Second Law Separation

INTRODUCTION

The second law of motion states that when a force is applied to an object, that force is equal to the objects mass times its acceleration. If the same force is applied to two objects of different masses how will each react? Many factors are part of this, but this experiment will primarily examine one factor, mass. Afterwards, a conclusion about the relationship between mass and acceleration can be made.

MATERIALS

• Two toy trucks (one large mass and the other small mass)
• Spring type clothespin
• Plastic Tab (BRead bag fastener cut in half lengthwise)
• Rubber bands
• Meter stick

WHAT TO DO

1. Attach the clothespin (open side down, the side that you squeeze with your fingers) to the larger truck using the rubber bands. This can be done by wrapping the rubber bands around the truck and then one of the teeth of the clothespin. Make sure that the rubber band does not restrict the movement of the tires.
2. Now squeeze the open end of the pin and place the plastic tab in the top to hold the clothespin open. This may take a few tries to get it to stay.
3. Place both cars face to face with each other, each touching the clothespin between them, and place the meter stick so that the 50cm mark is where the two trucks touch.
4. Flick the plastic tab so that the clothespin snaps open and creates a force that moves both of the cars in opposite directions. Observe the differences in each of the cars’ travels.

QUESTIONS

1. Which truck moves the farthest?
2. What caused each truck to move? Was it the same force on both?
3. What is different about the two trucks that would cause the difference in travel length?
4. How does this relate to Newton’s Second Law stating that F = m x a (force equals mass multiplied times acceleration)?

SUMMARY

The force applied to both trucks is the same because it comes from the same origin. The clothespin applies the same force to both trucks, but the one with less mass travels farther faster because of the relationship described in Newton’s Second Law. This experiment also shows an example of the Third Law of Motion -- for every action there is an equal but opposite reaction.

SOURCE

"Science is Fun" in "Playground Physics" Janet Tarino, The Ohio State University Research Foundation, Columbus, 1994, p. 129-132.

© S. Olesik, WOW Project, Ohio State University, 2002.