Homework Help: Why is it important to make sure the string attached to the block level?

1. Sep 30, 2011

paxian

1. The problem statement, all variables and given/known data
In a friction lab to determine the static and kinetic coefficient of friction, there is a block on a flat slope attached to a string which pulled down by a hanging mass. The lab question is: Why is it important to make sure that the string attached to the block is level?

2. Relevant equations

none

3. The attempt at a solution
is it bc at that angle, it only has one component x (horizontal component) and thus resulted in the lowest required force to move the mass with that string.

2. Sep 30, 2011

Staff: Mentor

Actually, the horizontal position doesn't result in the lowest required force to move the block. If the string angles upwards from the block, some of the force would reduce the effective weight of the block on the surface and reduce the friction.

Also, examine what happens to the angle as the block moves closer to the pulley. What happens?

In a lab situation it is usually desirable to minimize the number of variables that change t the same time. Ideally only one variable should be, well, variable! That way you can concentrate on measuring the effects of changing one thing at a time.

3. Sep 30, 2011

SammyS

Staff Emeritus
paxian, welcome to the PH forums.

How is the normal force affected?

What if the angle is not taken into account? How would this affect the resulting calculation of coefficient of friction?

4. Sep 30, 2011

paxian

Thanks so much for your quick responses!

The normal force would be smaller if the string is level?

Friction force acts parallel to the plane of interface. If the string is not leveled, the coefficient of friction is a lot smaller??

5. Sep 30, 2011

Staff: Mentor

The coefficient of friction is a characteristic of the materials involved. It doesn't change.

Draw a force diagram (FBD) for the block to determine what changes when the string angle changes.

6. Sep 30, 2011

paxian

After I draw the FBD...The Tension will now have x and y components when the string angle changes. Ff and N stay the same. If the string is leveled, it only has one component x (horizontal component).

You also helped me to answer one of my other lab questions. The coefficient of friction is the same on flat slope and inclined slope. Thanks so much.

By the way, I am so new to Physics and applying Physics theory... Please be a little bit more specific so that I can understand and learn Physics from you.

7. Sep 30, 2011

Staff: Mentor

Careful, that y-component of the string force is going to affect the normal force! Ff will change if the string angle changes!
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