# String in an experiment setup

Why does the string in an experiment setup fro friction experiment be parallel to the horizontal plane? What is its significance? (see picture attached)

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SteamKing
Staff Emeritus
Homework Helper
First, and most important, the reading on the scale will be the amount of force required to pull the doowhat on the left hand side. You won't need trig to figure out force components and whatnot.

But our teacher says that it is important that the string is parallel to the ground.. why?

jbriggs444
Homework Helper
But our teacher says that it is important that the string is parallel to the ground.. why?

Because the experiment is attempting to measure the frictional force that is produced by a particular normal force.

If the string were angled up slightly as it goes toward the scale then it would be pulling up on the weights. They would be pressing down with less than their full weight on the ground since they would be partially supported by the string. The normal force would be reduced.

If the string were angled down slightly as it goes toward the scale then it would be pulling down on the weights. They would be pressing down with more than their full weight on the ground. The normal force would be increased.

Because the experiment is attempting to measure the frictional force that is produced by a particular normal force.

If the string were angled up slightly as it goes toward the scale then it would be pulling up on the weights. They would be pressing down with less than their full weight on the ground since they would be partially supported by the string. The normal force would be reduced.

If the string were angled down slightly as it goes toward the scale then it would be pulling down on the weights. They would be pressing down with more than their full weight on the ground. The normal force would be increased.

I do not understand.. sorry.

Nugatory
Mentor
I do not understand.. sorry.

Think about the extreme case. Suppose you allowed the string to form a very steep angle with the surface, say ninety degrees. Then what would the scale be measuring?