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Friction & Newton’s Laws PhET WebLab Using HTML5 for Chromebooks
Pre-LAB: Define the following terms before you begin and attach:
1. Inertia
4. Static (Rolling) Friction, FStatic
7. Applied Force, FApplied
2. Force Due to Gravity, Fg
5. Kinetic (Sliding) Friction, FKinetic
8. Normal Force, FNormal
3. Gravitational Acceleration, g
6. Coefficient of Friction,
µstatic or µKinetic
9. Net Force, FNet or ΣF
Part 1: Overcoming Static Friction – Assume the track is NOT frictionless!
Open the PhET site and find the Forces and Motion: Basics
HTML5 simulation or go to:
https://phet.colorado.edu/sims/html/forces-and-motionbasics/latest/forces-and-motion-basics_en.html
Procedure Part 1: Choose
1. Choose <Forces><Sum of Forces><Values><Masses><Speed>
so that your screen looks like this:
2. Slowly begin applying a force onto the crate using the slider
until the crate barely begins to move. You have determined
the amount of applied force, Fapplied, necessary to overcome static friction, Fstatic and at this moment, they are nearly
equal forces, so assume Fapplied = Fstatic . Record the minimum applied force as Fstatic.
3. While the crate is in motion, let go of the slider, observe the change in the force vector. The crate will decelerate
to a stop.
3. Repeat this experiment for two crates, the refrigerator, the girl, the man, the trash can and the gift.
Calculations Part 1:
1. Weight or “Force due to Gravity,” Fg, equals mass x gravitational acceleration. Gravitational acceleration, g, on
Earth is accepted to be 9.8 m/s2, but for ease you may use a value of g = 10 m/s2. Oftentimes, the gravitational
acceleration is just called, “Gravity.” Calculate the Weight of each item and record as Fg in your table. Show work
for trial 1 only, but record all values in the table using 𝐹𝑔 = 𝑚𝑔
2. The coefficient of static friction, 𝜇𝑠𝑡𝑎𝑡𝑖𝑐 , is a value that describes the adherence or grip between an object and a
surface when they have a long time of contact. It applies to motionless objects when an initial force is applied and
also applies to rolling objects. Calculate the coefficient of static friction for each item. Show work for trial 1 only
but fill in all values in the data table, use:
𝜇𝑠𝑡𝑎𝑡𝑖𝑐 =
𝐹𝑠𝑡𝑎𝑡𝑖𝑐
𝐹𝑁𝑜𝑟𝑚𝑎𝑙
Data & Calculations Table Part 1:
Item
Mass
1 Crate
2 Crates
Refrigerator
Girl
Man
Trash Can
Gift
Fstatic
(min. applied force)
Fg
FNormal
𝝁𝒔𝒕𝒂𝒕𝒊𝒄
?
Questions Part 1:
1. Without calculating anything, look at the data and make an educated guess about the mass of the gift.
Mass of Gift = ______________________________ (You will not do any more calculations with the gift)
2. When the slider is released (Fapplied = 0), why does the item decelerate to a stop?
3. Since they are on level ground, the normal force will equal the weight for each item: Draw a free body diagram
for the crate with vectors showing all the forces (Fgravity, FNormal, Fstatic, Fapplied):
4. What force balances the weight? (Name it) ___________________, and (give its symbol)Fg = _______________.
Based upon the Free Body Diagram, record the value for the normal force in the table; you do not have to show
work since it must equal the weight.
5. Which of Newton’s Laws explains why the Normal Force must balance the Force due to Gravity(Weight)?
6. There is a correlation between the coefficient of static friction and the amount of force necessary to overcome
static friction? Specifically explain it using complete sentences:
7. Here’s a real-life example of when static friction must be overcome by an applied force to begin moving an
object: “You push horizontally on a heavy refrigerator to slide it across the kitchen floor.”
Now you give an example:
Part 2: Kinetic Friction for Different Masses – You will need a STOPWATCH OR TIMER!
Procedure Part 2:
1. Choose the
screen, click <Forces><Sum of
Forces><Values><Masses><Speed><Acceleration> so that
your screen looks like this:
2. Apply a 300 N force on the crate
and begin
timing until the crate reaches a speed of 40.0 m/s and the
dummy fails. Notice that the Friction Force value is given and record it as “Force of Kinetic Friction,” FKinetic. Also
record the sum of forces(𝚺𝑭), acceleration (a) and time (Δt) in your data table. Note: Sum of Forces is also called
“Net Force, FNet.”
3. Repeat Procedure #2 using Water, the Man and the Girl.
Calculations Part 2:
1. Calculate the coefficient of kinetic friction using: 𝜇𝑘𝑖𝑛𝑒𝑡𝑖𝑐 =
𝐹𝑘𝑖𝑛𝑒𝑡𝑖𝑐
.
