Lab 5 - Friction

Background: Forces are a quantifiable push or pull measurements on an object that is exerted through the presence of another object. There are two primary types of forces; contact forces and field forces. Contact forces exert the pressure on the object through physical contact with that object such as pushing a block across a table with one's hand. Field forces exert the push or pull from a distance, the primary examples being gravity and magnetism. Within the umbrella of contact forces, there are many more subtypes that describe exactly how the pressure is being exerted such as the applied force, force of tension, normal force, force of friction, spring force, and drag force. 

This experiment focused primarily on the force of friction. Friction is defined as the force that is present between two objects when one is resting on top of the other that resists the movement of the resting object. It is also defined by the equation; Ff = μ x Fn where Ff is the force of friction, μ is the friction coefficient, and Fn is the normal force. It is important to note that the coefficient of friction changes depending on if the object is sliding on the surface or standing still. The coefficient of static friction, when the object is still, is always greater than the coefficient of kinetic friction, when the object is in motion, because as described in Newton's first law it is easier to keep an object in motion than attempt to force it into motion when it is at rest. 

This experiment attempted to calculate the coefficients of static and kinetic friction between a wooden block and a plastic ramp using a series of trials in which the mass of the block changed and the mass required to move it when hanging off of a pulley system was measured. 

Methods:

Experiment 1 - Static Friction

1. A small pulley was attached to a plastic ramp.

2. A wood block with a hook was weighed to get an initial mass.

3. The block was then attached to a string and positioned on top of the ramp.

4. The string was looped over the pully to hang off the edge of the table. 

5. Weights were gradually added to the end of the rope until the block on the ramp began to move.

6. The last added weight was removed to and the total weight added was recorded.

Experiment 2 - Kinetic Friction

1. A small pulley was attached to a plastic ramp.

2. A wood block with a hook was weighed to get an initial mass.

3. The block was then attached to a string and positioned on top of the ramp.

4. The string was looped over the pully to hang off the edge of the table. 

5. Weights were gradually added to the end of the rope until the block on the ramp moved at a constant velocity.

6. The total weight added was then recorded.

Equations:

Fg = mass x gravity

Gravity = 9.8

μ = Fg2 / Fg1

Percent Error = ((measured value - true value) / true value) x 100

Results:

Coefficient of Static Friction Measured = 0.51 Coefficient of Kinetic Friction = 0.48

Literature Value = 0.50 Literature Value = 0.40

Percent Error = 2% Percent Error = 20%

Discussion: The coefficient of static friction was measured to be 0.51 with a percent error of 2% when compared to the known value of 0.50. Meanwhile, the coefficient of kinetic friction was measured to be 0.48 with a percent error of 20% when compared to the known value of 0.40. The measured value of static friction being larger than the measured value of kinetic friction was an important outcome. This meant that the experiment aligned with Newton's Laws and was therefore a reasonable outcome. It was also reasonable that the coefficient of kinetic friction had a larger percent error than that coefficient of static friction, because there were additional assumptions that had to be made during those trials. In order to be able to calculate the coefficient, it had to be assumed that the block was moving at a constant velocity and therefore the accelaration was 0 m/s2. While a constant velocity was visually monitored for during the trials it was probable that it resulted in many small mistakes that increased the error in the results through the assumption that the observations were correct. 

Error: The known values found in the literature were estimates instead of exact values because the coefficients of friction are expressed in ranges and can be dependent on the type of plastic which was not known in this experiment. Additionally, it was observed that the wood block occasionally became stuck on the ramp until too much weight was added to the hanging mass and the block suddenly accelarated rapidly. To account for anything the block was stuck on, when this occurred the ramp was wiped down with a cloth and the trial was repeated until a steady velocity was observed.