1
Name: ____________________________
Forces and Friction: PhET Lab
Introduction:
Friction is defined as the resistance that one surface or object encounters when moving over another. It is
a force that opposes motion or makes it hard for an object to move across a surface.
Friction between two surfaces arises from interaction between molecules on the surfaces. We have two
kinds of friction: kinetic friction and static friction.
Kinetic friction acts when a body slides over a surface. Static friction acts when there is no motion
etween bodies. Both friction forces act along the surface, in the opposite direction of motion or expected
motion.
Consider the box shown above in figures a through d. When it is at rest and no horizontal force is applied,
there is also no opposing friction force. Therefore, static friction in (a) is zero. If you apply horizontal
force, static friction is going to oppose that force to keep the object at rest. In (b), magnitude of tension is
equal to the static friction because the object is still at rest. We can keep increasing tension and the static
friction will also increase. The system will stay in equili
ium and at rest. But when we get to a point
where our tension force is equal to the maximum static friction (c), the object is ready to move and static
friction can not oppose to the force anymore. As soon as the motion starts (d), the friction turns to kinetic
friction which has a constant value.
The static friction force can vary between zero and its maximum value:
XXXXXXXXXX ≤ fs ≤ µsn.
fs(max) = µsn
In other words, maximum static friction is equal to static coefficient friction times the normal force. When
the system is using maximum static friction, it means object is just about to move. As soon as motion
egins, kinetic friction acts with the constant value :
fk= µkn
2
Procedure:
On your
owser, go to https:
phet.colorado.edu/
Click on “simulations” → “Physics” → “Motion” → “Forces and motion basics” → Download or run
Static and Kinetic Friction
1. In the menu, select the friction simulation.
2. Play with the sim and get used to it. Click on all the check boxes on the right to see what each of
them does and keep all of them checked for your experiments. Play with moving objects and
applying different forces.
3. Keep the friction button in the middle and don’t change it throughout this experiment.
4. Place the 50 kg box on the surface and then apply 50N force. Draw the free body diagram
(don’t forget to include weight and normal force) and explain what happens.
5. Now, apply 100 N of force on the 50 kg object. Draw the free body diagram (don’t forget to
include weight and normal force) and explain what happens.
6. Now, slowly increase the applied force (in one Newton steps using the button shown below), until
you start the motion. What is the maximum applied force that did not start any motion?
Maximum applied force before motion starts: ________________________
https:
phet.colorado.edu
3
This maximum force will be equal to maximum static friction force. Based on this
information, calculate the static coefficient of friction. Show your calculations below.
Calculations:
Static Coefficient of Friction: ________________________
7. Apply 150N on our 50kg object.
a) Draw the free body diagram (don’t forget to include weight, normal force and acceleration)
and explain what happens.
) What is the value of kinetic friction force? ____________________________
c) Calculate kinetic coefficient of friction. Show your calculations below.
Calculations:
Kinetic Coefficient of Friction: ________________________
d) Calculate the acceleration of the object. Show your calculations below.
Calculations:
Acceleration: ________________________
4
8. Repeat the experiment for the objects using simulations, determine or calculate unknowns
and fill in the blanks.
Mass (kg)
Max Static
Friction (N)
s
Kinetic
friction (N)
k
90 kg
(box and girl)
100 kg
(two boxes or thrash can)
130 kg
(box and man)
9. Now, let’s use the object with the unknown mass (gift box). By using the procedure we
developed, determine
Maximum static friction: ___________________________
s: _____________________________________________
Using the maximum static friction formula, find the unknown mass. Show your work:
Kinetic friction: __________________________________
k: _____________________________________________
Using the kinetic friction formula, find the unknown mass. Show your work. Does it
match with your above answer?
5
10. Kinetic friction and acceleration calculations: For the objects and applied forces given in
the table below, determine the kinetic friction and sum of forces from the simulation and
calculate k and acceleration to fill out the table.
Mass (kg)
Force
applied
(N)
Kinetic
friction (N)
Sum of
forces (N)
k
Acceleration
(N/kg)
90 kg
(box and girl)
350 N
100 kg
(two boxes or thrash
can)
350 N
130 kg
(box and man)
350 N
Questions
1. Which is higher: static friction or kinetic friction?
2. Which is higher: static friction coefficient or kinetic friction coefficient?
3. As the mass of an object increases, what happens to the amount of force needed to begin
moving the object? Explain.
4. Does the kinetic friction depend on applied force? Does it change if you increase the
applied force while object is moving? Explain.
5. Does the force of kinetic friction depend on weight of the object? Explain.
6. Does the coefficient of kinetic friction depend on the weight of the object?