7-Phys2211 - Static and Kinetic Friction - Sim
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Feb 20, 2024
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1 of 8 Phys2211K – Static and Kinetic Friction Name: Group Members: Date: TA’s Name: Learning Objectives: I.
Applying Newton’s second law to investigate the model of static and kinetic friction. II.
Understanding the factors that do and don’t affect the coefficients of friction. This experiment uses two simulations from the Physics Aviary. For the first part of the experiment, open the Friction Lab experiment: https://www.thephysicsaviary.com/Physics/Programs/Labs/ForceFriction/ Click on “Type of Surfaces” at the upper left to choose “Wood on Lab Table.” Adjust the “Object Mass” to 100 g
. Don’t hit the Start button yet!
Part A: Static Friction
1.
The wood block is sitting on the lab table. A string is attached to the block and a force applied to provide tension in the string. The force sensor measures the tension force. Imagine a tension force is applied but not enough to start the block moving. Identify the forces on the block and draw a free body diagram for the block. Also show your coordinate system. 2.
If the block is not moving, what is the acceleration of the block? ____________________ What is the net force on the block? _____________________________ Is your free-body diagram for the block consistent with those answers? ____________________ If not, make a change to your diagram. 3.
Using Newton’s 2nd Law in the y-direction, determine the normal force exerted by the table on the block.
2 of 8 4.
At a time when a tension force, T
, is applied but the block does not move, is the magnitude of the static friction force larger than
, equal to
, or smaller than
the magnitude of the tension force? Use Newton’s Second Law in the x-direction to explain your answer. 5.
Predict what will happen if the tension force is slowly increased. Hit the start button
and see if your prediction was accurate. Describe any differences from what you predicted. 6.
According the graph of tension force vs. time, we see that the tension force reaches a maximum value just before the block slides. Does this mean that the static friction force also reaches a maximum value? Explain.
7.
Use the graph of tension force vs. time displayed in the simulation to determine the maximum value that the static friction force achieves before the block slips. f
s,max
= _____________________________________ 8.
Copy your values for the normal force and the maximum static friction force into the table below. Change the mass of the block and use the simulation to complete the table for four additional values of the mass. Be sure to include units.
Type of Surface A Object Mass, M
Normal Force, n
Max Static Friction Force, f
s,max
Wood on Lab Table 100 g Wood on Lab Table Wood on Lab Table Wood on Lab Table Wood on Lab Table 9.
Compare the normal force and the maximum static friction force. Do you see a trend?
3 of 8 10.
Use Excel to make a graph of maximum static friction force (vertical axis) vs. normal force (horizontal axis). Fit a line to your data and determine the slope. Label the axes of your graph and make sure your equation is shown. Show your Excel graph to your TA before you go on (TA initials _________).
Write the equation for your trend line here ___________________________________________________________ 11.
The slope of this line tells us how much static friction is available to us for any value of the normal force. We call this the coefficient of static friction and use the symbol 𝜇𝜇
s
. From your experiment, what did you determine as the value for the coefficient of static friction for Wood on Lab Table?
𝜇𝜇
s
= ___________________
The units should be the same as the units of the slope of your graph of maximum static friction force vs. normal force. What units does the coefficient of static friction have? ____________________________ 12.
Friction is a force between two surfaces. Do you think it matters what the surfaces are made of?
13.
Pick a different Type of Surface in the simulation and repeat your previous experiment to test your prediction. Type of Surface B Object Mass, M
Normal Force, n
Max Static Friction Force, f
s,max
Show your Excel graph to your TA before you go on (TA initials _________).
Write the equation for your trend line here ___________________________________________________________ 14.
What do you conclude about whether the type of surfaces matter for the coefficient of static friction?
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4 of 8 Part B: Kinetic Friction
Reset the Type of Surface to Wood on Lab Table and adjust the Object Mass back to 100 g
. Run the simulation and observe that the applied force is reduced after the block begins to slide. The simulation adjusts the force to only what is needed to maintain a constant speed. 15.
For the period when the block is being pulled across the table at constant speed, identify the forces on the block and draw a free body diagram for the block. 16.
If the block is moving with constant speed, what is the acceleration of the block? ____________________ What is the net force on the block? _____________________________ Is your free-body diagram for the block consistent with those answers? If not, make a change to your diagram. 17.
For the period when the block is moving at constant speed, is the magnitude of the kinetic friction force larger than
, equal to
, or smaller than
the magnitude of the tension force? Use Newton’s Second Law in the x-direction to explain your answer. 18.
Use the graph of tension force vs. time displayed in the simulation to determine the magnitude of the kinetic friction force. f
k
= _____________________________________
5 of 8 19.
