Name ______________________________
Ms. Sabbatino
Date ___________
Period _________
Lab #8: Finding the Coefficient of Friction
Background:
The friction force acting on an object depends on three things.
1.
The normal force acting on the object
2. The type of surfaces in contact
3. And the initial state of the object’s motion (at rest or in motion)
The normal force acting on the object is determined by drawing a free body diagram and
setting up a vertical force statement like we have done in the past. The coefficient of
friction (μ) describes how the surface and the state of the object’s motion affects friction.
It is a number usually less than one and has no units. In this lab, we will use a graphical
analysis to determine four different coefficients of friction. One trial will take place on
the bare table top, the other will take place on sand paper. For each trial, you will
determine the coefficient of static friction, which describes the friction acting on a
stationary object, and the coefficient of kinetic friction, which describes the friction
acting on an object already in motion.
We will be using the ideas of equilibrium to measure the friction force. We know
that friction will act horizontally along the surface opposing motion. This means, if we
keep the object in equilibrium the force we apply horizontally on the block will be
equal to the friction force. We will use a spring scale oriented horizontally to pull the
block along the surface at a constant velocity to ensure the block remains in equilibrium
which, in turn resulting in the friction force being equal to the force seen on the spring
scale. When using a spring scale, make sure to use the one with the lowest threshold
until it is maxed out. This will ensure the most accurate data.
Goal: To determine the coefficients of static and kinetic friction between a wooden
block and the table and a wooden block and sand paper.
Predictions:
Rank the four frictional forces from least (1) to greatest (4)
___static table
___kinetic table
___static sand paper ___kinetic sand paper
Materials: List everything you use here
Lab Setup: Draw what you actually did here
Procedure:
1. Find the mass of the block in question
2. Place the block on the table top
3. Place a 200g mass on the top of the block
4. Hook a spring scale to the block and begin to pull HORIZONTALLY
5. Watch the spring scale carefully to see the maximum force exerted on the block
before it begins to move (this is the static friction force)
6. Record the force in the table below
a. You will see the force ‘jump’ back once the block begins to move
7. Pull the same block horizontally at a constant speed across the table top and read
the force on the scale. This will be the kinetic friction force.
8. Add 100g to the block and repeat steps 4-7 until you have 10 total trials=
9. Repeat steps 1-8 for the block on the sand paper
Data: Fill out whatever you need to in the table below (you don’t have to use the whole
table)
Table top
trial
Mass of block and
additional mass (Kg)
1
0.1
2
0.2
3
Weight of block and
mass combination (N)
Static
friction
force (N)
0.5
Sand paper
Kinetic
friction
force (N)
0.3
Static
friction
force (N)
1.0
Kinetic
friction
force (N)
0.8
1.0
0.7
1.9
1.6
0.3
1.4
0.95
3.1
2.5
4
0.4
2.1
1.2
4.0
3.1
5
0.5
2.5
1.45
4.9
3.9
6
0.6
3.0
1.8
6.1
4.8
7
0.7
3.6
2.15
7.1
5.5
8
0.8
4.0
2.5
8
6.2
9
0.9
4.4
2.75
9
7.1
10
1.0
5.0
3.0
9.9
7.9
Analysis:
You will create two graphs. Each graph will have two lines, one for the static friction
force, one for the kinetic friction force.
1. Friction forces on the table top.
a. X-axis:_________________
b. Y-axis:_________________
c. Slope:__________________
2. Friction forces on the sand paper.
a. X-axis:_________________
b. Y-axis:_________________
c. Slope:__________________
After the four (2 on each graph) are graphed, you will calculate the slope of each line.
The slope of each line will be the coefficient of friction for that object in that situation.
REMEMBER, the slope formula is:
Slope of static table
Slope of kinetic table
Slope of static sand paper
Slope of kinetic sand paper
Ranking:
Based on the slopes calculated above, rank the frictional forces from least to greatest:
1. ___________________
2. ___________________
3. ___________________
4. ___________________
Conclusion:
Are these rankings the same as your predictions? __________________________
Are there any generalizations you think you can make based on the data in your lab?
Example: Based on your results,
What force is needed to start a 20kg
wooden block sitting on a lab table?
What force is needed to keep a 7kg
wooden block moving on sand paper?