Ph101_Friction

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```					Ph101: Fundamentals of Physics Laboratory                                     1
Instructor: Tony Zable                                                10/4/2010

Laboratory #4: Friction
Purpose: To determine the coefficient of static and kinetic friction for wood on metal.

General Considerations:

When one drags an object along a horizontal surface at a uniform speed, he/she
experiences an opposing force. This resisting force is called friction. It is introduced by
the interlocking of the particles on the surface and by the cohesive forces between the
molecules. The frictional force depends upon:
1. the roughness of the surfaces
2. the kind of materials used
3. the force pressing the surfaces together, let’s call it the “normal” force or N.

The MAXIMUM frictional force is given by the relationship:
Frictional Force = Ffriction = * N
where  is the coefficient of friction between the two surfaces. Since there are 2 types of
friction that may occur between the surfaces, static and kinetic (“sliding”) friction, there
are also 2 coefficients of friction, static and kinetic.
In this experiment, we are going to determine the value of the coefficient of friction for
both static and kinetic friction.

Determining the coefficient of static friction:

One way of determining the coefficient of friction is to place a wooden block on an
inclined plane. Tilt the plane until the block just breaks free from the plane and begins
to slide down the plane (i.e. the sticking between the block and the plane is broken). The
angle of the tilt is related to the coefficient of static friction by the relationship:

static = tan (angle of tilt)
By measuring the angle of tilt, we can thus measure the coefficient of static friction.

Determining the coefficient of kinetic friction:

Similarly, we can determine the coefficient of friction by placing the wooden block on
the inclined plane. This time, if we tilt the plane until the block begin to slide down the

File Name: \ph101\Ph101_lab5-friction.doc
Ph101: Fundamentals of Physics Laboratory                                      2
Instructor: Tony Zable                                                 10/4/2010
plane at a constant speed. The angle of the tilt is related to the coefficient of kinetic
friction by the relationship:

kinetic = tan (angle of tilt)

By measuring the angle of tilt, we can thus measure the coefficient of static friction.

Part A: Static Friction

1. Place a wooden block on metal plane (in the horizontal position). Draw the free body
diagram for the block.

2. Carefully tilt the plane until the block breaks free and begins to slide down the plane.
Draw the free body diagram for the block.

3. Record the angle of the tilt in the table below. Calculate the coefficient of static
friction and record this value in the table. Repeat this measurement 2 more times.

Angle of tilt                   static

File Name: \ph101\Ph101_lab5-friction.doc
Ph101: Fundamentals of Physics Laboratory                                      3
Instructor: Tony Zable                                                 10/4/2010
Part B: Kinetic (“sliding”) Friction

1. Place the wooden block on the metal plane (in the horizontal position). Carefully tilt
the plane until the block slides down the plane at a constant speed. Note that you may
want to give the block a small nudge to overcome the static friction while the block is
resting against the metal plane. Draw the free body diagram for the block.

3. Record the angle of the tilt in the table below. Calculate the coefficient of static
friction and record this value in the table. Repeat the measurement 2 more times.

Angle of tilt               static

Part C: Conclusion

1. Calculate the average value for the coefficient static and kinetic friction and record
these values in the table below.

Friction               Average 
static
kinetic

2. How do these values compare?

File Name: \ph101\Ph101_lab5-friction.doc

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