March 16, 2015: Modeling Friction Forces

Purpose:
The purpose of this lab is to get a better understanding of static friction and kinetic friction. Thru experiment we hope to come up with a model to be able to predict values of acceleration.

Set-up:

Water was added and weighed

Here are all the blocks we used and the balance we used

We weighed each block

We set up our mass on the table, and hanging of the ledge our cup with water

Static Friction:

We took a block and tied string to it and let a cup with water hang on one side, water was added until it the blocks began moving as this would indicate our maximum static friction. The same block was always used in the same position on the bottom to ensure the most accuracy with measuring static friction.

The values we obtained for the masses of the blocks and the required mass to make them move is the following:

We then took the values of normal force and graphed them against a static force required to move them. The using a linear fit we obtained a line with the slope of the coefficient of the static friction.

Kinetic Friction:

For this part of the lab we changed the set-up instead of having the block pulled by water, we connected a force sensor to the string and pulled the sensor (and therefore moving the block) at a constant velocity. We did this 4 times and recorded the data. This data was then analyzed and the mean for each of them was found.

The means were graphed along side the force applied to each of these and the following graph was obtained with the coefficient of static friction being 0.2482 .

Static Friction at a sloped surface:

 We set up the block on a ramp and elevated it until the point were it just started to slide.

In our case, we found the angle of static friction to be 12.7 degrees. Then we drew up a free-body-diagram and solved for the static friction. As we can see here, items just canceled out leaving us with a static coefficient on this surface of 0.325 .

Kinetic friction of downhill block:

We used the same setup a the previous one except we hooked up a motion sensor to the top and increased the angle to ensure a downward acceleration. We choose 22.3 degrees and took measurements of our block sliding down the ramp. What we got was the following data set: 

Our value for acceleration as seen here is of a= 0.320(m/s^2). Since we already knew the mass of our block, again we used newtons laws and solved for our kinetic friction:

As we see here, our final value for our kinetic friction was 0.3745 . 

Prediction the Acceleration:

For this experiment we had to use what we knew about this block in order to predict what would happen if a force where to be applied to it. So we set up a scenario, this included a frictionless pulley and a mass. We knew that a mass of at lease .047kg is required to move our block so we used a 0.74kg mass. 

The following formula was derived for our prediction:

The actual values we got where:

Conclusion:

What we learned in this lab is that static friction is higher than kinetic friction. Once an object starts moving, it takes less force to keep it in motion. We also learned that different surfaces have different friction coefficients, not all materials are the same and have different surfaces. Of course we also had measurement error, but what we saw if fundamentally this: Static friction is higher than kinetic friction.