Physics Simulation
A Free Fall Problem
[Lab Index]
BHS
-> Staff
-> Mr. Stanbrough ->
AP Physics -> Kinematics
-> this page
The Problem:
Ask the instructor for your values for the distance "y" and the
speed "v" for this problem:
Two balls are _y__ meters above level ground. Ball
1 is thrown upward with an initial speed of __v__ m/s. At what
time after Ball 1 is thrown should Ball 2 be released from rest so
that both balls hit the ground at the same time?
The Simulation:
This is a little more complex simulation, but very interesting and
instructive. Here is a way to set it up:
Preliminaries:
- Open the Interactive Physics program.
- It will be really handy to resize the window to make it taller
and thinner, but that's up to you.
- Set
the accuracy. You want the simulation to be as accurate as
possible, so select "Accurate" and change the Animation Step to
0.010 s. .
Create the Objects:
- Create a 2 new
objects - size and shape don't matter.
- Object 1 will represent Ball 1 - the one thrown upward.
Open its
Properties Window and set:
- y = 0.0 m
- vx = 0.0 m/s
- vy = the value for "v" given by your
instructor.
Sample Properties Window for Ball 1 - Here, v = 15
m/s. The value of x can be anything you like, only y,
vx, and vy are critical.
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Sample Properties Window for Ball 2 - Critical values
are y, vx, vy, and mass. The others
can be anything that's convenient.
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Object 2 will represent Ball 2 - the one dropped from rest. Open
its Properties Window and set:
- y = 0.0 m
- vx = 0.0 m/s
- vy = 0.0 m/s
- mass = 1.0 kg (This is important!)
The completed simulation.
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It is really convenient to have some "ground" for the balls to
hit, so create a rectangle to represent the ground.
- Use the mouse to shape it into a long, thin
rectangle that both balls will hit when they fall.
- Open its Properties window and set y = -the height given by
your instructor..
- Anchor it
()
to prevent it from falling.
Add the Meters:
- Add a clock (Time
meter) to the simulation.
- Add a Position
meter (Y direction) for each ball.
The "Trick":
If you run the simulation now, you find that Ball 2 begins to fall
immediately and hits the ground well before Ball 1- how do you keep
it where it is until you are ready to drop it? Ha!
Properties Window for the force.
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Create a force
()
acting vertically upward on Ball 2.
- Open its Properties Window and set:
- Fx = 0.0 N
- Fy = 9.81 N - This is equal to the weight of Ball 2 (That's
why you set its mass to 1.0 kg.), so it supports Ball 2 to keep
it from falling. Run the simulation now to try this out.
- How do
you get the force to "let go" of Ball 2? At the bottom of the
force's Properties Window, notice the box labeled "Active when".
Click the radio button below "Always" and enter "time<1.00" in
the field. This means that the force will only be active when the
time is less than 1.00 seconds, then Ball 2 will be in free fall.
This is the first time we have used formulas, a very
powerful feature of Interactive Physics.
Run the Simulation:
If you haven't done so already, zoom
out ()
until you can see both balls and the ground in the workspace.
Run the simulation, and adjust the value in the "Active when"
field of the force until both balls hit the ground
simultaneously.
Add your name to the simulation using
the text tool (),
and turn it in - along with your analytical solution, of course!
[Lab Index]
BHS
-> Staff
-> Mr. Stanbrough ->
AP Physics -> Kinematics
-> this page
last
update June 16, 2000 by JL
Stanbrough