Frictional force opposes relative motion?

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Discussion Overview

The discussion revolves around the nature of frictional force, specifically whether it opposes relative motion rather than just motion. Participants explore examples and clarify concepts related to static and kinetic friction in various scenarios.

Discussion Character

  • Conceptual clarification
  • Technical explanation
  • Exploratory

Main Points Raised

  • One participant asks for an example to illustrate the statement that frictional force opposes relative motion.
  • Another participant provides an example of a conveyor belt, explaining that when the belt moves, static friction between the belt and objects on it causes those objects to accelerate.
  • A participant questions the type of friction involved, suggesting that static friction is at play between the belt and the object, while kinetic friction is relevant between the ground and the object.
  • It is noted that if the object does not move with respect to the belt, static friction is responsible for its motion relative to the ground, and there is no interaction with the ground except for gravity.
  • A further example involving a car accelerating on level ground is discussed, highlighting that static friction between the road and the tyres is what allows the car to accelerate, opposing relative motion between the tyre and the road.

Areas of Agreement / Disagreement

Participants generally agree on the role of static friction in opposing relative motion, but there are nuances in the examples and definitions that remain open for further discussion.

Contextual Notes

Some assumptions about the conditions under which friction acts, such as the state of motion of the objects involved, are not fully explored. The discussion also touches on different types of friction without resolving the implications of these distinctions.

kay
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Frictional force opposes relative motion and not just motion. Can you please give an example to further explain this statement? :confused:
 
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Think of as conveyor belt, initially at rest. There are things on it, also in rest. The belt gets moving, and the things on it will move together with the belt. They must have been accelerated, what was the force?

ehild
 
so when we consider the friction between the belt and the 'thing', it's static friction?
and when we consider the friction b/w ground and 'thing', it is kinetic friction?
 
If the object does not move with respect to the belt: it is static friction between the belt and the object. It opposes the motion relative to the belt and makes the object move with respect to the ground.

There is no interaction between the ground and the object on the belt (except gravity). Friction is a contact force.

ehild
 
Last edited:
Thanks a lot! :D :thumbup:
 
A car accelerating on level ground is an interesting example. The force accelerating the car (but NOT the energy it acquires) is from the road. The force involved is static friction between the road and the tyres. If the tyres slip on the road the car doesn't accelerate. So although the force opposes relative motion between the bottom of the tyre and the road, by doing so the force accelerates the car! [I've sometimes thought that there were two brilliant discoveries/inventions concerning the wheel. The first was that fitting wheels to a cart, chariot etc. enabled it to be dragged with little resistance, the second was that turning the wheel moved the cart forward.]
 

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