LAB: FRICTIONAL FORCES AND COEFFICIENTS OF FRICTION

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PRE– LAB

 

A wooden block of mass m is at rest on an inclined plane.

1) Can you label the vectors I have traced?

Using: weight (W) , normal (N), static friction (Fs), the x-component of the weight (Wx) and y-component of the weight (Wy) .

 

The wooden block of mass m is pulled by a spring scale along a horizontal surface at a constant speed.

3) Can you label the forces acting on the block :

Weight (W), normal, (N), kinetic friction (Fk), pulling force (P).

4) we have : Fk = ____________

                     N = _____________

A friction force F opposes the motion. It is always proportional to the normal force N.

Friction =μ  x  Normal      μ is the coefficient of friction.

5) If normal = 50N and μ = 0.2  then friction = _________ N

 

The frictional force that prevents the block from sliding is called the static frictional force Fs: 

 Fs = μs x Normal     μs is the static coefficient of friction  (first diagram)

Once the static fricional force is overcomed, the blocks slides but is still acted up by a frictional force now called the kinetic frictional force Fk. 

Fk = μk x Normal   μk is the kinetic coefficient of friction (second diagram)

Fs is always greater than Fk. (once something starts sliding, it is easier to keep it in motion)

In this lab, you will compute using 2 experiments the coefficients of friction: μs and μk

 

6) In 2)  you found that N = mg cos(θ)   , Fs = mgsin(θ).

 plugging these expressions in Fs = μs N  show that μs = tan(θ)

We will use this ratio in our lab. Show your work.

 

7) in 3) you found that Fk = P and N = mg

Plugging these expressions in Fk = μk N  you find μk = ____________________

2) Find the static frictional force Fs and the normal N using only mg and sinθ or cos(θ)

(Try without hint)

Fs= _______________

N=_______________

 

Hint: The block is in equilibrium. So:

Along the X-axis : Sum (x-components) = 0

Wx + Fs = 0

Wx = - mg sin(θ) so Fs = ___________

 

Along the Y-axs: Sum (y-component) = 0

Wy + N = 0

Wy = - mg cos(θ) so N =___________

 

PROCEDURE:

1) Select a block and a flat board for that experiment. Describe the object and the board:

______________________________________ (compute the surface area and describe the material your block is made of)

 

2) Hang one end of the block on a spring scale to find the mass. Record the mass (in grams) of the block in table 1.

 

3) Place the flat board on a horizontal surface. Hold the spring scale and, with the string held parallel to the level board, pull the object along the board at a constant speed. With the spring scale, measure the amount of force required to keep the object moving at an uniform rate. Repeat this procedure several times, average your results and record every thing in table 1 as the force of kinetic friction between the surface of the board and the surface of the object.

 

4) Detach from the spring scale. Place the object on the flat board. Slowly lift one end of the board. Continue increasing the angle of the board with the horizontal until the object starts to slide. Use the protractor to measure this angle. Record the value of this angle in table 2 as the angle for static friction.

The tangent of this angle is the coefficient of static friction. see pre-lab 6)

 

5) Move the object to one end of the board. Again, slowly lift this end of the board while your lab partner lightly taps the objects. Adjust the angle of the board until the object slides at a constant speed after it has received an initial tap. Use the protractor to measure this angle and record it in table2 as the angle of kinetic friction. The tangent of this angle is the coefficient of kinetic friction.

ANALYSIS

1) From table 2, calculate the coefficients of static and kinetic friction for the object used. Record these values in table 2.

2) Explain any difference between the values for the coefficient of static and kinetic friction.

 

 

3) Using the data in table 1 , calculate the coefficient of kinetic friction. Show your work.

(see pre Lab 7). (forget about g. Since you are computing a ratio, μ = P (in grams) / mass (grams) )

 

 

4) Are your values for kinetic coefficient from questions 1 and 3 are equal ?

 

 

5) Find a block, made of the same material than the previous one, but with a larger base. Now the surface area in contact with the board is largerand is equal to _____________.  Find the angle at which the block starts to slide (step 4).  Do you find the same angle as before (table 2) (about) ?

 

Then the coefficient of friction does not depend on _____________

 

 

 

6) Glue a mirror on one side of the block. Repeat step 4 ) from the procedure. Do you find the same angle ? What is the coefficient of static friction then?

 

 

Then the coefficient of friction depends on ____________________

 

 

 

7) Repeat step 4 ) with a block that has a different mass but that is made of the same materiel (wood). Do you find the same angle ? What is the coefficient of static friction then?

 

Then the coefficient of friction does not depend on __________________

 

 

8) what factors influence the force of friction? What factors do not influence the force of friction?

 

 

9) Was the purpose of the lab accomplished ? What did you learn?