Laboratory Activity 2: Velocity
Graphs
Group member names:
-Greg Strickler
-Chris Marino
-
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
Velocity
Position
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.
Time
Time
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.
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.
It was different because when you walk your never really at a constant velocity,
that’s why our graph resembles a sin wave.
Prediction 2a: How will the graph be look if you walk away from the detector at a
faster pace?
Position
Velocity
Time
Time
Velocity
Position
Prediction 2b: How will the graph be look if you walk towards the detector at a
slow, steady pace?
Time
Time
Velocity
Position
Prediction 2b: How will the graph be look if you walk towards the detector at a
fast, steady pace?
Time
Time
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.
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 distance away tells us the speed of the object. If your walking toward your
velocity is negative and if your walking away your velocity is positive. If you
stand still, you have no velocity and your graph will have a line at 0.
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.
Velocity
Position
Time
Time
Question 3: Try walking the way described. How does it compare to your
predictions?
Is basically the same as we predicted, but not straight lines.
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.
Stand still for 4 seconds. Then walk away at a constant speed for 4 seconds.
Then stop again for 4 seconds, and walk back toward the detector at a constant
speed for 6 seconds, then stop.
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 moved like we had predicted. When we walked away the velocity was
positive, and it was negative when we walked toward. When we were standing
still we had no velocity.
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?
Yes, the first run we started to close to the detector so when we walk back
toward it we ran out of room. Our other trials we started farther away. No,
velocity graph only tells us our speed and direction.
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?
The horizontal distance from the axis tells the amount of time. The vertical
distance tells the velocity. If the line is above the axis, it means that the velocity is
positive and that the object is moving away from the point of reference. If it is
below, it is moving towards the point of reference. If it is on the axis, it isn’t
moving at all.
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?
Position and velocity graphs both tell speed and direction. The steepness of a
position graph determines the speed of the object, whereas the vertical distance
from the axis in a velocity graph also tells the speed, but more accurately. The
direction of motion is determined in a position graph by whether the slope is
positive or negative. In a velocity graph, it is determined by whether the velocity
is positive or negative.
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 know our statements are true because we tested them using the motion
detector. As we walked to and from the detector, the slope on the position graph
and the line on our velocity graph matched our statements.