Group member names: -Jenni Shafer Laboratory Activity 2: Velocity -Tassall Hughes Graphs -Eric Koenn
Objectives Explore how motions are related to velocity-time graphs Relate velocity-time graphs and position-time graphs
Equipment Computer Ultrasonic motion sensor USB Link
Activity One: Simple graphs Prediction 1: Using the line tool on the drawing toolbar of Word, draw a line on both the position and velocity graph below predicting the shape of a curve for a person walking away at a slow, steady pace. y n t i o i c t i o l s e o V P
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Set up the equipment: Obtain a USB Link and a motion detector as in the previous lab. Open the experiment file called VelocityGraphs.ds to display velocity vs. time axes. You can find it by going to the networked disk “PLAB” (usually disk G:), in that opening the folder “Plab”, then finding the folder for Physics 201, and then looking in the folder for Lab2. Experiment: Set the detector up as in the previous lab and walk away backwards at a slow, steady pace, while watching the graph on the screen. When you have a nice graph, copy the graph and paste it in the section below. -paste graph here-
Question 1: How did the result compare to your prediction? If it was different, do you now understand why it is the way it is? Note: you should not go back and change predictions. There is absolutely nothing wrong with making a prediction and then finding out it wasn’t quite correct. In fact, that can show that you are learning. Our prediction proved to be correct.
Prediction 2a: How will the graph be look if you walk away from the detector at a faster pace? y n t i o i c t i o l s e o V P
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Prediction 2b: How will the graph be look if you walk towards the detector at a slow, steady pace? y n t i o i c t i o l s e o V P
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Prediction 2b: How will the graph be look if you walk towards the detector at a fast, steady pace? y n t i o i c t i o l s e o V P
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Experiment: Without erasing your last data run, start about ½ meter from the detector and walk away at twice the rate. Then take another run, starting away from the detector and walking toward it slowly. Finally, walk toward the detector quickly. Label each run as to which it is by clicking twice (not too fast) on the labels “Run 1” etc. in the menu list on the upper left-hand window and giving them more descriptive names. Then copy and paste your graph in the space below the same way as above. - paste graph
here-
Question 2: What does distance away from the horizontal (time axis) tell you about the motion? What does the sign (above or below the axis) tell you about the motion? What would a graph look like for someone standing still? The horizontal (time axis) will tell you how much time has passed during the motion. Above the axis is away from the detector and toward the detector is below the axis. It would be a horizontal line. Activity Two: More complicated motion Prediction 3: Sketch on the graph below a prediction for the following motion: walking away from the detector quickly, stopping for a while, and then walking toward the detector slowly. y n t i o i c t i o l s e o V P
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Question 3: Try walking the way described. How does it compare to your predictions? Our prediction was correct.
Open the experiment file called VelocityMatch.ds. A velocity graph like that shown below will appear on the screen.
(from RealTime Physics (Electronic Version) by Prisilla Laws, et al., John Wiley and Sons, NY, 1999)
Prediction 4a: Describe in words how you would move in front of the motion detector in order to create the graph. Discuss your answer with your group until you all agree. You stand still for 4 seconds then walk away for 4 seconds. Then for another 4 seconds you stand still and then walk toward the detector for 6 seconds. For the remaining time you stand still.
Experiment: Walk in front of the motion detector and try to create the graph. You may try a number of times. It helps to work in a team. Get the times right. Get the speeds right. Question 4a: How did your prediction match how you had to move? How did you have to move when the line was above the axis? Below the axis? On the axis? Our prediction matched how we had to move. You have to move away from the motion detector. Moving towards the detector will create a line below the axis. Standing still will create a line on the axis.
Question 4b: Did you run out of room for the motion? Can you tell from a velocity graph where you need to start? Why or why not? No. On a velocity graph it does not matter where you start because you are measuring speed instead of distance.
Summary The following questions will help you get the main ideas out of this lab. You should find these straightforward questions, but take the time to talk it over with your team and write complete answers to these questions. You may find your answers here to be the most useful part of this lab down the road. Summary 1: In a velocity graph, what does the horizontal distance from the axis of the graph mean? What does the vertical distance from the axis mean? What does it mean if the line is above the axis, on the axis, or below the axis? Horizontal distance is the time. The vertical line is the velocity. Moving away will create a line above, and moving toward the detector will create a line below the axis.
Summary 2: How are position and velocity graphs related? What aspect of a position graph and what aspect of a velocity graph tells us how fast something is moving? What aspects of each graph tell us what direction something is moving? In a position graph the slope of the line is the velocity, and in a velocity graph the slope is the acceleration. In a position graph the slope will tell how fast the object is moving, and in a velocity graph how fast it moves will be a positive or negative direction. On a position graph the slope will tell us the direction, and on a velocity graph if the line is above or below the axis will tell the direction.
Summary 3: How do you know your above statements are true? What experimental observations and/or logical reasoning can you give to justify what you said in summary questions 1&2? Our predictions were proved by the graph. When standing still you have a zero velocity. If moving away the velocity will be positive, and moving toward the motion detector will create a negative velocity.