# Horiz.-Launched Projectiles Lab

 Materials: Computer and School Network Time Allotment: 3 Class Days

### Overview:

In this activity, you will study the characteristics of the falling motion of a basketball and then compare it to the characteristics of the motion of a projectile which falls vertically while moving horizontally.

1. Log on to the school server in the usual manner.
2. Open the Physics Explorer application (One Body model) by double-clicking on its icon; it is found in the directory Science-Math/Science Apps/Physics.
3. Choose Open... from the File menu and navigate to the Unit 3 folder in the Physics 163 folder.
4. Open the file titled Exploring One Body. A multi-part Physics Explorer simulation is opened. Open the simulation for this lab by clicking on the "Accelerated Motion" button at the bottom of the monitor. The Accelerated Motion Data window on the left side of the monitor should then display the following initial values. If not, modify them by hilighting and typing new values.

### That's How the Ball Bounces:

1. Run the simulation and observe the motion of the ball. In reality, do you expect a falling basketball to bounce back to the initial height as it does in this simulation? ____________ Why or why not? Explain your answer.

2. Stop the simulation. The input variable Restitution Coefficient controls the "elasticity" of the collision between the basketball and its surroundings (the floor). Vary its value and run until the simulation seems more "real" to you. (Hint: Normally, a ball will bounce each time to about two-thirds of the initial height.) What value seems to be realistic? ____________

### Velocity Change During Free-Fall:

Let us look at the motion of the ball during its free-fall.

• Press the "Before Hitting Ground" button.
• Display the velocity vector by selecting "Velocity (V)" from the Vector Shown pop-up menu in the window on the left side of the monitor. Run the simulation with the velocity vector displayed. Use complete sentences to describe what happens to the basketball's velocity as it is free-falling.

1. Click on the "Display Vy Graph" button. Run the simulation and observe the slope of the vy vs. time graph.
1. How would you describe the slope of the graph? Is it positive or negative or zero?_________ Is it constant, decreasing or increasing? __________

2. What do the above features of the graph indicate about the acceleration of a free-falling basketball? Be extensive in you discussion.

3. Calculate the acceleration of the basketball during its free-fall. PSYW

4. What type of force do you suppose causes this acceleration?

5. What is the acceleration of the basketball at 2 seconds? __________ Explain or PSYW.

2. On the graph below, draw the velocity graph you would expect to see as it falls, bounces, and return to its peak. For your own benefit, make your prediction before you procede to the next step of this activity.

3. Press the "Let it Bounce" button and then click on the GO tool to run the simulation. Check and correct your answer to the above question (if necessary). Clearly label (or color-code) any corrections which you make as the "Corrected Sketch." In several sentences, describe the velocity in your own words in the space below and explain the changes which are observed. Use the velocity vector display to assist in your explanation. Be complete (describing the entire down and up cycle of motion) and clear.

### Combined X,Y Motion:

Click on the Combined X,Y Motion" button at the bottom of the monitor. A new window titled "Combined X, Y Motion" should appear on the left side of the monitor. See the diagram at the right.

1. Click on the GO Tool and observe the motion of the basketball.

How long did it take the basketball to hit the ground? ___________

2. The basketball had no starting velocity. If it did have an initial horizontal velocity, do you think it will take more, less, or the same time to hit the ground? _______________ Justify your prediction with a clearly-written explanation.

Once you have recorded your prediction above, run the simulation with several positive and negative values for the initial horizontal velocities between +15 m/s and -15 m/s. (Do not clear the trace after each run.) What do you observe? Does the horizontal velocity of the basketball effect the time which it takes to fall vertically? Explain.

3. In the simulations, the lab was set to leave a trace of dots as the basketball dropped. In the space below, use the language of physics to describe several features of the dot patterns you observed.

4. Set the Vector Display menu to "V, Vx, Vy" by selecting this option from the Vector Display pop-up menu. Run a simulation with vx = +10 m/s, vy = 0 m/s. Describe the behavior of the basketball while it falls. Comment on both the horizontal and vertical velocity components.

5. Consider your answers to the previous questions. How does an initial horizontal velocity affect the vertical velocity of the dropping object?

6. Do you think your answers to the previous question would change if there were an initial negative vertical velocity? ___________ To see, clear the trace in the Model window (click on the "Clear Trace" butoon) and run some simulations with vy set to -10 m/s using different vx values.
1. What differences do you see in the results now?

2. Describe what motions stay the same.

### Application Questions:

1. A cliff diver leaps off a high cliff into the water below. The diver leaves the cliff with a horizontal speed of 8 m/s. Determine the horizontal and vertical velocities of the diver after 1 second, 2 seconds, and 3 seconds.

 Horiz. Velocity (m/s) Vert. Velocity (m/s) After 1 second _____________ _____________ After 2 seconds _____________ _____________ After 3 seconds _____________ _____________

2. A blue ball rolling at a high speed across the horizontal surface of an elevated table leaves the table at the same time that a red ball drops off the same table from rest. Which ball (blue, red or neither) will hit the ground first? ________________ Justify your answer with good reasoning and the language of physics.

3. An airplane, flying at a high altitude, drops a flare from below its cargo area. After releasing the flare, the plane continues in its straight-line constant speed motion as the flare falls towards the ground. Assuming that there is a negligible amount of air resistance, five seconds later the flare will be positioned ...
1. ... directly below the position of the cargo area.
2. ... slightly behind the position of the cargo area.
3. ... slightly ahead the position of the cargo area.

### Conclusion:

Use a well-written paragraph to summarize your observations in this simulation. In your discussion, be sure to describe the horizontal and vertical components of motion (both the velocity and the acceleration, etc.) for a horizontally launched projectile. Do a bang-up job!