Investigating the constancy of gravity: Free-fall using a water-filled plastic bottle.

This activity is designed to have students to observe the constancy of gravity in a variety of different situations.

After the study of distance, velocity, and acceleration in one dimension, this activity may be used as segue into forces associated with projectile motion; and preferably to be done after labs using motion detectors as balls are rolled towards the motion detectors, away from the detectors, up a ramp towards a detector, down a ramp away from a detector, and tossed up and caught above a motion detector.

This activity can be done with any class size and requires simple equipment: a plastic pop bottle, a garbage can to catch the water, and paper towels to clean up any mess.

Activity 1: Throw, straight up, an empty plastic water bottle with a hole near the bottom up over a motion detector and catch it at the same height it was released. Ask students to predict if the upward speed will be greater or less than the downward speed. Optional: vectors along the path can be reviewed here.

Activity 2: Set the motion detector aside, and fill the water bottle with water, placing your finger over the hole. Ask students to predict what happens if you move your finger? Will the fluid stay the bottle?

Activity 3: Standing on your front desk under the mounted motion detector on the ceiling, with a filled water bottle and your finger over the hole, ask students to predict whether the fluid will stay in the bottle as you drop the bottle. Have them explain their reasoning.

Activity 4: Standing on the ground under the mounted motion detector, with a filled water bottle and your finger over the hole, ask the students to predict whether the fluid will stay in the bottle or not as you toss the bottle straight up and catch it at the same height.

Activity 5: Standing on the ground, with a filled water bottle and your finger over the hole, ask students to predict what will happen if you toss the water bottle into a garbage can in the corner - will the liquid stay in the bottle?

For each of the demonstrations described above use Sokoloff/Duffy's 8 steps for an ILD (interactive lecture demonstration) given below:
1. Teacher describes the experiment, and carries it out without recording data.
2. Students record their predictions of the outcome on a Prediction Sheet.
3. Peer discussion follows, with the students discussing their predictions in small groups.
4. Teacher engages class, soliciting predictions and highlighting common predictions.
5. Students record their final prediction on the Prediction Sheet (this is collected).
6. The experiment is run. Real data is recorded and plotted by the computer, with the results displayed graphically for all to see.
7. Teacher engages class, discussing what students say about their predictions and focusing in particular on any common misconceptions. Students record the results on a Results Sheet, which they keep as notes.
8. Teacher discusses variations of the experiment and similar physical situations based on the same underlying concepts. Water bottle (Microsoft Word 163kB Aug25 09) Results sheet for water bottle (Microsoft Word 163kB Aug25 09)

Have a garbage can underneath the water bottle at all times.
Students typically think that the water should escape horizontally during vertical trips. This isolation of 'x' and 'y' axis is an excellent segue into projectile motion

Students will turn in their prediction sheets and keep their results sheets.
Move around the room during group discussion of starter sentences and ask clarifying questions.
Class discussion of the starter sentences as well as journal entries will be viewed.

9P.2.2.1.1 "Use vectors and free-body diagrams to describe force, position, velocity, and acceleration of objects in two-dimensional space."