PHYSICS LAB 4 COEFFICIENT OF FRICTION

The coefficient of kinetic friction is the ratio of the force required to slide an object at uniform speed over a horizontal surface to the weight of the object itself. The coefficient of static friction applies to a non-moving object and relates to the force required to initiate sliding. These coefficients can be found experimentally by weighing the object and then measuring the pulling force needed to slide the object at slow, uniform speed.

OBJECTIVE:

1. Measure the coefficients of kinetic and static friction of different objects on different surfaces.

2. Compare the coefficient of friction found from a sliding mass on an incline plane and one found by pulling the mass on a horizontal surface.

3. Determine what parameters have an effect on the coefficient of friction and the force of friction.

Materials: 1 black wooden box; 1 wooden block; 1 wooden board; 1 set of weights;

1 spring scale; 1 scale balance; 1 protractor

Diagram:

 

Measure and record the mass of the black box and the wooden block. You will be repeating everything three times to get an average for your calculation of the coefficient.

Part 1: This part will be used to calculate the coefficient of static friction based on the angle of inclination.
    1. Place the black box with a mass inside on the wooden board.
    2. Lift one end of the board slowly until the box begins to slide.
    3. Measure and record the angle between the board and the table.
    4. Repeat for a total of 3 trials.
    5. Add a different mass to the box and repeat the procedure.
    6. Repeat the procedure with a different mass. Be sure to record the mass used in each case.

Mass of the box: ________

trial

Mass in box

Angle of inclination 1

Angle of inclination 2

Angle of inclination 3

1

  

 

 

 

2

 

 

 

 

3

 

 

 

 

 

 

Part 2: This part will be used to calculate the coefficient of static and kinetic friction based on the pulling force.

    1. Place the black box on the wooden board on a level table.
    2. Attach the spring scale and pull horizontally on the black box slowly until it begins to slide. Repeat for two more trials and record the force each time. Make sure they are roughly the same amount each time.
    3. Using the same masses inside the box, repeat the trials by pulling on the black box slowly as the box slides at a constant velocity. This is to measure the coefficient for kinetic friction. Do this for three trials.
    4. Repeat steps 1 to 3 with the same masses used in part 1 placed in the black box, three trials for each different mass.

Static case kinetic case

trial

Mass in box

Force1/g

Force 2/g

Force 3/g

Force 1/g

Force 2/g

Force 3/g

1

  

 

 

 

 

 

 

2

 

 

 

 

 

 

 

3

 

 

 

 

 

 

 

 

Part 3: 1. Place the wooden block on a level table.

    1. Attach the spring scale and pull on the wooden block slowly until it begins to slide. Repeat two more times and record the force each time. Make sure they are roughly the same amount each time
    2. Repeat with the wooden block turned on its side.
    3. Repeat once more using the black box with masses placed inside so that the total

mass (including the box) is almost the same as that of the wooden block.

Mass of the wooden block: __________

trial

Force1 / g

Force 2 / g

Force 3 / g

Block

 

 

 

Block

on its side

 

 

 

Black box

With masses

 

 

 

 

 

 

Analysis:

    1. Using the average of the three trials in each case, calculate the coefficient of friction for each case. For part 1, use tangent of the angle. For parts 2 and 3, divide the value from the force scale by (the mass of the box and the mass in the box). Make sure you use the same units for both.
    2. Find the percent difference between the static coefficient of friction found in Part 2 compared to the results found in Part 1. (difference divided by the average of the two values)

Results Tables:

trial

Coefficient for part 1

Static coefficient for part 2

% difference

Kinetic coefficient

1

 

 

 

 

2

 

 

 

 

3

 

 

 

 

trial

Coefficient of friction

Block

 

Block

on its side

 

Black box

With masses

 
  1. From the results of parts 1 and 2, does the mass affect the coefficient of friction?
  2. From the results in part 2, how do the coefficients for kinetic friction compare with those for static friction?
  3. What can you conclude from the results of part 3 regarding just the wooden block?
  4. What can you conclude from the results of part 3 regarding the block and the black box with the same mass as the block?
  5. According to your results, what factors affect the value of the coefficient of friction? Be specific.

 

 

Conclusion:

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