We discussed how a force can be a push or a pull and then spent a little time talking about examples of pushes and pulls in our daily lives (pushing or pulling a door open or closed, bending over to pick something up, stretching a rubber band, squeezing a tube of toothpaste, etc).
The students completed three hands-on experiments to observe how things can pull on each other. Our first project, "Floating Sticks," involved the students placing two toothpicks side-by-side in a bowl of water. The students dipped a third toothpick into dish washing detergent and then carefully dipped this third stick into the water between the two floating toothpicks. This lab showed how dish washing detergent breaks up the attraction of water molecules. This causes some water molecules to pull on others, taking the toothpicks with them.
Our second project for the day was "Tug of War." For this one, each student was given a sheet of aluminum foil, 1/2 cup of water that was mixed with blue food coloring, and 1/4 cup of rubbing alcohol. The students placed the foil on the table and then poured a thin layer of the blue water onto the foil. They then used an eyedropper to add a drop of rubbing alcohol to the center of the layer of water.The water molecules on the surface of the water normally pull equally in all directions. However, when we added the rubbing alcohol, the two liquids immediately separate. The alcohol pulls away from the water while the water pulls away from the alcohol.
We also tried a lab called "Powder Dunk." This one didn't have the desired effect so we may re-try this one at some point during the semester. Each student had two soup bowls filled with water. We sprinkled a thin layer of talcum powder over the surface of the water in each bowl. The students dipped one of their toothpicks in shampoo and the other in the dish soap. They then touched the soapy end of each toothpick to the center of the powder in each bowl. We should have seen the shampoo cause the talcum powder to break up into large floating blocks and the dish soap to cause it to rush to the sides of the bowl and then sink.
We moved on to discuss velocity. Velocity describes the speed and direction of an object. We were going to go outside to move around at different speeds and in different directions to demonstrate changes in velocity but the thunder and rain kept us in the classroom. Instead, I used the example of cars traveling on a road to explain this concept. If two objects (cars) are moving at the same speed and in the same direction, we can say they have the same velocity. However, if one car slows down or speeds up, the velocity of that car has changed. We also discussed cars traveling at the same speed in opposite directions (even though the speed is the same, their velocity is different).
After discussing velocity, we spent a little time talking about balanced and unbalanced forces. Forces on an object that are equal in size but opposite in direction are balanced forces. To explain this, I used the example of playing tug-of-war with a dog. When you plant yourself on the ground, the ground actually pushes back up on you. If you do not move backward or forward, the force of the dog pulling must be equal to the force of the ground pushing on you. The forces of the dog and the ground, then, are equal or balanced.
Now, imagine you are playing tug-of-war with the dog but your foot hits a slippery spot on the ground. The ground is now unable to exert as much force back on you but the dog is still able to pull just as much. Since the pull of the dog is now greater than the push of the ground, the forces acting against you are unbalanced. We also tied in velocity. When the forces are balanced and you are not changing either speed or direction, your velocity remains constant. When you hit the slippery patch of ground and the dog is able to pull you over, you accelerate in the direction of the greater force (in this case, the dog). Therefore, your velocity has changed.
There was one more quick lab to close out the class session. This one, "Fly Away," involved the movement of air due to unequal pressure. Each student was given a small glass soda bottle which they laid on its side on the table. They then squeezed a small piece of notebook paper into a small ball. The students placed the wad of paper just inside the opening of the bottle and then crouched down in front of the bottle to try to blow the paper into the bottle. We had mixed results (probably due to the pieces of paper being too big) but we should have observed the paper ball flying out of the bottle. This happens due to changes in air pressure. Before the students blew into the bottles, the amount of air inside and outside of the bottle are the same. When they blew into the bottles, the amount of air inside the bottle increased, thus increasing the air pressure. That extra air has to go somewhere so it exits through the neck of the bottle, pushing the paper ball out with it.
To look forward to next week:
Inertia and Newton's Laws of MotionWe will be making paddle boats and balloon rockets!
The labs "Floating Sticks," "Tug of War," and "Powder Dunk" are all from Janice VanCleave's Chemistry for Every Kid.
VanCleave, J. (1989). Chemistry for Every Kid: 101 Easy Experiments That Really Work. San Francisco: Jossey-Bass.
The lab "Fly Away" is from Physics for Every Kid.
VanCleave, J. (1991). Physics for Every Kid: 101 Easy Experiments in Motion, Heat, Light, Machines, and Sound. San Francisco: Jossey-Bass.
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