Laboratory Activity 2: Velocity Graphs

/ Group member names:
-Brandon Bratcher
-Derrick Worley
-Jeremy Lange

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.

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.

The graphs we predicted actually matched up really well with the results after our “run.” The only minor fluctuation was the up-and-down motion of the velocity which probably had something to do with our variable velocity while walking.

Prediction 2a: How will the graph be look if you walk away from the detector at a faster pace?

Prediction 2b: How will the graph be look if you walk towards the detector at a slow, steady pace?

Prediction 2b: How will the graph be look if you walk towards the detector at a fast, steady pace?

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?

Distance away from the horizontal line (time) is actually the distance away from the detector and not time measurement. The sign of the motion signifies whether the person is moving toward or away from the origin. Someone standing still will align perfectly with 0 in velocity, in other words will be on the time axis.

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.

Question 3: Try walking the way described. How does it compare to your predictions?

We be right

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.

For 4 seconds the object stays still, then it goes to ½ m/s away from the detector, then stands still away from the detector before walking back toward the detector at ½ m/s for 6 seconds. Then you stay at rest. You move two meters away from the origin at first, then after staying still, you move three meters away.

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?

We were right again. Above the axis = away from the origin, Below the axis = toward the origin. On the axis = standing still.

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?

Yeah we ran out of room. We determined early on that we needed to start 2 meters further than we ended up, because we walk toward the detector longer than we walked away.

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 measures the velocity, such as m/s. In terms of vertical distance from the axis, a positive number (above the axis) is away from origin, a negative number (below the axis) is toward the origin.

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?

Both position and velocity graphs show something moving in association with time. Position graphs show us velocity, while velocity graphs show us acceleration. The slope of a position graph shows us the speed, while the position above or below the x-axis on a velocity graph shows us the speed. The position graphs show us direction of movement in terms of positive and negative slope. A velocity graph shows us direction in terms of position above or below the x-axis.

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?

We tested out all of these assertions through our last two labs. The detectors backed up our predictions.