Physics - Friction Lab



Name ___________________________________ Date ________________ Per____

Lab#4 Friction Lab

FRICTION

In this activity you will examine how static and kinetic frictional forces vary as the normal force between an object and a surface is changed. You will apply a horizontal force to a block of wood loaded with weights in order to pull the block at constant velocity across a horizontal plane. With your results you will estimate the coefficients of static and kinetic friction for the surfaces involved.

Prediction:

Complete the following Free Body Diagram:

∆V = 0

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1) According to a force diagram explain why:

a. The magnitude of the force of the pull will equal the magnitude of the force of kinetic friction.

b. The magnitude of the weight will equal the magnitude of the normal force.

2) How do you expect both the static and kinetic forces on the block to change with increasing block mass? If possible, write your answer mathematically, including as much information as you can.

Purpose: Measure the kinetic friction and static friction for wood blocks on wood surfaces.

Materials: Wooden block, wooden plank, spring scale, various masses.

Part I Coefficient of static / kinetic friction

Procedure:

1) Weigh the block using a three beam balance scale

Measure the maximum static friction force by using the Newton scale. Note the maximum reading just before the block starts to move. Do this for at five different masses listed below.

2) Measure the kinetic friction force in Newtons by pulling the block over the plank at a steady, moderate speed. Be sure to hold the spring scale horizontally.

Do this for at five different masses listed below.

3) Complete the table of the total weight (combined weight of block and mass) vs. the Pull Force.

Data:

Weight of wooden block ________________kg

|Added Mass (kg) |Total Weight (N) = (added mass |Maximum Pull Force prior to motion |Pull Force during constant velocity |

| |+ block)(gravity) |(static) |(kinetic) |

| | |(N) |(N) |

|0.100 |  | |  |

|0.300 |  | |  |

|0.400 |  | |  |

|0.700 |  | |  |

|1.000 |  | |  |

Analysis:

1) On a separate sheet of paper create a graph of

FRICTION FORCE vs NORMAL FORCE

A) Using two separate colors plot the static and kinetic friction force (y-axis) as a function of normal force (x- axis) and perform a linear regression analysis (best fit line). Make sure to make a legend.

If y = mx + b

And Ff = µf Fn

According to the two above equations how could you find µf (coefficient of friction)?

Our measured coefficient of friction µs _______________ µk _______________

2) Why is it important to hold the spring scale horizontal?

3) Try pulling the Newton scale at different speeds. What do you find?

4) Using a 500g mass on your block, measure the friction force for an additional two surfaces, lab table and floor. Summarize your results below:

5) Turn your block sideways and repeat with the 500g mass on all three surfaces (wooden plank, lab desk, and floor). What did you find?

6) Based on your result the previous question, are you skeptical of claims that wide tires give better traction? Explain.

Part II Coefficient of static friction

In the experiment you just completed, the weight of the block was equal to normal force of the table on the block, and the horizontal pull was equal to the friction force. In the next experiment, you will incline the friction plane. In this part of the experiment, you will use a simple (more accurate?) technique to measure the coefficient of static friction. The method involves placing a block on an inclined ramp and raising the ramp gradually until the block just begins to slide (the moment the x component of the force of gravity is greater than static friction). The angle at which this occurs is called the critical angle, θc.

Prediction:

1) Draw a FBD of the block of wood.

2) In this situation, how will the normal force be related to the weight? Provide the algebraic equation.

3) How is the maximum of static friction related to the weight? Provide the algebraic equation.

Procedure:

1) Place the wooden block on the wooden plane.

2) Slowly lift the end of the plane creating a steeper and steeper incline until the wooden block starts to slide. Immediately stop lifting.

3) Measure the angle of the incline plane by measuring the length of the adjacent and opposite leg of the critical angle, θc.

Data:

Length of Adjacent Leg ______________cm

Length of Opposite Leg ______________cm

Analysis:

Find the critical angle:

1) tan (Θc) = opp/adj Find Θc

Θc = ____________

2) Following the usual force problem procedure (using a force diagram, net force equations, etc.), What is the maximum of static friction (the maximum angle where the block starts to slide) Note: since µs = Fs/Fn, θc = tan-1(µs),

3) Compare the results of θc you from part I to the above calculations. Which is more accurate and why?

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