Probeware Workshop SEPS/AAPT April 2, 2011 University of the Sciences Bill Berner, Univ. Pennsylvania Barry Feierman,

1. Probeware Workshop SEPS/AAPT April 2, 2011 University of the Sciences Bill Berner, Univ. Pennsylvania Barry Feierman, Westtown School

2. We will review in this session how to use the following Vernier probes: Motion sensor - one dimension Force probe - dual range Acceleration probe – one dimension Microphone Voltage and Current probes

3. SETUP sensors

4. DATA COLLECTIONchoices

5. SET UP your Windowswith tables, meters, graphs

6. What can you do with a “motion sensor”? • Record position (+ / - 1 mm) • Calculate velocity • Calculate acceleration • Plot graphs instantly

7. Start with the easy stuff ….. Ask students to first predict the shape of the position-time graph for an object which is AT REST Will they know that the graph of position-time is a horizontal line for an object at rest?

8. Next, ask students to predict the position-time graph for an object moving at constant velocity. Most of the “learning” takes place here where students get to test out their initial ideas and assumptions

9. Here is the graph of a Vernier cart moving along a horizontal track after being given an initial push. The data are sampled at 10 Hz.

10. cart moving at constant speed

11. Ask students to figure out the speed of the cart from the graph itself Some will estimate the number of meters covered during each second Some will estimate the time needed to cover each meter. some might think of “slope”

12. slope of graph indicates the average speed of the cart about 0.5 m/s

13. Logger Pro can display graphs of position - time velocity – time acceleration – time Logger Pro will also “fit” a variety of mathematical functions to the data

14. average speed about 0.52 m/s

15. Acceleration is one of the toughest concepts to understand since it is a “rate of a rate” Acceleration occurs whenever the speed or direction changes Always begin with the motion of an object with constant acceleration

16. Investigate the motion of a cart moving up and down an inclined plane. The cart begins at the bottom, is given a push up the plane, and then let go. Ask students to first predict the shape of the position-time graph for both the uphill and downhill motion of the cart. Will the graph be a line or a curve?

17. This is a challenge for students, since they can see the speed of the cart changing throughout the experiment. They know the cart slows down on the way uphill, stops for a moment, and then speeds up on the way downhill. The motion sensor is located at the TOP of the inclined plane, facing downwards.

18. cart at rest cart at rest PUSH CART cart moving down plane cart moving up plane slowing down speeding up cart at the top of the plane: speed is zero

19. Now predict the velocity-time graph for this same event. note on +/- sign convention Any object moving towards the motion sensor has a “negative velocity”. Any object moving away from the motion sensor has a “positive” velocity.

20. cart moving downhill speeding up cart at rest at top of inclined plane cart pushed by hand cart moving uphill slowing down

21. What is the acceleration of the cart when moving uphill? What is the acceleration of the cart when it comes to rest at the top? What is the acceleration of the cart when moving downhill? Is the acceleration reasonably constant?

22. slope = average acceleration acc = 1.0 m/s/s

23. Physics classses often measure the acceleration of gravity by various methods In this next demo, we drop a ball from a height of about 2 meters and record the ball’s position and calculate the ball’s instantaneous velocity. The motion sensor is in the ceiling facing downwards

24. First, ask students to PREDICT the shapes of the position-time graph and the velocity-time graph for a ball bouncing on the floor a few times. Question: is the deceleration of the ball when rising the same value as the acceleration of the ball when falling?

25. ball hits floor ball hits floor ball falling max height max height above floor ball falling falling hit floor ball rising

26. ball falling ball at rest ball rising average acceleration about 9.5 m/s2

27. You could use a motion sensor to investigate the potential and kinetic energy of a falling object. Here is a tray falling from a height of about 1.5 meters. The motion sensor is near the ceiling, facing down.

28. First ask students to PREDICT the shapes of the potential energy graph, the kinetic energy graph, and the total energy graph for a falling object. Hint: think conservation of energy

30. Here is an interesting question. If a tray is dropped, what would the graph of the PE vs. KE look like? As the tray falls, it loses PE and it gains KE producing a graph with the shape of a ________________ ?

32. What about the PE, KE and total mechanical energy of a bouncing ball? Let’s look at the gravitational PE first, found by plotting the height of the ball above the floor The ball has zero PE when on the floor

34. Now let’s examine the KE of the same bouncing ball. When the ball is AT REST its kinetic energy is zero. Predict the shape of this graph first.

36. Now predict the total mechanical energy of the bouncing ball The SUM of the PE + KE plotted against time.

38. One could ask whether the same PERCENT of energy is “lost” (to heat and sound) on each bounce. Looks like an exponential decay! At times the total energy is near zero. So if not PE nor KE, what kind of energy does the ball possess?

39. What could you do with TWO motion detectors? How about test for the conservation of momentum in an elastic collision of two toy carts of equal mass. Each cart’s velocity is determined by a motion sensor both before and after a head-on collision.

40. Here is a snapshot of the velocity of each cart a moment before they make a head-on collision Both velocities are “positive” since each cart is moving away from its own motion sensor

42. Now let’s look at the velocity of each cart just after the head-on collision. Each cart has a “negative” velocity since each cart rebounded and is moving back towards the motion sensor.

44. change in velocity of cart 1 = 0.73 m/s change in velocity of cart 2 = 0.72 m/s carts had equal mass: 1.0 kilogram The change in momentum of cart 1 closely matches the change in momentum of cart 2

45. FORCE PROBES Force probes can measure forces from 0 – 10 N at high resolution +/- 0.001 N and from 0 – 50 N at a lower resolution +/- 0.01 N

46. Forces can be measured at high sampling rates. Predict the FORCE – TIME graph for lifting a 1000 gram mass (10N) very slowly ………. and then lowering it very slowly.

47. the force is constant if there is no acceleration

48. Now predict the force-time graph for QUICKLY raising a 500 gram mass, holding it steady for a moment, and then then lowering it QUICKLY Hint: the weight must accelerate and then decelerate, then stop.

49. raise quickly lower quickly hold steady hold steady hold steady

50. Holding it steady is a constant 5 N Accelerating upwards the force moves up to 10N maximum, then drops to about 2N when the weight decelerates Then steady again at 5 N Lowering it quickly reduces the force to 2 N, and then catching it increases the force back up to 10 N maximum