Direction of friction on a rolling body

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SUMMARY

The discussion centers on the dynamics of a rolling cylinder on a horizontal surface and the role of friction in its deceleration. When the cylinder rolls anticlockwise and slows down, friction acts to the right, generating an anti-clockwise torque that contradicts the cylinder's stopping motion. The key insight is that the deceleration is primarily due to rolling resistance, which involves surface deformation rather than simple friction. This results in a clockwise angular acceleration, highlighting the complex interplay between friction, surface deformation, and aerodynamic drag.

PREREQUISITES
  • Understanding of basic mechanics, specifically torque and angular acceleration.
  • Familiarity with the concept of rolling resistance and its effects on motion.
  • Knowledge of the relationship between linear and angular motion.
  • Basic principles of friction and surface interactions.
NEXT STEPS
  • Research the concept of rolling resistance in detail, including its mathematical modeling.
  • Explore the effects of surface deformation on rolling objects and how it influences motion.
  • Study the relationship between torque, angular acceleration, and linear motion in rolling bodies.
  • Investigate aerodynamic drag and its impact on the motion of rolling cylinders.
USEFUL FOR

Physics students, mechanical engineers, and anyone interested in the dynamics of rolling motion and the factors affecting it, particularly in relation to friction and resistance.

Happiness
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A cylinder rolls on a horizontal, flat surface without sliding towards the left, so it must be rolling anticlockwise about its center of mass (CM). Suppose it slows down to a stop due to friction. What is the direction of the friction at the point of contact?

Since it slows down, friction must be acting to the right. But this rightward friction produces an anti-clockwise torque ##\tau## about the CM. Since ##\tau=I\alpha##, this anti-clockwise ##\tau## produces an anti-clockwise angular acceleration ##\alpha## about the CM. Since the cylinder does not slide, a faster rotation means its CM moves faster. This contradicts the premise that the cylinder comes to a stop. What's wrong?

I am guessing any cylinder that comes to a stop must slide. For a cylinder that is not observed to be sliding, it is still sliding but not noticeably. Is this true?
 
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Happiness said:
Since it slows down, friction must be acting to the right. But this rightward friction produces an anti-clockwise torque ##\tau## about the CM. Since ##\tau=I\alpha##, this anti-clockwise ##\tau## produces an anti-clockwise angular acceleration ##\alpha## about the CM. Since the cylinder does not slide, a faster rotation means its CM moves faster. This contradicts the premise that the cylinder comes to a stop. What's wrong?
It's not simple friction that causes the rolling cylinder to slow and stop, but deformation of the surface. (This effect is called "rolling resistance".) The effect of surface deformation ends up with the reaction force of the surface acting ahead of the center of the cylinder, thus creating a clockwise angular acceleration.
 
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Doc Al said:
It's not simple friction that causes the rolling cylinder to slow and stop, but deformation of the surface...
... and deformation of the cylinder, and aerodynamic drag.
 

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