Download Forces in 1

Forces in 1-dimension
Exploring Newton’s Second Law F= m a
Go to the URL:
http://phet.colorado.edu/sims/forces-1d/forces-1d.jnlp
The window above should open. Uncheck the friction check box so our first experiments
will be without friction. Select the file cabinet so you have a mass of 200 kg to push.
Youtube Introductory Video (3:30 basic intro to activity)
Prediction: What would you expect the acceleration of the file cabinet for applied forces
of 200 N, 400 N, 600N, and 800 N ?
Applied Force
(N)
Predicted
acceleration
(m/s2)
200
400
600
800
1
2
3
4
Prediction Verification: Run the simulation (click on Go) for each of the four forces.
(Control the force with the slider just to the left of the force graph) What are the
simulated accelerations of the file cabinet for applied forces of 200 N, 400 N, 600N, and
800 N ? You may not be able to get the exact force values but get as close as possible.
Applied Force
200
400
600
800
(N)
Simulated
acceleration
1
2
3
4
(m/s2)
For a force of 400 N, graph forces, acceleration, and velocity. Sketch these on the axes
below. Pause the simulation after 8 seconds.
How much does the velocity change in the first 8 seconds of this simulation? 16 m/s
By how much does the acceleration change when the force is doubled? It doubles
By how much does the acceleration change when the force is tripled? It triples
We say that a variable Y is proportional to X if a percent increase in X results in the same
percent increase in Y.
Is it appropriate to say that acceleration is proportional to force? Explain. Yes. When
the force is larger by a certain factor the acceleration is also larger by the same
factor.
Prediction: Using a 400 N force, if the mass doubles (from 200 kg to 400 kg) what will
happen to the acceleration?
Acceleration will drop to 1 m/s/s (or cut in half)
Prediction Verification: Run the simulation using the refrigerator as the 400 kg mass.
What is the simulated acceleration for the 400 kg refrigerator compare to the 200 kg
refrigerator?
1.0 m/s/s
Prediction: Using a 400 N force, if the mass drops to 1/8 its value (from 200 kg to 25
kg) what will happen to the acceleration?
Acceleration will increase 8 fold to 16 m/s/s
Prediction Verification: Run the simulation using the dog as the 25 kg mass.
What is the simulated acceleration for the 25 kg dog compared to the 200 kg refrigerator?
Acceleration is 16 m/s/s
Is it appropriate to say that acceleration is inversely proportional to mass (proportional
1
to /mass)? Explain
Yes as the mass decrease by a certain factor the acceleration increases by that same
factor.
Friction: Watch the video on 1-D forces Friction to obtain the information required for
the questions below.
What is the mass of the file cabinet? (include correct units) M= ____200 kg__________
What is the maximum magnitude (ignore +/-) of the static friction force? (include units)
Static Friction (maximum applied force before sliding begins) =about 600N___
What is the magnitude of the kinetic friction force?
Kinetic friction force (friction while sliding) =_____392 N____________
The numerical value on the graph above are the values of all numbers at 12.8 seconds.
Below show how the numbers go together and conjunction with Newton’s second Law
(Net force= mass x acceleration) to give an acceleration of 1.4 m/s/s. Include all steps so
you could repeat this later.
Net force = Mass (acceleration)
280 N=200 kg (1.4 m/s/s)
280 N=280 N
Left and right side of F=ma agree
Question: John pushes on a 400 kg refrigerator with a force of 800 N. While sliding a
friction force of 300 N acts on the refrigerator. What is the acceleration of this
refrigerator? Show all steps.
Net Force = Mass (acceleration)
800N-300N=400kg (a)
500N=400 kg (a)
A=1.25 m/s/s