Why Must the String Be Parallel to the Surface in Friction Experiments?

[munchy35]

Homework Statement

Alright, I understand that friction needs to be parallel to the surface, but I'm confused on why.

So if a string is attached to a block that connects to hanging masses in a lab, and you're pulling the block to measure the friction force, why does it have to be parallel to the surface?

and would the frictional force change if the string were not parallel?

Homework Equations

no equations.

The Attempt at a Solution

I think that it has to be parallel so the object does not slide or move? and the frictional force would change it the string were not parallel i just don't know why... maybe it would cause the force needed to pull the block to increase? I'm confused on this concept.
help?

 

[berkeman]

Homework Statement

Alright, I understand that friction needs to be parallel to the surface, but I'm confused on why.

So if a string is attached to a block that connects to hanging masses in a lab, and you're pulling the block to measure the friction force, why does it have to be parallel to the surface?

and would the frictional force change if the string were not parallel?

Homework Equations

no equations.

The Attempt at a Solution

I think that it has to be parallel so the object does not slide or move? and the frictional force would change it the string were not parallel i just don't know why... maybe it would cause the force needed to pull the block to increase? I'm confused on this concept.
help?

When you pull the block with a string like that, the vertical component of the force from the string on the block serves to lighten the block (lower its normal force on the surface), and the horizontal component of the string force is what opposes the frictional force, and provides sideways acceleration if that component is larger than mu*N.