PHYSICS 220 LAB #1: ONE-DIMENSIONAL MOTIONnewton.uor.edu/FacultyFolder/eric_hill/Phys220L/Lab... ·...

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PHYSICS 220 LAB #1: ONE-DIMENSIONAL MOTION OBJECTIVES

1. To learn about three complementary ways to describe motion in one dimension—words, graphs, and vector diagrams.

2. To acquire an intuitive understanding of speed, velocity, and acceleration in one dimension.

3. To recognize the patterns of position vs. time, velocity vs. time, and acceleration vs. time different types of motion.

OVERVIEW

You’ll begin your studies of motion with a simulated “moving man” and with a real cart on a track. To detect the motion of the real cart, you’ll use an ultrasonic motion detector attached to a computer. The rules for calculating velocity and acceleration from distance and time measurements are programmed into the motion software, so you can get a feel for what velocity and acceleration mean and how they are represented graphically. PART ONE: (mostly) Constant Velocity In the first part of the lab, you’ll get familiar with how constant velocity motion is represented on Position, Velocity, and Acceleration vs. Time plots. First you’ll use a simulated “Moving Man” and then you’ll use a real cart and motion detector. Simulation

Set-up. From the Start menu, open the program “PhET Simulations;” this will open a page within Internet Explorer. Click on the “Go To Simulations” button, and then scroll down to find and select “The Moving Man” by clicking on its graphic.

Slow, Forward Motion You’ll pull the simulated man back to the pine tree and then set him moving slowly toward the house at a constant rate. Then you’ll sketch the Position vs. Time plot.

1. Ignore the bobbing “Drag the Man” message; instead, use the blue

arrow by the Position axis to back the man up to the tree.

/53 pts

Physics 220 Lab #1: One-Dimensional Motion

2. Now raise the red arrow by the Velocity axis to some small, positive value.

3. When you’re all set, hit one of the “Go!” buttons (it doesn’t matter which). When the man arrives at the house, hit one of the “Pause” buttons (again, it doesn’t matter which.)

4. Reproduce the Position vs. Time and Velocity vs. Time plots below. Slow Forward Motion

Fast, Forward Motion Repeat the procedure above, but set the Velocity at a fairly large, positive value.

Fast Forward Motion

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Slow, Backward Motion This time, set the man moving back from the house to the tree at a fairly low velocity.

Slow Backward Motion

Question: What is the difference between the position graphs you made by moving the man slowly to the house and the one made by moving it more quickly?

Question: What is the difference between the position graphs made by moving the man toward the house and the one made moving it away?

Question: What is the difference between the velocity graphs you made by moving the man slowly to the house and the one made by moving it more quickly?

Question: What is the difference between the velocity graphs made by moving the man toward the house and the one made moving it away?

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Minimize the simulation window; you’ll return to it later. Experiment Now you’ll repeat these motions (slow & forward, fast & forward, and slow & backward) with the real cart. Note the similarities and differences between the real and simulated motions.

Using the Motion Detector. The detector works much like a bat locating its prey – it emits sound pulses, detects the reflected sound, and uses the time lag between emission and detection to determine the distance to the cart. There are several things to watch out for when using a motion detector: a. Do not place the cart closer than 0.1 meters from the detector’s face

because it cannot record reflected pulses that come back too soon.

b. The ultrasonic waves come out in a cone of about 15°. It will see the closest object (including your arms if you swing them). Be sure there is a clear path between the object whose motion you want to track and the motion detector.

c. The motion detector is very sensitive and will detect slight motions. You should try to move the cart smoothly, but don't be surprised to see small bumps in velocity graphs and larger ones in acceleration graphs.

1. Set-up

a. Make sure a LabPro interface is connected to your computer, and a motion detector is plugged into the port labeled DIG/SONIC 1. Open Cart (in “Physics Experiments/Physics 220-221/ One-Dimensional Motion”). You’ll be confronted with a pop-up box verifying that the motion detector is plugged in; go ahead and hit “ok.”

b. Place the motion detector at the end of a level track as shown

below. Switch the detector to its narrower sensor range (switch on top of detector). You’ll move a cart along a track, in front of a motion detector. Place the “sail” on the end of the cart near the detector.

2. You’ll make position-vs.-time and velocity-vs.-time graphs for

different cart speeds and directions by clicking on the “Collect” button at the bottom of the screen and moving the cart by hand in front of the motion detector at distances that are no closer than 0.1


m. Try the following motions and sketch the graph you observe in each case.

Notes:

Acceleration Plots. To toggle between displaying velocity and acceleration graphs, click on the Y-axis label “Velocity” and when the Y-Axis Selection box comes up select “Acceleration.”

Acquisition Time. If you wish to increase or decrease the acquisition time, go to the “Experiment” menu and select “data collection…”

(a) Starting 0.2 m from the origin (i.e. the detector), move the cart away from the origin slowly and steadily (i.e. constant speed).

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(b) Starting 0.2 m from the origin, move the cart away from the origin more quickly but still steadily.

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(c) Starting 1.0 m from the origin, move the cart toward the origin slowly and steadily.

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Question: Qualitatively, in what ways are these plots similar to those acquired for the simulated “walking man?”

Question: The plots of the actual motion are probably a little bumpier than those of the simulated motion. Why is that?

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3. When you’re done, close the program “Cart” and open “Distance” from the same folder.

To double check that you understand how to interpret position-vs.-time graphs, you’ll translate a verbal description of motion into a predicted graph and follow-up by actually measuring and graphing the motion when it is carried out.

4. Sketch your prediction for the following motion on the axes below using a dashed line. Suppose the cart were to start 0.1 m in front of the detector and move away slowly and steadily for 4 seconds, stop for 4 seconds, and then move toward the detector quickly.

