area?
Friction does not depend on area, but it does depend on the contact material and the plane inclination .
You can see how the inclination dependency comes to play by drawing a force schematics on a problem similar to the one shown in the video , for some angle alpha .
The contacy material affects friction since every surface has different friction coefficint (mu) , and static friction follows f
I am not getting completely. my ques is that if torque is provided by friction so why the wheel stopped aftr sometime as friction is always there.
jagmohan gautam in reality when an object rolls there is also a normal reaction acting on it. All real objects a compressible, therefore this normal reaction does not act exactly through the center of mass. Instead it acts through a point slightly to the right or left to the center of mass according to the direction of rolling. Torque produced due to this decelerates the object and brings it to rest
friction only acts as long as there is nonzero velocity: if you slide a block down a table, it doe not start to reverse its direction due to friction
Hi, it would be very helpful if someone could help me with this question:
Let's say we have a wheel, we give it a starting speed (Ucm let's call it) and it rolls on a surface with friction.
Now, everyday life observation says the wheel must stop at some point, right?
Well, how does it stop? I can't seem to find out a way where the friction's torque does not accelerate one movement and decelerate the other...
Yeah I just saw that he actually explains it, thanks a lot!
I'm late, also a real wheel isn't perfectly circular and the contact point isn't a point, but rather an ellipse. They both cause the wheel to slow down.
its because nothing in real life is perfectly circular. And every point on the wheel is irregular with respect to the surface. Hence, friction is irregular on each part causing different linear accelerations which oppose and support each other.
I have this exact same doubt and still can’t understand it
sir which software you use? really nice video liked it
I think there's something wrong with your explanation, Sir.
I agree
@@awwadsharaf the fs is a force that is opposite to slipping, hence it should be pointing down the incline, not upwards. The rolling movement will tend to slip relatively to the plane...the wheel surface point of contact will not slip backwards and therefore, rolls, because fs prevents that from happening...or does it?
Everything is correct
Since its rolling
ty very much sir
what about Dynamic friction, doesn't it also act on the wheel after it starts moving?
No
That's the whole point no, the bottom point that is in contact with the surface is at instantaneous rest! If that portion is not moving at all then how would you apply Kinetic friction?!
@@piyush2404 thanks for the explanation!
@@abadibi You're welcome buddy, have a nice day🤗
If u write from your side on a transparent thing, to us on this side the words should appear laterally inverted....!!!
video is flipped
Best
Wtf static friction while rolling???
The peculiarity of pure rolling is in the fact that the contact point has velocity equal to zero, and for this reason the force exchanged between the wheel and the plane is of static friction.
@@antoniomangiapane8922 So you are saying when it's rolling with sliding we should use rolling friction and when it's pure rolling static friction should be used?
I think he means rolling friction when he says static. When there’s sliding we use dynamic friction.
@@antoniomangiapane8922 Yeah he should have meant rolling friction.
Sir if static friction is 0 how the wheel rotate sir
He says the wheel has an initial velocity (Vcm) for the horizontal case (in a vacuum). Therefore, it will continue to roll if no other forces are acting on the wheel. I hope this clears that up a little bit. Good luck!
Jyst put it in laymen terms In other word's
The bloody wheel wants to stay home an eat ice cream.
Is this dude writing backwards?!?
video is flipped
@@zuboragabora8966 or a mirror
he must be right-handed, however he writes using the left hand...so, the image is horizontally flipped.
this 2 people chat XD
Sir can you please do a problem for a wheelchair move up a slope of 5°, with total weight is 150kg
How is this video shot!!
Anyone in class 11 🤔
Teaching is not good....
It seems like Indian students are much more into theoritical education than the Mitians. Though we indians dont get practical education but the depth of questions we reach to is better than americans
you really didn't have to brag about that.
Exactly you didnt have to do that
Well, it is not the only nation that does such "difficult" stuff😝
That is a gross assertion that I’m confident you have no evidence for. Speaks volumes about your intellect and dare I say ego.
It seems to me that people from a third world country like iNdia shit on the streets
i can clearly say that indians have good education system than americans
Bro, your grammar is trash. I wouldn't be making fun of another education system. Yours has clearly failed you.
This is the most stupid and ignorant comment that I have seen in a while.
I don't think like that
What makes you say that !! while some students excel in their careers in STEM,there are lots of students who are failed and commit sucide.Indians only give respect to STEM subjects which has really destroyed a lot of talent in India.
ohh..really ? Tell me any major invention done by indian . facebook - abroad ,google-abroad and amny more . We crave to study into MIT .. so why don't u just stfu !
The good thing is Americans focus on learning and absorbing whereas we indians focus more in depth of the subject + cramming if we dont get it