𝐹𝑁𝑜𝑟𝑚𝑎𝑙
Show work for trial 1 only below, but fill in
the table for all items:
2. Calculate the horizontal net force using Newton’s 2nd Law: FNet = ma. Show work for trial 1 only below, but fill in
the table for all items:
Data & Calculations Table Part 2:
Item
Fapplied
1 Crate 300.0 N
Water
300.0 N
Man
300.0 N
Girl
300.0 N
Questions Part 2:
m
50.0 kg
100.0 kg
80.0 kg
40.0 kg
vf
40.0 m/s
40.0 m/s
40.0 m/s
40.0 m/s
𝚺𝑭
FKinetic
a
Δt
𝝁𝒌𝒊𝒏𝒆𝒕𝒊𝒄
FNet
1. Which item took the most time to reach the target velocity? _____________________ Which item took the least
time?______________________________________. Using complete sentences, explain why in terms of Newton’s
2nd Law:
2. Explain why the sum of forces, 𝚺𝑭, is greatest when the item’s mass is smallest using Newton’s 2nd Law:
3. YES or NO? Did your calculated net force, FNet match the website’s value for sum of forces, Σ𝐹?
Hint: In Physics, “sum” and “net” both mean the same thing, we use FNet and Σ𝐹 interchangeably.
4. What physical quantity does the amount of kinetic friction seem to depend upon?
5. How does the coefficient of kinetic friction change as you tried pushing heavier masses?
6. What physical aspect does the coefficient of kinetic friction seem to depend upon? Hint:
Slowly move the friction slider between “None” and “Lots” and watching the screen for changes
on the track.
Part 3: Balancing Friction
Procedure Part 3:
1. Still in
choose <Forces><Sum of
Forces><Values><Masses><Speed><Acceleration> and
reduce the Friction by sliding the bar to the left
somewhat.
2. Apply a force onto the crate to just barely get it
started. Once the crate begins to move, adjust the Friction Slider until the Sum of Forces = 0. Notice any changes in
velocity and acceleration. Record the final velocity, vf.
3. Record the applied and (kinetic) friction force values, FApplied and FKineticFriction. Note: the kinetic friction value
should be recorded as (-) because it points in the direction opposite motion!
Calculations Part 3: These calculations pertain only to the constant velocity motion.
1. Add the applied force and kinetic friction force together to find the horizontal Net Force. Show the work for trial
1 only, but record all values in the table:
2. Using FNet = ma, solve for the acceleration. Show the work for trial 1 only, but record all values in the table
Data & Calculations Table Part 3:
Item
Mass
1 Crate
50.0 kg
Water
100.0 kg
Man
80.0 kg
Girl
40.0 kg
Questions Part 3:
vf
FApplied
FKineticFriction
FNet
a
-
1. To balance friction, which item required the most applied force?__________________The least? _____________
Explain why:
2. As you moved the friction slider to the right, what happens to the acceleration of the item?
3. When the item accelerated, the forces were unbalanced. Describe the amounts of each force:
4. When the item decelerated, the forces were unbalanced. Describe the amounts of each force:
5. Describe the forces on the item when the acceleration reached zero:
6. When the item accelerates, which of Newton’s Laws applies? _______________________________
Explain how:
7. When the item decelerates, which of Newton’s Laws applies? _______________________________
Explain how:
8. When the acceleration is zero, which of Newton’s Laws applies? ______________________________
Explain how:
9. When friction was suddenly increased, the speed of a moving item began decreasing. Test to see if the opposite
is also true. Explain your experiment and its results:
Part 4: Understanding Net Force
1. Change to the
screen. Click <Sum of
Forces><Values> so that your screen looks like this:
2. You are going to try 2 scenarios to create the given outcomes. Sketch the dummies and the force vector for the
losing team in each scenario.
3. For each scenario, show the complete horizontal Net force equation with symbols and then substitute in the
appropriate values assuming the direction of motion to be (+).
4. For each scenario, show work to predict the acceleration of the cart using Newton’s 2nd law if it’s mass = 100 kg.
Scenario #1: Outcome - Red Wins with a Right Force = 250 N where the Sum of Forces = 200 N Right
ΣF or FNet Equations
(Show Symbols Only):
(Show Substitution):
a=
Scenario #2: Outcome - Blue Wins with a Left Force = 350 N where the Sum of Forces = 50 N Left
ΣF or FNet Equation
(Show Symbols Only):
(Show Substitution):
a=
Questions Part 4:
1. There are 3 sizes of dummies, small, medium and large. How many smalls would equal the pulling force of the
large? Is this possible in this simulation?
2. If you wanted to create a pulling force of 200 N, explain at least two ways to combine dummies:
#1:
#2: