Station 3
Friction Forces
Rube Goldberg Machine Design Contest Teacher Training Program
January 29, 2005
Introduction
Friction is a force that can either help us or hurt us. Sneakers and boots use friction to their
advantage to prevent people from slipping on smooth or icy surfaces. But 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 and how to minimize frictional losses in your
Rube Goldberg machine.
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 that we will deal with here. 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, or an index of how much friction there is between two surfaces, varies
for different contact 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 raise the
bottom object at an angle to the horizontal until the top object just about 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 of thumb, the coefficient of kinetic friction is approximately two-thirds that of static friction.
Experiment Time!
Purpose
When you design a Rube Goldberg machine, you need to take into account friction so you can
predict how your machine will perform in reality. At this station, you will determine the
coefficient of static, kinetic, and rolling friction between several surfaces.
Procedure
Section A
1. Choose an object and a surface to put the object on. Record in table.
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2. Place the object near the top end of the track and slowly incline the board until the object
just about begins to slide down the track. Measure and record the angle at which this
happens.
3. Calculate the coefficient of static friction (tangent of angle) and record in the table.
4. Repeat for different objects and different tracks. Which combination of surfaces has the
least amount of static friction? The most?
Object
Track
Angle
Coeff. of Friction
1
2
3
4
5
6
7
8
Section B
1. Choose a non-rolling 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 about 5-10 degrees and repeat this step. If not,
increase the incline by about 5-10 degrees and repeat this step. (Only repeat this step a
maximum of 3 times or else 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 and compare to that of coefficient of static
friction.
Object
Track
Angle
Coeff. of Friction
Section C
1. Choose a non-rolling object and track that you have already chosen in Section A. Record
in table.
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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 about 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
Coeff. of Friction
If there is time, you can experiment with more objects. Please ask your Station Leader.
Congratulations! Now you know which factors affect friction and how to determine the amount
of friction between two surfaces.
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