# 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:

1. Open the Interactive Physics program.
2. It will be really handy to resize the window to make it taller and thinner, but that's up to you.
3. 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:

1. Create a 2 new objects - size and shape don't matter.
2. Object 1 will represent Ball 1 - the one thrown upward. Open its Properties Window and set:
1. y = 0.0 m
2. vx = 0.0 m/s
3. vy = the value for "v" given by your instructor.
3.  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. Sample Properties Window for Ball 2 - Critical values are y, vx, vy, and mass. The others can be anything that's convenient.
Object 2 will represent Ball 2 - the one dropped from rest. Open its Properties Window and set:
1. y = 0.0 m
2. vx = 0.0 m/s
3. vy = 0.0 m/s
4. mass = 1.0 kg (This is important!)
4.  The completed simulation.
It is really convenient to have some "ground" for the balls to hit, so create a rectangle to represent the ground.
1. Use the mouse to shape it into a long, thin rectangle that both balls will hit when they fall.
2. Open its Properties window and set y = -the height given by your instructor..
3. Anchor it () to prevent it from falling.

1. Add a clock (Time meter) to the simulation.
2. 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!

1.  Properties Window for the force.
Create a force () acting vertically upward on Ball 2.
2. Open its Properties Window and set:
1. Fx = 0.0 N
2. 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.
3. 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