Momentum Laboratory: Introduction

20 points

This lab demonstrates conservation of momentum. Momentum is the product of the mass and velocity of a moving object. Studying momentum helps physicists understand the relationship between the motions of two interacting objects.
First, design an experiment you could construct that might measure conservation of momentum.

  • What materials would you use?
  • What would you measure?
  • What results would you expect?
  • What if the results were different; what would that indicate?

Momentum Laboratory: Description

Developing a Hypothesis:
In this experiment you will study the momentum of two balls with various masses. The balls will be pushed toward each other and will then undergo either elastic or inelastic collisions. You will find the mass and velocity of the balls prior to and following the collision in order to compare the momentum before and after the balls move apart.
Create hypotheses of what you think the results of this experiment will be.

  • How will the momentum of the balls change before and after the collision?
  • According to the Law of Conservation of Momentum the momentum will be the same before and after both elastic and inelastic collisions
  • What differences can you expect between inelastic and elastic collisions?
  • An inelastic collision has both object joined together after the final collision so they have the same velocity

Defend your hypotheses with your knowledge of momentum. Your answers will be a part of your lab write-up that you submit to your teacher.

Objectives:

  1. Generate data for both elastic and inelastic collisions
  2. Calculate the momentum of each object in different collisions
  3. Evaluate data to verify the law of conservation of momentum.

To view the items that need to be included in your lab write-up along with a grading rubric, please refer to the Guidelines for the Laboratory section.

Preparation

  1. Read the entire lab procedure and plan the steps you will take.
  2. The virtual lab can be found at You will see a screen with two balls, and two boxes. The green box on the side contains various labels and the control for the amount of elasticity in a collision. The yellow box at the bottom allows you to control the initial conditions of the collisions. Spend some time playing with the virtual lab to become familiar with it. In the green box, click Show Values. You can adjust the speed of the red ball by clicking on the circled V at the end of the velocity arrow and making it longer or shorter. You can adjust the mass of the balls by using the sliders in the box at the bottom. Hit the Play button to begin moving the red ball. Hit the Pause button once the collision has taken place.
  3. Prepare two data tables in your spreadsheet program. Table 1 is for trials in which the collisions are elastic. Label columns: Trial, m1 (kg), m2 (kg), v1,i (m/s), v2,i (m/s), v1,f (m/s), and v2,f(m/s). In the first column number trials 1 through 10. Table 2 is for trials in which the collisions are inelastic. Label columns: Trial, m1 (kg), m2 (kg), v1,i (m/s), v2,i (m/s), and vf (m/s). In the first column number trials 1 through 10.

Elastic Collisions

  1. Slide the elasticity bar in the green to the right so that it is 100% elastic. Also make sure that the velocity vector box at the top of the box is chosen.
  2. Select the More Data button at the bottom. Input the following values: m1 = 0.5 kg, p1 = 0, v1,i = 0.2 m/s, m2 = 0.5 kg, p2 = 1, and v2,i = 0 m/s. Record these values in your data table. Select the play button. Observe the carts balls collide, and record their final velocities in your data table. Press the Pause button after the balls have collided.
  3. Press the blue reset button. Change the mass of one of the balls by typing in a number in one of the white boxes labeled Mass. Repeat the experiment. Change the mass of the other ball, and repeat the experiment. Change the initial velocities of both balls, and repeat the experiment. Notice that positive movement is to the right and negative movement is to the left. Continue changing initial conditions until you have tried at least ten different types of initial conditions. You might want to try to make the balls behave in different ways like both stopping after the collision or both moving in the same direction after the collision. Be careful to note that the initial values you input are accepted by the virtual laboratory. If the balls will not collide given your choices, the laboratory will return to default settings.

Elastic Collisions
trial / m1 (kg( / m2 (kg) / v1,i (m/s), / v2,i (m/s / v2,f(m/s) / v2,f(m/s)
1 / 0.5 / 0.5 / 0.2 / 0 / 0 / 0.2
2 / 0.5 / 1 / 0.2 / 0 / -0.0667 / 0.133
3 / 1 / 0.5 / 0.2 / 0 / 0.0667 / 0.267
4 / 0.5 / 0.5 / 0.2 / -0.2 / -0.2 / 0.2
5 / 1 / 1 / 0.2 / -0.3 / -0.3 / 0.2
6 / 1 / 1 / 0.5 / -0.1 / -0.1 / 0.5
7 / 0.6 / 0.2 / 0.5 / -0.10 / 0.2 / 0.8
8 / 0.5 / 0.2 / 0.2 / -0.5 / -0.2 / 0.2
9 / 0.7 / 0.3 / 0.5 / -0.3 / 0.02 / 0.82
10 / 0.7 / 0.3 / 0.5 / 0.3 / 0.38 / 0.58

Inelastic Collisions

  1. Slide the elasticity bar in the green to the left so that it is 0% elastic. Also make sure that the velocity vector box at the top of the box is chosen.
  2. Copy the initial conditions that you used to study elastic collisions in your second data table. Repeat the trials for inelastic conditions. Record the values of vf in your second data table.