4. After making your prediction, click on the “Collect” button and move

the cart in the way described above. Sketch the actual graph of the cart’s actual motion with a solid line.

Question: Is your prediction the same as the final result? If not, describe how you would move the cart to make a graph that looks like your prediction?

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Now you’ll do it the other way around – given a graph of motion, you’ll translate it into a verbal description, and then you’ll follow up by measuring the motion actually being carried out.

5. Pull down the File menu and select Open, then select the experiment file called “Position Match” (on the Desktop in the folder “Physics Experiments/Physics220–221/One-Dimensional Motion”). A position graph like that shown below should appear on the screen.

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Question: Describe in your own words how you would move the cart in order to match this graph.

6. Click the “Start” button and move to match the graph on the computer screen. Repeat this until you get the times and positions (close enough to) right. It helps to work in a team. Go ahead and sketch what you got on the graph above. Correct your answer above if necessary.

Now, to make sure you really understand Velocity vs. Time plots, you’ll translate from a verbal description of motion to a Velocity vs. Time plot.

2pts


7. Suppose your cart were to undergo the following sequence of motions: move away from the origin slowly and steadily for 4 seconds; stand still for 4 seconds; move toward the origin steadily about twice as fast as before. Sketch your prediction of the velocity-vs.-time graph on the axes below using a dashed line.

8. After making your prediction, click on the “Start” button and move

the cart in the way described above. Sketch the actual graph of the cart’s actual motion with a solid line.

Question: Is your prediction the same as the final result? If not, describe how you would move the cart to make a graph that looks like your prediction?

Now for the other way around, you’ll translate a Velocity vs. Time graph into a verbal description of the motion.

9. Open the file Velocity Match, (in “Physics Experiments/Physics220–221/One-Dimensional Motion”). A velocity graph like that shown below should appear on the screen.

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Question: Describe in your own words how you would move the cart in order to match this velocity graph.

10. Try to move the cart in such a way that you can reproduce the graph shown. You may have to practice a number of times to get the movements right. Work as a team and plan your movements. Get the times right. Get the velocities right. Then draw in your group's best match on the axes above.

Question: Is it possible for an object to move so that it produces an absolutely vertical line on a velocity-vs.-time graph? Explain.

PART TWO: Non-Constant Velocity You probably noticed that, aside form jitter, there wasn’t much happening in the Acceleration vs. Time plots for constant-velocity motion. Now you’ll look at non-constant velocity motion. Position, Velocity, and Acceleration plots should look a little more interesting. First you’ll use the simulation, then you’ll use the real cart.

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Simulation You’ll look at motions and plots for different combinations of initial velocities and accelerations. 1. Return to the “Moving Man” Simulation and reset it by clicking one

of the “Clear” buttons and answering “Yes” in the pop-up. 2. Use the blue position arrow and the green acceleration arrow to

give the man the initial conditions shown below (note: Acceleration is 1.0 m/s2).

3. Hit “go” and copy the man’s position, velocity, and acceleration plots onto the graphs below.

Question: How do these distance, velocity, and acceleration graphs differ from those on page 2?

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4. Now give the man a greater acceleration, 3.2 m/s2 and copy the resulting plots below.

Question: How do the Position and Velocity plots differ from those on the previous page?

5. Now, consider what the plots would look like if you started the man

at the tree, gave him a positive initial velocity and a negative constant acceleration. Before actually doing that, for each part of the motion after you set the man going—away from the tree, at the turning point, and toward the tree—predict in the table that follows whether the velocity and acceleration will be positive, zero or negative.

Moving Away Turning Around Moving Toward

Velocity

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6. Okay, now give the man a negative acceleration (-1.0 m/s2) and a positive initial velocity (5.0 m/s), and hit “go.” Correct any errors you may have made in the table above.

Question: Carefully explain the direction (sign) of the acceleration at the point where the man turns around.

Minimize the simulation window, you may wish to refer back to it.

Experiment Now you’ll reproduce some of these motions with the real cart. Note the similarities and differences between the real plots and the simulated plots.

Set-up Open Cart (in “Physics Experiments/Physics 220-221/ One-Dimensional Motion”). Put a cart on the track near the detector. Set the cart’s fan on “high” so that pushes away from the detector.

7. Click on the “Collect” button, and let the cart go. Always catch the cart before it crashes at the end! Record the resulting graphs below. Note: we’re particularly interested in the motion after you’ve stopped pushing and before you’ve caught the cart at the other end.

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Question: How do these distance and velocity graphs compare with those for the “moving man”?

8. Sketch your prediction of the fan-powered cart’s acceleration on the axes that follow using a dashed line.

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9. After making your prediction, display the acceleration graph. Sketch it on the axes above using a solid line.

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Question: How does the acceleration behave as the cart speeds up? How could you predict this based on the velocity graph?

10. Set the fan to “low” and turn the cart around so that it would be blown toward the detector if released. Practice giving the cart a push away from the motion detector. It should move toward the end of the ramp, slows down, reverses direction and then moves back toward the detector. Be sure to stop the cart before it hits the motion detector!

11. Collect data for this motion and sketch the graphs on the next page. Label the times: “A” - where the push ended (where your hand left the cart), “B” - where the cart reversed direction, and “C” - where you stopped the cart. Also label where the cart is “Moving Away” from the detector and “Moving Toward” the detector.

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Question: In what ways are these plots similar to and different from those you’d gotten when the “moving man” had similar initial conditions? They should still be displayed in the simulation’s window.

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PHYSICS 220 LAB #1: ONE-DIMENSIONAL MOTIONnewton.uor.edu/FacultyFolder/eric_hill/Phys220L/Lab... ·...

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