Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Slide the wooden support feet onto the ends of the metal V-track according to Figure 2.
   {11874_Preparation_Figure_2}
2. Wrap the rubberband around the end of the wooden feet as shown in Figure 2. The rubberband will act as a stopper for the ball bearings.
3. Place the metal track on a flat, level tabletop.

Safety Precautions

The materials included in this kit are nonhazardous. Please follow all normal laboratory safety precautions.

Disposal

Save the materials for future use. The tracks may be disassembled and stored in the original shipping tube.

Lab Hints

• Enough materials are provided with this classroom set for eight student groups. Additional kits may be purchased to accommodate classroom needs. This laboratory activity can reasonably be completed in one 50-minute class period.

Teacher Tips

• This laboratory activity can be completed as an inquiry-based lab, in which students experiment with collisions before they are given any Background information about conservation of energy and momentum. Or, this laboratory activity can be completed as a reinforcement tool after the topics have been discussed in class. In this case, the Background information and any textbook information should be provided before the experiment.
• Additional experiments include rolling one ball at one stationary ball, rolling two balls at one stationary ball, rolling one ball at two stationary balls and rolling two balls at two stationary balls. Students may also want to experiment with collisions between two moving ball bearings. Students may experiment with different speeds and compare the speed and direction of each ball after the collision. For each additional experiment, students should create and record their observation in a data table.
• If time or equipment is limited, this activity can also be performed as a demonstration. In this case, each student should record his or her observations in the data table and then complete the Post-Lab Questions.

Sample Data

Answers to Questions

1. What type of collision did the ball bearings experience—elastic or inelastic? Explain.

   The ball bearings experience elastic collisions because the balls separate after the collision.

2. What happened to the colliding ball bearing(s) after the collision?

   The colliding ball stopped after it collided with the stationary ball bearings. It did not bounce back or recoil after the collision.

3. How did the number of stationary ball bearings affect the collision results?

   The number of stationary ball bearings did not affect the number of balls that were knocked away. Only the number of colliding balls affected the number of balls that were knocked away.

4. How did the speed of the colliding ball affect the speed and number of ball bearings knocked away?

   The speed of the colliding ball bearing only affected how fast the knocked-away ball traveled. A faster moving colliding ball still only knocked away one ball. Two fast-moving balls knocked away two balls from the series.

5. How did the number of colliding ball bearings affect the number of ball bearings knocked away?

   One ball bearing could only knock away one from the series—no matter how fast it traveled. Two ball bearings could only knock away two balls from the series. The speed of the colliding balls affected the speed of the knocked-away balls, but not the amount.

6. (Optional) If the colliding ball bearings had more mass than the individual stationary ball bearings, how would this affect the results of the collisions? (Would more ball bearings be knocked away? Fewer? Would the colliding ball bearing stop after the collision? How would the speed of the balls that are knocked away be affected?)

   A more massive colliding ball may knock away more than one ball from the series. The total momentum and kinetic energy must still be conserved however. If one ball bearing is knocked away, it will most likely have a higher velocity than the colliding ball because it will be less massive and, in order to conserve momentum, its speed must be faster.

Introduction

Discover some of the basic laws of physics by studying simple one-dimensional collisions. Smash one ball into a series of three or four balls and observe what happens. Predict how colliding two ball bearings at the same time will affect the collision results?

Concepts

• Collisions
• Conservation of energy
• Conservation of momentum

Background

When an object is set in motion, the object has a property known as momentum. Momentum is calculated by multiplying the mass of the object by its velocity. A fundamental principle of physics is that the momentum of a system of objects always remains constant. This principle is known as the conservation of momentum. If objects within a system collide, the momentum of the individual objects before and after a collision may change, but the total momentum of the system will remain constant.

There are two types of collisions—elastic and inelastic. An elastic collision occurs when the objects that collide separate after the collision. An example of an elastic collision is the collision between a baseball and a bat. An inelastic collision occurs when the objects that collide stick together and move as one object after the collision. An example of an inelastic collision is when the baseball hits the catcher’s mitt and stops. In every collision, elastic or inelastic, momentum is always conserved. The main difference between the two types of collisions is that for an elastic collision, the kinetic energy of the system also remains the same. The conservation of energy principle does not apply to an inelastic collision because in an inelastic collision much of the energy is lost as heat and sound due to frictional forces that arise when the objects deform and “stick” together.

In this activity, elastic collisions occur because the ball bearings separate and one set continues to move after the collision. Since these are elastic collisions, both the conservation of momentum and the conservation of kinetic energy principles apply. The conservation of energy principle limits the number of ball bearings that can be knocked away from the stationary series. No matter how fast a single colliding ball bearing hits the stationary series of ball bearings only one ball bearing will be knocked away(provided they are the same mass). If two ball bearings collide with the stationary ball bearings, two ball bearings will be knocked away.

Example
A moving ball with mass (M) and velocity (V) collides into a stationary series of three ball bearings, each with the same mass as the colliding ball. The momentum and kinetic energy of the colliding ball is MV and ½MV², respectively. The colliding ball comes to a complete stop after the collision and its momentum and kinetic energy are transferred to the ball bearing at the end of the series. Momentum is conserved during every collision so the ball bearing is knocked away with velocity V (momentum equal to MV). The kinetic energy of the ball bearing is equal to ½MV², clearly showing that energy has also been conserved.

Assume instead that two ball bearings were knocked away by the one colliding ball bearing. In order to conserve momentum, the two ball bearings (2M) would be knocked away from the series with half the velocity of the colliding ball [MV = 2M(½)V].However, the kinetic energy of this two-ball system would then be equal to ½(2M)(V/2)², or ¼MV². The kinetic energy of the two-ball system is one-fourth the original kinetic energy and is clearly not conserved as it should be during an elastic collision. Therefore, this result is not possible. One colliding ball will knock away only one ball (provided the masses are equal). One ball cannot knock away two or more balls no matter how fast it is traveling.

Materials

Safety Precautions

The materials used in this activity are nonhazardous. Please follow all normal laboratory safety precautions.

Procedure

1. Place three steel ball bearings in the middle of the V-track. The three ball bearings must be in contact with each other (see Figure 1).
2. Roll one ball bearing into the three-ball bearing system from the left. Record the results of the collision in the data table. (Did the colliding ball stop or recoil? How many ball bearings were knocked away from the stationary ball-bearing series? How did the speed of the knocked-away ball compare to the initial speed of the colliding ball?)
   {11874_Procedure_Figure_1}
3. Repeat steps 1 and 2. For step 2, however, roll the colliding ball with more speed at the stationary ball bearings. Record the results of the collision in the data table.
4. Repeat step 1.
5. Roll a system of two, nearly touching ball bearings into the three-ball system. To ensure that the ball bearings stay in contact as they roll, push on the ball bearing of the two-ball series that is farthest away from the series of stationary ball bearings. This way both ball bearings will roll with the same speed and remain close together. Also, release the ball bearings a short distance away from the stationary ball bearings. Record the results of the collision in the data table.
6. Repeat steps 4 and 5. For step 5, increase the speed of the two colliding ball bearings. Make sure the two rolling ball bearings are nearly touching as they make contact with the stationary ball bearings. Record the results of the collision in the data table.
7. Place four steel ball bearings in the middle of the angle track. The four ball bearings must be in contact with each other.
8. Repeat steps 2 and 3. Record the results of the collisions in the data table.
9. Answer the Post-Lab Questions. 
10. Consult your instructor for appropriate storage procedures.

Student Worksheet PDF

11874_Student1.pdf