October 14 - Gravity

We began class by discussing what the students already know about gravity. I also asked them how gravity affects human beings.

Each student was then given a sheet of notebook paper and a hardcover textbook. The students were asked to predict what would happen if they dropped the book and paper separately. Would the book or paper fall fastest? We tried this and then predicted what would happen if the paper was placed on top of the book. Again, the students tried the experiment and discussed their observations.

Gravity pulled equally on the book and the paper. So, even though the book hit the ground before the paper when we dropped them separately, gravity's pull on each object was equal. The paper fell at a slower speed due to air resistance. The book's weight overcame the force of the air but the paper, being so much lighter, had little effect on the air's push. This made it fall at a much slower rate.

Gravity is a force that attracts all objects to each other. This attraction is called Gravitational Pull. On Earth, gravity keeps an atmosphere around the planet. It also causes things to fall to the ground, causes the ocean's tides, and causes hot air to rise while cool air falls (which leads to winds).

Gravity pulls things to the center of the Earth. Gravity also gives people and objects weight. Since the force of gravity is different on other planets, people and objects can weigh more or less than they do on Earth on other planets.
Here is a link to a fun website you can use to discover weights of people, pets, or objects on other planets: http://www.exploratorium.edu/ronh/weight/

We completed several other experiments about gravity this week. Here are the details.
*Bigger - In this lab, students made two parachutes each to determine if size affects the speed of a falling parachute.
Each student was given a 12-inch square of plastic (cut from a plastic bag), a 24-inch square of plastic, 8 20-inch pieces of string, 2 washers, and 2 4-inch pieces of string. They used these materials to construct parachutes and then predicted which they thought would hit the ground first (similar to the book and paper experiment from the beginning of the class session). We found that the parachute made from the larger bag fell to the ground more slowly than the smaller parachute. Just like in the book and paper experiment, this is due to air resistance. The larger parachute has a larger surface area and a small weight so it has more air resistance. 

*Gravity Won - The students learned about water's surface tension in a previous lesson. This lab demonstrated the effect of gravity on weak surface tension.
We filled a baby food jar with rubbing alcohol then colored it with food coloring. A straw was placed into the jar of alcohol and held in place with a small piece of modeling clay. When we tipped the jar upside down, the alcohol flowed out of the jar and the straw.
Alcohol has a weaker surface tension than water meaning the attraction between the alcohol molecules is not very strong. The air pressure inside the straw was not enough to hold liquid in the straw so the pull of gravity caused the liquid to flow out of the straw.

*Anti-Gravity - This was very similar to the last lab except we used water instead of rubbing alcohol. Since water has a greater surface tension than alcohol, the air pressure inside the straw pushed up on the water when the jar was tipped upside down while the water molecules pulled from side to side. These pushing and pulling forces were greater than the pull of gravity so the water remained in the straw.

The labs "Same Speed" (book and paper) and "Bigger" are 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.

The labs "Gravity Won" and "Anti-Gravity" are from Chemistry for Every Kid.
VanCleave, J. (1989). Chemistry for Every Kid: 101 Easy Experiments That Really Work. San Francisco: Jossey-Bass.