While reading my textbook I came across an activity which told :

Try rolling a barrel from concrete onto grass, what happens does the barrell reverses its direction of motion ? Is this because of greater resistance offered by grass ?

And then later it answers :

rolling barrel from concrete onto grass reverses its direction of motion as grass offers greater resistance.

Now as said I did the activity but nothing like reversing of direction was observed by me , on contrary the barrel stopped.

I can't understand how direction of motion would reverse on I believe that due to friction barrel slows down but that contradicts what's written , where I am going wrong ?

  • $\begingroup$ Was there a diagram in your book? The barrel rolling analogy is sometimes used to explain refraction. books.google.co.uk/… $\endgroup$ – Farcher May 10 '17 at 5:06
  • $\begingroup$ @Farcher , no ; there wasn't any diagram, although looking at the link you provided I understood that barrel changes direction but how come it "reverses" its direction of motion ? $\endgroup$ – Physicsapproval May 10 '17 at 5:14
  • $\begingroup$ That doesn't sound right to me either. I can't say I've ever seen grass that is springy enough to make a rolling barrel reverse course. $\endgroup$ – JMac May 10 '17 at 11:05
  • $\begingroup$ I did the activity again and this time I noticed something that there was a litte bit backward motion after it stopped very quickly but I am not able to feel that even , can it seen mathematically ? assuming resistance due to grass larger than concrete , can it be proved ? $\endgroup$ – Physicsapproval May 10 '17 at 11:09
  • $\begingroup$ -1. What text? If you quote from a source, you ought to identify it so that we can judge how reliable it is. It does not sound reliable. $\endgroup$ – sammy gerbil May 12 '17 at 0:58

You are (probably) more likely to observe the barrel rolling backwards just before stopping when it rolls on a hard surface such as concrete, steel or marble. This happens because the centre of mass (CM) might not be exactly at the geometrical centre (centroid) of the barrel.

If the CM is behind the point of contact with the concrete when the barrel stops, the torque on the barrel could be big enough to turn it backwards. It might again overshoot and then rock forwards and backwards until its energy is dissipated by friction and the CM lies vertically above the point of contact.

This is much less likely to happen on grass because the rolling resistance to motion is higher. There is more deformation of the ground compared with a hard surface like concrete, so the torque due to rolling resistance is more likely to be higher than that due to the asymmetric CM, unless the CM is very far from the centroid.

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The textbook is probably alluding to rolling resistance, a backward force which results from deformation of the barrel and the ground. This deformation is partially elastic and stores energy. While the barrel is rolling, the surface ahead is more deformed than the surface behind, so there is a backward force on the barrel. When the barrel stops, the surface ahead is still more deformed, causing the barrel to roll backward until these deformations are equal.


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