I Why does friction act in the normal direction

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1. Feb 15, 2017

zachdr1

For a particle undergoing a constant velocity circular motion. I thought friction always acted in the direction of motion which would be in the tangential direction.

2. Feb 15, 2017

A.T.

Kinetic friction acts opposite to relative motion at the contact. Static friction can point in any direction parallel to the contact plane.

3. Feb 15, 2017

zachdr1

So how do you determine the direction of static friction?

4. Feb 15, 2017

A.T.

Depends on what else is known.

5. Feb 15, 2017

zachdr1

Cartons having a mass of 5kg are required to move along the assembly line at a constant speed of 8 m/s. Determine the smallest radius of curvature for the conveyor so the cartons do not slop. The coefficients of static and kinetic friction between a carton and the conveyor are .7 and .5 respectively

6. Feb 15, 2017

A.T.

Since friction is the only horizontal force its direction should be obvious.

7. Feb 15, 2017

Staff: Mentor

Show us your free body diagram.

8. Feb 15, 2017

ZapperZ

Staff Emeritus
Boys and girls. This example is why we insist that you present your question clearly and fully, and with the CONTEXT. Otherwise, we will give you apples, when what you're asking are oranges!

When the OP asked the problem, it appears as if this is a normal, sliding mass problem, in which of course the frictional force is NOT in the normal direction.

However, it is ONLY after the OP presented the ACTUAL problem do we see what it really is! In this problem, the frictional force is the one providing the centripetal force! So this is not the same problem and with a different context than earlier. In this case, the frictional force/centripetal force points inwards (not in the normal direction).

The OP needs to present this in the HW section of the forum and, as Chestermiller has stated, he/she needs to show the free-body diagram.

So moral of the story: when we ask you to post your question in full and as clearly as possible, this is the VERY reason why! Without context, one can have a widely-varying scenario that is possible from a vague question.

Zz.

9. Feb 15, 2017

A.T.

Yes, "normal" is ambiguous. In the context of friction it usually means normal to the contact surface, but the OP meant normal to the path of the object.

10. Feb 15, 2017

ZapperZ

Staff Emeritus
Which again, is another reason why we insist on a clear and complete post. I think a lot of people somehow assumed that we can read what's in their heads, whereas many of the terms being used in physics already have a clear definition which may be different than what they are thinking of.

The art of clear communication is something that one can acquire only via practice, and this forum is a very good place to practice.

Zz.