36.4 Worked Example - Yoyo Pulled Along the Ground

MIT 8.01 Classical Mechanics, Fall 2016
View the complete course: ocw.mit.edu/8-0...
Instructor: Dr. Peter Dourmashkin
License: Creative Commons BY-NC-SA
More information at ocw.mit.edu/terms
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[@Postermaestro]

What was not intuitive to me was that a force that is acting not through the center of mass still results in the same change of linear momentum as a force acting through the center of mass, *in addition* to the change in angular momentum created by the torque from that same force.

[@shubhibilgaiyan6680]

Sir, if force on the yoyo is in the +x direction, it's rotating clockwise, and if that's the case, isn't the velocity of the yoyo in the -x direction at the point of contact with the ground? If it is so, shouldn't friction act in the +x direction and not in the -x direction?

[@tmpace9]

This is an old comment but, no, because of the direction of angular acceleration, Fs is in the -x direction. If there was no friction, and the yoyo slid across the ground, the tangential acceleration would be larger with the same pull force. This means there is a frictional force in the opposite direction of motion.

[@Upgradezz]

At point of contact velocity is zero.

[@KoonFox6560]

@@Upgradezz translational v is zero at pt of contact only because of friction (not sliding)

[@BasicallyaboutBasil]

Mixing left and right handed coordinate systems but did manage to get all the signs right. The video maker used left handed coordinates for the rotational motion and right handed for the linear motion.

[@hermanosmc4272]

why when we take moments, the force of friction is positive and the other force: F is negative, shouldnt be otherwise?

[@djuroradusinovic5475]

He has chosen j direction downards to be positive and i direction to the right to be positive. When you apply right hand rule now you will see that if j-finger is down(positive) and i-finger is to the right(positive) than your k-finger(thumb) would be inwards which is not positive since other two arr also positive. If he took j upwards as positive than it would be reverse.

[@tro4127]

Congusing. It woul make more sense if friction was negative and the tension was positive direction....

[@bostangpalaguna228]

well, the force that causing the rotation motion is the friction force. you can imagine that there would not be any rotation if there is no friction. so, any force that has the same direction with the friction would be +, and forces that anti-parallel to it would be (-)

[@Postermaestro]

@@bostangpalaguna228 There would be rotation without the "rolling without slipping"-friction, but counterclockwise as a result from the pulling force F and the moment arm b.

[@deathxunnat4693]

Jee students ...welcome

[@amarjyoth3000]

😄😄😄yay

[@hypocratus7341]

Yo my jee pals😂

[@abhishekchatterjee7184]

Lucky are people studying in a World Class college as MIT, MASSACHUSETTS.

[@sayanjitb]

You are also lucky that you are getting mit educational videos in your home with the advent of technology.

[@abhishekchatterjee7184]

@@sayanjitb True

[@sahilbansal4958]

Sir, i wanna know,what happend if our static friction cross its maximun values?And how you explain that motion?

[@Zonnymaka]

It'll start both rolling and slipping

[@aditya_saha]

@@Zonnymaka if the static friction is less than its maximum value, it must be equal to the force applied. so how can body have linear acceleration, shouldn't 'a'=0?

[@naman3231]

i dont get it, i did the same thing, got the same ans but my teacher marked it wrong(took Icm as mR^2/2) can someone help?

[@iamnoob4232]

I think when
fs = u Mg
Then, should
F-fs = 0
Fmaks = fs
Cause when fs = u Mg
That mean yoyo should stop

[@kylewolfe_]

0 acceleration can mean constant velocity. So when fmax is reached, if the force stays constant, it will slide without rolling at a constant velocity.