Station 3: Friction Forces
Rube Goldberg Machine Design Contest Teacher Training Program
January 27, 2007
Introduction
Friction is a force that helps us and hurts us. Sneakers and boots use friction to their advantage to
prevent people from slipping on smooth or icy surfaces. When you’re trying to move a couch
from one location to another, friction can hinder the movement. At this station, you will learn
about some properties of friction.
Background
Friction opposes motion. If you are pushing your couch to the left, friction force will act to the
right. If a block is sliding down a ramp, friction will cause the block to stick or slide down
slowly.
There are two types of friction: one is called static friction, the other is called kinetic friction.
When a block is resting on a ramp, static friction must be overcome in order to set the block in
motion on the ramp. However, if the block is already in motion, then kinetic friction must be
overcome in order to keep the block in motion on the ramp (otherwise it’ll eventually stop
sliding down the ramp). The force needed to overcome static friction is much greater than the
force needed to overcome kinetic friction.
The coefficient of friction μ is an index of how much friction there is between two surfaces. A
simple way of determining the maximum coefficient of static friction between two surfaces is to
place these two objects on top of one another and increase the angle relative to the horizontal
until the top object just begins to slide down the incline. The tangent of the angle that the bottom
object makes with the horizontal is equivalent to the coefficient of maximum static friction
between the two surfaces. Additionally, as a general rule, the coefficient of kinetic friction is
approximately two-thirds that of static friction.
Experiment Time!
Purpose: When designing a Rube Goldberg machine, one needs to take into account friction to
troubleshoot sticky and nonfunctional mechanism. At this station, you will determine the
coefficients of static and kinetic friction between several surfaces.
Procedure
Section A
1. Choose an object and a surface to put the object on. Record in table.
2. Place the object near the top end of the track and slowly incline the track until the object
just begins to slide down the track. Record the angle at which this happens.
3. Calculate the coefficient of static friction (tangent of angle) and record in the table.
4. Repeat with different objects and tracks. Which combination of surfaces has the least
amount of static friction? The most? What other characteristics of the materials might
influence this measurement?
Page 1 of 2
Object
Track
Angle Θ
Coeff. of Friction
(tan Θ)
1
2
3
Section B
1. Choose an object and a surface. Record in table.
2. Place the object near the end of the track and incline the track to 2/3 the angle that you
determined previously for these surfaces.
3. Gently give the object a push to start it in motion. Does the object continue moving for at
least a second? If so, lower the incline by 5-10 degrees and repeat this step. If not,
increase the incline by 5-10 degrees and repeat this step. (Only repeat this step a few
times or you’ll run out of time.) Record the angle at which the object stays in motion but
is on the verge of immediately stopping.
4. Calculate the coefficient of kinetic friction. Compare it to that of coefficient of static
friction.
Object
Track
Angle Θ
Coeff. of Friction
(tan Θ)
1
2
3
Section C
1. Choose an object and track that you have already chosen in Sections A and B.
2. Record how many additional weights were added to the object.
3. Place the object near the top end of the track and slowly incline the board until the object
just begins to slide down the track. Measure and record the angle at which this happens.
4. Calculate the coefficient of static friction and record in the table.
5. Repeat steps 2-4 for two more different masses. Does mass have a significant effect on
friction?
# of add’l weights
Angle Θ
Page 2 of 2
Coeff. of Friction
(tan Θ)