Phys221 – Lab Week 2Changing Motion
Introduction
In this lab activity you will use a cart, a track/ramp, and a motion detector to gain a better understanding of how position, velocity and acceleration are related.
A. You will conduct a number of experiments in which your cart will undergo some motion and you will:
1. Predict position vs time (x-t), velocity vs time (v-t), and acceleration vs time (a-t).
2. Run the experiment and experimentally determinex-t, v-t, and a-t graphs.
3. Compare your prediction and experimental result.
B. If there is enough time we will get quantitative and explore how the mathematical description, the graphical result, and the actual motion are consistent with each other and ‘tell the same story’.
Prep Work at beginning of lab
Open the document linked under ‘LoggerPro Instructions’ on the course website. There should be a hardcopy on your table as well (please leave that, don’t take it home). The document helps you
- connect and set up the equipment. If at any moment you are unable to follow, please get my attention immediately.
- openthe configuration file that we will use for ALL activities.
What to do in the lab
Follow the instructions on the next few pages. You will NOT have to turn in your work.
Please make sure to FIRST predict and THEN measure. We want to ensure that you commit to an answer and then check whether your thinking was correct. If your prediction and measurement do not match, discuss within your group and seek my help when necessary.
Make sure to save the graphsthat you will generate. Note: you do not need the graphs for the lab report but you might have a discussion in your group that requires you to revisit the data.
What to turn in as your lab report
I will give you a handout in class (one per group). This will be our lab report to be completed and turned in next week. The document is also available on the course website. One lab report per group.
Activity 1:
Most of you have never used a motion detector before. Whenever you use new equipment, it is a good idea to run a few very simple experiments to check that you understand what the equipment actually measures. Activities 1 and 2 will hopefully help with this.
Setup
- Place track on table horizontally.
- Motion detector MD should be located at the end of track closer to the front of the classroom and facing along the track.
- Place cart somewhere on the track (more than 20 cm away from MD) and let it sit there.
Predict x-t, v-t, and a-t
Activity 2:
Setup
- Place track on table horizontally.
- Place cart somewhere on the track (about 20 cm away from MD), give it a quick push and observe it rolling along table. (We are not interested in the motion during pushing.)
Predict x-t, v-t, and a-t
Questions:
1. What feature of your position graph signifies that the motion was away from the motion detector?
2. What feature of your velocity graph signifies that the motion was away from the motion detector?
Activity 3:
Setup
- Place track on table horizontally.
- Place cart on the far end of the track, give it a quick push toward the MD, and observe it rolling along table. (We are not interested in the motion during pushing.)
Predict x-t, v-t, and a-t
Questions:
1. What feature of your position graph signifies that the motion was toward the motion detector?
2. What feature of your velocity graph signifies that the motion was toward the the motion detector?
Activity 4:
Setup
- Use one wood block to create a ramp that is higher at the MD end of the track.
- Place MD at the top, as shown.
- Place cart near the bottom of the track,give it a quick push, and observe it rolling up the ramp. Don’t let it roll back down (grab it at the top).
Predict x-t, v-t, and a-t
Questions:
1. What feature of your velocity graph signifies that the motion was toward the motion detector?
2. What feature of your acceleration graph signifies that the motion was towardthe motion detector?
3. As the cart slows down, is the velocity positive, or negative?
4. As the cart slows down, is the acceleration positive, or negative? Does it vary or is it constant?
Activity 5:
Setup
- Use one wood block to create a ramp that is higher at the MD end of the track.
- Place MD at the top, as shown.
- Place cart near the top of the track (about 20 cm away from MD), let go (don’t push), and observe it rolling down the ramp. Make sure to stop it at the bottom.
Predict x-t, v-t, and a-t
Questions:
1. As the cart speeds up, is the velocity positive, or negative?
2. As the cart speeds up, is the acceleration positive, or negative? Does it vary or is it constant?
Activity 6:
Setup
- Use one wood block to create a ramp that is higher at the MD end of the track.
- Place MD at the top, as shown.
- Place cart near the bottom of the track, give it a quick push, and observe it rolling up the ramp AND then back down.
Predict x-t, v-t, and a-t
Note:
If you combine Activities 4 and 5, you get Activity 6. Make sure you are clear on all positive and negative velocities and accelerations.
Activity 7:
Setup
- Use one wood block to create a ramp that is higher at the end of the track opposite to the MD.
- Place MD at the bottom, as shown.
- Place cart near the top of the track, let go (don’t push), and observe it rolling down the ramp. Make sure to stop it at the bottom before it hits the MD.
Predict x-t, v-t, and a-t
Activity 8: Quantitative Analysis
Use your data from Activity 7.
Note: The motion you observed has constant acceleration. In chapter 2 you will find that the magnitude of the acceleration on a ramp (without friction) is given by , where is the angle of the incline. The direction of the acceleration is downhill.
1. Find the average acceleration of the cart from your acceleration graph. Use the analysis feature in LoggerPro to read 6 values from your graph. Use these the find the average (mean) acceleration:
Acceleration values (m/s2)Average (mean) acceleration: _____ m/s2.
2. Find the average acceleration of the cart from your velocity graph. Use the analysis feature to read the velocity and time values for two typical points on the velocity graph.
v (m/s) / t (s)Point 1
Point 2
Find the average acceleration
(m/s)
(s)
avg a (m/s2)
3. Fitting your graphs (we’ll do this one as a group)