9/18/2014 Modeling Friction Forces

Static Friction

Purpose:

On this lab we will measure the static force of an object which is the force created by friction when object is held in place by another from friction.

Experiment:

One of the items we needed was a wooden block which we measured it mass on a scale. Then we got a pulley which we put at the end of a table. With the a wire we attach the block to loop over the pulley down the table and hung a cup of for adding water as a mass. With this setup we added water until the block begin to move on the track. We then measured the mass of the cup of water. We repeated this experiment five times adding another wooden block on our original block. Here is a picture of our experiment:

Here is a picture of our recorded data after five tests:

I wasn't sure on this lab if our scale was weighing mass or weight so unfortunately this data is bad even though it comes out looking right.

With our data table we used Logger Pro to graph max static friction force vs normal force. With the graph we can find the coefficient of static by finding the slop of our graph. Here is our graph of  max static friction force vs normal force.

The value we have for the slope which is our coefficient of static is 0.3866 which I believe is inaccurate and we should have done another set of data.

Kinetic Friction:

Purpose:

The purpose of this portion of the lab is to measure kinetic friction which is the force created by friction of an object that is moving.

Experiment:
On this lab we used a force sensor which measure the amount of force created by pulling on the sensor or pushing on the sensor. To get good collection of data we first had to zero the force sensor on Logger Pro. Then we attached a string and a known objects mass to measure the force to make sure we had good data collection. Then we the same blocks we used on the static friction part of this lab we pulled the block across the table measuring the amount of force it took. Here is a picture of out setup:

Here is an example of a reading from the force sensor which we used to get an average force from a 

graphs slope of velocity vs time.

We did this experiment five times increasing the number of block on top of the original block four times. We measured the mass of the block and calculated the the normal force the block created with the table. We data reading from the force sensor we had a average kinetic friction force from Logger Pro. We took all this data and created a table as shown below.

With this data table we are now able to once again graph max kinetic friction force vs normal force of the block. Here is the graph showing are slope which is the coefficient of kinetic friction.

We now have a reading for our kinetic friction coefficient as 0.2887 which I think is a good collection of data and we have a good result for our kinetic friction coefficient.

Kinetic Friction and Static Friction on a slope:

Purpose:

On the last lab we once again measure static friction first then kinetic friction again, yet this time we did it on a track which was on a slope.

Experiment:

To find the static friction we set up our ramp to an angle where the block wood was just about to slide down the ramp. Then we measure the height and length of the ramp to calculate our angle. With that all we had to do was create a FBD with the forces and solve for us which ended up being the tangent of our angle. Here is the setup and our calculations.

As you can see from our FBD and equation we found the coefficient of static friction to be 0.3 which is not that far off from our first experiment.

For the static friction part we  raised the ramp to have the block slide down the ramp. We measured the angle again by using the height and distance of the track and we also got the mass of the block from a scale. Then we used a motion sensor to measure the velocity of the block which we graphed on Logger Pro to find the acceleration by graphing velocity vs time. With that data and our mass of the block we where able to create another FBD and solve our equation for the coefficient of kinetic friction. Here is our FBD and our equation.

As the picture shows our coefficient for kinetic friction is 0.386.

Results:

While measuring the static friction was simple it was not an effective way to get a measurement because the block we be moved by a smaller weight and then the same block would take almost twice the weight to move. So our mass of the object that made it move has a large range of error.

The measurement of the kinetic friction was easier to move the object but to move the object at a constant speed or acceleration was very hard to do. Out of all our measurements I think our last experiment produced the best result for reliable data.