Topic:Free FallLab

Date:2/7/2013

Page #76

Seed Question:You toss a ball straight up in the air. How much time will it take to travel from your hand to the high point compared with the time it will take to fall from the high point back to your hand? (Is the graph of x vs t symmetrical?)

Exploration:

Part 1: Getting familiar with LoggerPro.(Groups of 2…)

LoggerPro is the software that runs LabPro interface that is connected to the motion detector.

Let’s get familiar with the set-up. Groups of 2…

  1. Make sure everything is connected as shown above.
  1. Make sure Motion Detector in inside protective cradle on floor.
  1. Click on the Collect button (top right of screen) to begin graphing.
  1. To do and Notice: Bring your hand toward detector until the position vs time graph levels off. LoggerPro won’t see an object any closer than this to the detector. Make sure your object is always farther away than this minimum distance.
  1. To Do and Notice: Start with your hand directly over detector (about 0.5 m away.) Move your hand sideways until position vs time graph jumps. Your hand is out of the sonic beam! The sonic pulse comes out of the Motion Detector at about a 15o angle. You must stay inside that angle.
  1. To Do and Notice: Experiment with collecting data by placing your hand over the detector moving it up and down. If you are paying attention to cautions from 4 and 5, you should get nice, smooth graphs.

Part 2: Do the Freefall Lab

Use the motion detector to capture the motion of a ball that is tossed upward.

  1. Remembering the cautions from 4 and 5 above, try tossing the ball upward, above the detector until you get nice, smooth motion graphs of x vs t and v vs t.
  1. When you get a set of beautiful graphs (at least for the freefall portion) rescale the time axis to focus in on the freefall portion of your graph. To rescale, click on the number at the end of you axis, and replace with a more optimal value. Make sure that you rescale the vertical axis as well to maximize your graphs. You want something that looks like this:
  1. Highlight the freefall part of the velocity vs time graph only (by clicking and dragging) and use the R= button to get the slope. Be careful not to include any other parts of the

v vs t graph.

  1. To print these graphs:
  2. Click on the data table, then delete it.
  3. Click on File, then Print…A dialogue box will pop up…
  4. Click on the box forPrint Footer. Type in your and your partner’s names in the dialogue box, click OK.
  5. Select printer P87_Rm110. Print 2 copies, one for each of you…
  6. Trim and tape these graphs into your PJ under Exploration on page 76.

On your printed graphs:

  1. Indicate the freefall portion of your position vs time graph by tracing over it in red ink. Remember, freefall means that gravity is the only force acting on the object.
  1. Indicate the freefall portion of your velocity vs time graph by tracing over it in blue.
  1. What is the acceleration due to gravity according to your velocity vs timegraph?
  1. Indicate when the object reaches the high point above the ground on your velocity vs time graph.

On page 77:

  1. What is the ball’s velocity at its highest point above the ground?
  1. Why is the acceleration due to gravity negative?
  1. What is the value of the freefall acceleration? What does this correspond to on your velocity vs time graph?
  1. Is the freefall acceleration the same before and after the high point? Explain, referring to your graph.
  1. Notice that the sign of the velocity changes from + to – around the ball’s high point. Does the sign of the acceleration change too?
  1. What is the instantaneous acceleration of the ball at its high point?

Big Idea:

The slope of the tangent on the x vs t graph is the instantaneous velocity, which is the v vs t graph below. (Trace the top graph with a pen…)

The acceleration due to gravity is 9.8 m/s2 for objects in free fall near the surface of the Earth, regardless of the motion of those objects.

The acceleration due to gravity for objects thrown straight up when they reach their highest point above the ground is ______.

Instantaneous velocity is the instantaneous rate of change of position.

Instantaneous acceleration is the instantaneous rate of change of velocity.

For Further Contemplation: