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.

 

[tomcue]

I can provide some clarification on the concept of friction and why it is important for the string to be parallel to the surface in this scenario. Friction is a force that opposes motion between two surfaces that are in contact with each other. In order for an object to move, the force applied to it must overcome the force of friction.

When pulling the block with a string, the force of friction acts in the opposite direction of the pulling force. If the string is not parallel to the surface, the force of friction will not be acting in the direction that opposes the motion of the block. This means that the block may slide or move in a direction that is not intended, making it difficult to accurately measure the frictional force.

Additionally, the amount of frictional force depends on the coefficient of friction between the two surfaces in contact. The coefficient of friction is a measure of how rough or smooth the surfaces are. If the string is not parallel to the surface, the contact area between the string and the block may change, affecting the coefficient of friction and thus the amount of frictional force.

In summary, the string must be parallel to the surface in order to accurately measure the frictional force and to ensure that the force of friction is acting in the opposite direction of the pulling force. Any deviation from this parallel alignment can affect the accuracy of the measurement. I hope this helps clarify the concept for you.