Experiment 2: Inertia & Tensile Strength

I love inertia, it's so fun to play with! This experiment will greatly get these kids thinking. Have them brainstorm as a team to come up with a hypothesis (prediction) of what they think will happen and WHY. We have experiment journals, here the kids keep track of the supplies, process, hypothesis, and results from all our daily experiments. Most of our experiments use things you should have readily available around the house.

  SUPPLIES:

• Water bottle (or small soda bottle)
• 4 Pieces of string about 40 cm long (cotton is best, but you can judge the tensile strength of various fibers in another experiment)

PROCESS:

1. tie one end of 2 pieces of string around the neck of the bottle
2. tie the other end of ONE of the strings onto something solid like a railing, so the bottle will hang down
3. holding the other end of the other string in your hand. Pull slowly. Where will the string break?
4. repeat steps one-three, but this time jerk the string quickly. Where does it break now?

Understanding Inertia: Inertia is a part of Newtons First Law of Motion. "An object at rest wants to stay at rest, whereas an object in motion wants to remain in motion." Inertia is the measurement of how hard it is to get an object to change motion. For instance. One example of inertia would be how hard do you have to push the breaks to stop a car that is rolling downhill in neutral. Or, an example for kids, how much pressure do you have to apply to get your matchbox car to zoom across the room. In this instance. Inertia is proven because, in effect, the motion of your arm wants to continue on, but it comes to the length of the string. In order to maintain inertia, it breaks the string.....now, why do you think they broke in different places?

Understanding Tensile Strength: Tensile strength refers to The amount of stretching strength a material has before breaking. For instance, if you have a waterski rope, and try to tow a house with it, it will break because the force/weight of the object exceeds the tensile strength of the rope. But if you tow a boat with a thick chain, then it will probably be ok, because a steal chain has a high tensile strength. But if you add in inertia and momentum, then it decreases the tensile strength of an object because of Newtons Second law "Every action has an equal and opposite reaction"" so as you pull left an equal force yanks the rope left. The faster you pull the greater the force/momentum, the smaller the tensile strength...get it?