18-Mar-2015: Friction Lab

This lab contained five different experiments involving friction.

Part 1: Static friction

Static friction is the force acting between two bodies when they are not moving relative to one another. In this experiment, we added water to a cup  a little bit at a time until the block just started to slip.

The threshold of motion in this experiment is defined by the coefficient of static friction

u static = f static maximum / N 

since

f static maximum = u static * N

The block has a velvet material. To determine the coefficient of static friction of that material on the table, we need to find its normal force which depends upon its mass.

Mass of One Block

Then we filled the cup attached to the string to find the force that equaled the static friction maximum, or the amount of water it took just before the block moved. We used the mass of the water in the cup times gravity to find the force equal to the static friction maximum.

Mass of Water for One Block

We then repeated this process but after adding a new block onto the block for a total of four trials. 

Mass of Two Blocks

Mass of Water for Two Blocks

Mass of Three Blocks

Mass of Water for Three Blocks

Mass of Four Blocks

Mass of Water for Four Blocks

Using this data we created a data table that first had the mass of the blocks and then mass of the cups of water. We then made a column that represented normal force (mass of block*gravity) and one that represented the static friction force maximum (mass of water*gravity).

(picture of data but my lab partner is laggin it....)

We then graphed the data so that we could find the slope. Therefore, the experiment was successful since the slope showed us the coefficient of static friction for the red velvet on the table surface. 

(picture of graph.....lab partner again...awk)

Part 2: Kinetic Friction

f kinetic = u kinetic*N

Using the logger pro we connected the block to the Force Sensor. We got a second block on top of the first and repeated the experiment until there were three more blocks on top like the previous experiment. The graph below showed the force which equalled the force of kinetic friction since velocity was constant.

(graph)

The data received from the mean of each line was used for the force of kinetic friction which was then divided by normal to get the kinetic coefficient of friction.

(data)

The coefficient of friction was then averaged to be equal to (coefficient).

Part 3: Static Friction From A Sloped Surface

We placed a block on a horizontal surface. After slowly raising one end of the surface, we tilted it until the block started to slip. We then took the angle measurement.

We then used the angle to calculate the coefficient of static friction.

Part 4: Kinetic Friction From Sliding A Block Down And Incline

With a motion detector at the top of an incline steep enough that a block accelerates down the incline, we measured the angle of the incline and the acceleration of the block and determined the coefficient of the kinetic friction between the block and the surface from our measurements. 

Free Body Diagram

Lab with Motion Detector

Coefficient of Kinetic Friction 

Part 5: Predicting The Acceleration of a Two-Mass System 

Using this coefficient of kinetic friction we derived an expression for what the acceleration of the block would be if a mass was hanging from the other end of a pulley. 

Then the actual acceleration was recorded on the graph below.

(picture of graph, if my partner would help...)

Therefore, the percent error was %, a close match.