LAB A7: KINETIC AND STATIC FRICTION

10/19/00

Equipment required: Wooden box, set of standard masses, 5 or 10 N spring scale, friction plane

A. Relationship between friction and normal force (horizontal surface)

Prelab problem: In this experiment, you will apply a horizontal force to a box loaded with weights, in order to pull the box at constant velocity across a horizontal plane. Follow the usual force diagram procedure to explain why

1) the magnitude of the force of the pull will equal the magnitude of the force of kinetic friction, and 2) the magnitude of the weight will equal the magnitude of the normal force.
 
 



 
 
 
 
 
Added Mass (kg)
Total Weight (N)
Pull Force

(N)

0.200
   
0.400
   
0.600
   
0.800
   
1.000
   

1) Use a 5 N spring scale to measure the weight of the wooden box, and record your result. Then, using the spring scale, apply a HORIZONTAL force to the box to pull the box at a constant, slow velocity along the HORIZONTAL friction plane (NOT along the classroom table !). Try to make the box's velocity as constant as you can - start with a gentle pull and increase your pull gradually until you can move the box with constant speed. Read the force of pull for five values of mass (0 to 1 kg in increments of 0.2 kg) placed in the box. Distribute the weight in the box as uniformly as possible. Make a table of the weight of the loaded box (box + added weight) and the corresponding pull force.

2) As you were carrying out the measurements above, what did you notice about the reading on the scale just as the box began to move? Make a conclusion about the difference between the friction force on an object at rest and that on the object while moving.

3) Use your TI calculator to plot and fit graph of FRICTION FORCE vs NORMAL FORCE. (Where will the data come from?) Sketch the graph in your lab journal, and write the equation of the fit in the usual way (math equation, physics equation, matching table, physics with fit coefficients -- numbers and units -- substituted, etc.).

4) If 0.700 kg of mass were placed in the box, what would you expect the force of friction to be? Use your equation of fit to calculate the answer. Then check your answer by making the measurement. Quantitatively compare the measured value to the calculated value. Show all work for predictions, and record all measurements in your lab journal. Compare the prediction with measurement (% difference?).

B) Relationship between friction force and normal force (inclined surface)

Prelab problems: In the experiment you just completed, the weight of the box was equal to normal force of the table on the box, and the horizontal pull was equal to the friction force. In the next experiment, you will incline the friction plane. In this situation, how will the normal force be related to the weight? the pull related to the friction force? To answer these questions, do the following in advance of the lab.

a) On a diagram in your lab journal, draw and label the forces acting on the box. The box is being pulled at constant velocity upward along the inclined plane and parallel to it. Use axes parallel and perpendicular to the plane.

b) Write the net force equation for the axis perpendicular to the plane. Then solve for the normal force in terms of the weight of the box, mg, and the angle of inclination, q . Make it clear on your diagram how the angle is used in finding force components.

c) Write the net force equation for the axis parallel to the plane. Then solve for the friction force in terms of the force of the pull, mg, and q .

When your answers to the prelab problems are complete, do the following.


1. Using a box loaded with 400 g, pull the box at constant velocity upward along the plane for angles

of 0 to 50° in increments of 10° . Be sure to keep the pull parallel to the plane. Record your results

in a table similar to the one below. Use your results from b) and c) to calculate the normal and

friction forces.

Record the weight of the loaded box.
 
Angle

(°)

Pull Force (N)
Normal Force (N)
Friction Force (N)
       
       
       
       
       

 

2. Using your TI calculator, fit a graph of FRICTION FORCE vs NORMAL FORCE. Sketch the graph in your lab journal (label axes with quantities plotted and units) and give the results of the fit in the usual way.

3. Compare your results to those of step 3 in Part A.
 
 
 
 
 
 
 
 
 
 
 
 

C) More about Normal and Friction Force

1) Now put the box, loaded with 400 g, on a horizontal friction plane. Pull the box with the spring scale at constant velocity, exerting the force at an angle of 30° to the horizontal. Give the value of the pull force that you measured. Then calculate what the friction force must have been. Explain your answer using a force diagram and net force equation.

2) Given the friction force determined in step 1, what would you expect the normal force to be? Use a result from Part A or B in determining your answer.

3) Calculate the normal force by a second method, using your force diagram from step 1 and another net force equation.

4) Quantitatively compare your answers to 2 and 3.

D) Coefficient of static friction

Prelab problem: In this experiment, you will use a simple technique to measure the coefficient of static friction. The method involves placing a box on an inclined ramp and raising the ramp gradually until the box just begins to slide. The angle at which this occurs is called the critical angle, q c. Following the usual force problem procedure (using a force diagram, net force equations, etc.), show that the maximum angle for which the box will not slide down the ramp is q c = tan-1(µs), where µs is the coefficient of static friction.

1) Use the method described in the pre-lab to determine µs for a box loaded with 400 g. How does your result compare to the coefficient of kinetic friction obtained in parts A and B ?