penelopehills
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If a rod is at an angle theta to a horizontal plane and is in limiting equilibrium, will the friction always act horizontally? (Rather than at an angle) If so, why?
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penelopehills
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I get that friction opposes motion but I always thought it would act at an angle as the rod will move downwards at an angle if it is not in limiting equilibrium?

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Red is where I thought the friction would be, green is the correct one (I think)
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mqb2766
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Along the rod it would be red, if there was a mass on the rod and you were considering the friction acting on the mass by the rod. This is because the (hypothetical) mass wants to slide down the bar.

But the ground friction pushes horizontally on the end of the bar. There is also the vertical ground reaction at that point. The rod end would slide horizontally left without friction. Friction must push horizontally right.
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penelopehills
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(Original post by mqb2766)
Along the rod it would be red, if there was a mass on the rod and you were considering the friction acting on the mass by the rod. This is because the (hypothetical) mass wants to slide down the bar.

But the ground friction pushes horizontally on the end of the bar. There is also the vertical ground reaction at that point. The rod end would slide horizontally left without friction. Friction must push horizontally right.
Hm ok so basically if there was a mass anywhere on the rod, the friction would act up the rod /red one and no mass means horizontal so green one
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mqb2766
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The rod - ground friction is horizontal right because the end of rod wants to move horizontal left.

If there was a mass on the rid, the mass - rod friction would be along the rod. However, the horizontal ground - rod friction would still exist.
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DFranklin
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(Original post by penelopehills)
Hm ok so basically if there was a mass anywhere on the rod, the friction would act up the rod /red one and no mass means horizontal so green one
When a surface exerts friction it always acts along (tangential to) the surface.

So the friction between the ground and the rod will always be horizontal. If you had a brick resting on the rod, then friction between the brick and the rod would act in the direction of the rod, but the friction between the rod and the ground would remain horizontal.

[And just in case you're wondering why we couldn't flip the roles of rod and ground to say that the friction exerted by the rod on the ground acts along the surface of the rod, the difference is that the contact between rod and ground doesn't involve the *surface* of the rod - just the end. So the contact point on the rod doesn't have a clearly defined "direction"].
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penelopehills
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(Original post by DFranklin)
When a surface exerts friction it always acts along (tangential to) the surface.

So the friction between the ground and the rod will always be horizontal. If you had a brick resting on the rod, then friction between the brick and the rod would act in the direction of the rod, but the friction between the rod and the ground would remain horizontal.

[And just in case you're wondering why we couldn't flip the roles of rod and ground to say that the friction exerted by the rod on the ground acts along the surface of the rod, the difference is that the contact between rod and ground doesn't involve the *surface* of the rod - just the end. So the contact point on the rod doesn't have a clearly defined "direction"].
Thank you so much!! It makes sense now!
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