Inelastic Collisions
trial / m1 (kg) / m2 (kg) / v1,i (m/s), / v2,i (m/s / vf(m/s)
1 / 0.5 / 0.5 / 0.2 / 0 / 0.1
2 / 1 / 0.5 / 0.2 / 0 / 0.133
3 / 1 / 0.5 / 0.2 / 0 / 0.333
4 / 0.5 / 0.5 / 0.2 / -0.2 / 0
5 / 1 / 1 / 0.2 / -0.3 / -0.05
6 / 1 / 1 / 0.5 / -0.1 / 0.2
7 / 0.6 / 0.2 / 0.5 / -0.10 / 0.35
8 / 0.5 / 0.2 / 0.2 / -0.5 / 0
9 / 0.7 / 0.3 / 0.5 / -0.3 / 0.167
10 / 0.7 / 0.3 / 0.5 / 0.3 / 0.44
  1. Organizing Data:For each trial under elastic conditions, calculate the momentum of each ball before the collision and after the collision by multiplying its mass by its velocity. Remember that velocity and momentum are vectors, so the sign of the value is important in your calculations. You may want to add columns to your data tables or build new tables in your spreadsheet for these calculations.

Eleastic Collisions
toal before / total after
0.1 / 0.1
0.1 / 0.1
0.2 / 0.2
0 / 0
-0.1 / -0.1
0.4 / 0.1
0.28 / 0.28
0 / 0
0.26 / 0.26
0.44 / 0.44
  1. Organizing Data: For each trial under inelastic conditions, find the momentum of each of the two balls before the collision and after the collision.

Total before / total after
0.1 / 0.1
0.1 / 0.1
0.2 / 0.2
0 / 0
-0.1 / -0.1
0.4 / 0.1
0.28 / 0.28
0 / 0
0.26 / 0.26
0.44 / 0.44
  1. Organizing Data: For each trial under inelastic conditions, find the momentum of each of the two balls before the collision and momentum of the joined balls after the collision.
  2. In both elastic and inelastic collisions the velocity in not always conserved as see in the tables below

Elastic
total v before / total velocity after
0.2 / 0.2
0.2 / 0.0667
0.2 / 0.3337
0 / 0
-0.1 / -0.1
0.4 / 0.4
0.4 / 1
-0.3 / 0
0.2 / 0.84
0.8 / 0.96
Inelastic
total v before / total velocity after
0.2 / 0.1
0.2 / 0.133
0.2 / 0.333
0 / 0
-0.1 / -0.05
0.4 / 0.2
0.4 / 0.35
-0.3 / 0
0.2 / 0.167
0.8 / 0.44
  1. data will vary depending on what the students choose to use as their initial conditions
  1. Drawing conclusions:In this situation, conservation of velocity would mean that the total velocity for both balls is the same after the collision as it was before the collision. Is velocity conserved in elastic conditions? Is it conserved in inelastic conditions? Support your answer with data from your experiment.
  2. In all cases the momentum was the same for before and after each elastic collision
  3. Drawing Conclusions: Based on your data, is momentum conserved in elastic conditions? Support your answer with data from the experiment.
  4. In all cases the momentum was the same for before and after each inelastic collision
  5. Drawing Conclusions: Based on your data, is momentum conserved in inelastic conditions? Support your answer with data from the experiment.
  6. In all cases the momentum was the same for before and after each elastic collision
  7. Making Predictions: How did the results from your data compare with your hypothesis?
  8. The results confirmed the hypothesis, momentum was conserved in both the elastic and inelastic collisions as seen by the total initial and total final momentums were equal in each trial
  9. Evaluating Methods: How would performing this experiment in a hands-on laboratory affect the data you collected? What sources of error could you expect in real-life conditions that you could neglect in a virtual setting?
  10. Friction would have prevent having examples of perfect elastic an inelastic collisions but momentum should still be conserved.