Need help calculating friction and normal force for Measuring Friction Lab

[jillime]

Need help calculating friction and normal force for "Measuring Friction" Lab

Hey guys, I'm not great at physics and have a lot of work ahead of me so I could use some help.

The lab setup is a wooden block sliding on a wooden surface. There is a weight attached to the block by a string that's hanging from a pulley at the end of the wooden surface.

This guy right here ----> http://imageshack.us/photo/my-images/820/40999170.jpg/ if I'm being unclear.

I have found the weight it takes to start the object moving (static friction) and the weight it takes to move the block at a constant velocity (kinetic friction). I did this a bunch of different times with different weights and surface areas and now I get to do a bunch of calculations.

SO, I could really use a walk-through on simply how to calculate the friction and the normal force, because I have to do it 12 different times.

For example, my first calculation was for the wood on wood block (weighing 128.1g) with a 100g weight on top, it took 81.1 grams to start it moving (static friction). If someone could just give me basic steps to work this out, it would help a lot. (Find friction force and normal force).

This is probably simple but I don't have a physics brain and I'm new to the course. So thanks for the help.

 

[AdrianMay]

The normal force is just the total mass of your block including whatever you piled on top, times gravity (9.8N/kg).

The force in the string is similarly the mass of the thing hanging over the end of the desk times g.

If you've adjusted the latter to make it just start to move, or move with constant velocity, then you just need to divide the string force by the normal force and that's your coefficient of static/dynamic friction. This is a property of the two materials you are pushing together (assuming their equally smooth/oily/dry/etc). So it should come out the same for any given pair of materials, but be different if you use plastic or metal instead of wood.

Adrian.