Copy your values for the normal force and the kinetic friction force into the table below. Change the mass of the block and use the simulation to complete the table for four additional values of the mass. Be sure to include units.
Type of Surfaces Object Mass, M
Normal Force, n
Kinetic Friction Force, f
k
Wood on Lab Table 100 g Wood on Lab Table Wood on Lab Table Wood on Lab Table Wood on Lab Table 20.
Use Excel to make a graph of kinetic friction force (vertical axis) vs. normal force (horizontal axis). Fit a line to your data and determine the slope. Label the axes of your graph and make sure your equation is shown. Show your Excel graph to your TA before you go on (TA initials _________).
Write the equation for your trend line here ___________________________________________________________ 21.
The slope of this line tells us how much kinetic friction there will be for any value of the normal force. We call this the coefficient of kinetic friction and use the symbol 𝜇𝜇
k
. From your experiment, what did you determine as the value for the coefficient of kinetic friction for Wood on Lab Table?
𝜇𝜇
k
= ___________________
The units should be the same as the units of the slope of your graph of kinetic friction force vs. normal force. What units does the coefficient of kinetic friction have? ____________________________ 22.
Compare the coefficient of kinetic friction with the coefficient of static friction for Wood on Lab Table. How do they compare? 23.
What feature of the graph of force vs. time displayed in the simulation is related to the comparison of the two coefficients?
6 of 8 Part C: Static Friction and a Block on an Inclined Plane
For the next part of the experiment, open the Forces on an Incline Lab experiment: https://www.thephysicsaviary.com/Physics/Programs/Labs/ForcesOnInclineLab/ Set the Coefficient of Static Friction to a value around 0.4
of and adjust the Block Mass to about 800 g
. Make sure all the force vectors are displayed including the gravity components. 24.
When the board is level, read the magnitude of the forces.
Normal Force n
= _________________ y
-component of Gravitational Force F
Gy
= _______________ Static Friction Force f
s
= _________________ x
-component of Gravitational Force F
Gx
= ________________ 25.
Click to Change Angle and let it increase to about 15
°
and stop it, then again read the magnitude of the forces.
Normal Force n
= ________________ y
-component of Gravitational Force F
Gy
= _______________ Static Friction Force f
s
= _________________ x
-component of Gravitational Force F
Gx
= ________________ 26.
Has the magnitude of the Normal Force increased, decreased or stayed the same? ______________
Based on the earlier experiment, did the maximum static friction available change when the angle was increased? Explain.
27.
Click to Change Angle again and let it run. What happens?
28.
Determine the angle that the block begins to slide down the inclined surface. 𝛳𝛳
max
= _________________
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7 of 8 29.
Run the simulation several times until you can stop it just before the block slides, that is, 𝛳𝛳
~ 𝛳𝛳
max
. Determine the magnitude of each of the forces (note that f
s ~
f
s,max
at this point).
Normal Force n
= _________________ y
-component of Gravitational Force F
Gy
= _______________ Maximum Static Friction Force f
s,max
= _________________ x
-component of Gravitational Force F
Gx
= ________________ 30.
Use the values you determined for the normal force and maximum static friction force to determine the coefficient of static friction. Show your calculations. 𝜇𝜇
s
= ___________________
31.
Using Newton’s Second Law we could show that at an angle 𝛳𝛳
max
the normal force would equal n = mg cos
𝛳𝛳
max
.
Also, we could show that to keep the block from sliding the frictional force must equal f
s,max
= mg sin
𝛳𝛳
max
.
Combining these gives us 𝜇𝜇
s
= f
s,max
/ n = (mg sin
𝛳𝛳
max )
/ (mg cos
𝛳𝛳
max
) 𝜇𝜇
s
= tan
𝛳𝛳
max
.
Use the angle you determined for 𝛳𝛳
max
to calculate 𝜇𝜇
s
. 𝜇𝜇
s
= ___________________
32.
Does the value for the coefficient of static friction you determined from the forces in Question 30 approximately agree with the value you determined from the angle in Question 31? Explain.
8 of 8 Part D: Conclusions
33.
What does the coefficient of static friction NOT depend on? That is, what can we change about the situation and still have the coefficient stay the same? 34.
What determines the coefficient of static friction in a particular situation? That is, what can we change about the situation that would make the coefficient change? Give an example. 35.
How is the static frictional force related to the coefficient of static friction? 36.
Can the static frictional force change even though the coefficient of static friction stays the same? If so, give an example from this experiment. 37.
How does the coefficient of kinetic friction compare to the coefficient of static friction for the same surfaces?
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