In this activity, students will investigate two very important forces. The first is the force of gravity. This force acts at a distance and it always pulls objects towards the center of the Earth. The second type is an impact force. In this activity, the impact force will be the impact of fast moving air pushing against the balloon.
Each group needs:
- 2 balloons
- String (not too weak, so that it can be pulled tight without breaking – fishing line works well)
- 2 plastic drinking straws
- 2 elastic bands (small) or twist ties
- Tape of any sort.
Gravity is one of the forces at a distance which we are most familiar with. It is responsible for keeping our feet on the ground, and for keeping the Earth in orbit around the sun, among numerous other phenomena in our world. Gravity, however, is different from other forces at a distance, because it only attracts, and it acts on any object that has mass. The other force which we are very familiar with is the impact force of matter pressing on matter. We are most familiar with solids pushing on solids, like pushing a car to make it roll, or solids pushing liquids, like paddling a canoe, but in this activity the impact force that will be causing the motion of the balloon rockets is the impact of fast moving air.
Part 1 of this activity is just the set up. You want to make sure that one of the strings is almost horizontal, and that one has an angle upwards. This is because the balloon that is travelling upwards is trying to overcome the force of gravity pulling it downwards. In Part 2, when you actually start letting the rockets fly, you will notice that since the rockets on the angled string are trying to overcome gravity, they can not travel as far as the rockets on the horizontal string. This will be true as long as you put the same amount of air in both balloons. An idea you can play with in this activity is seeing how much more air you need to make the rocket on an angle go the same distance as the horizontal rocket (you will need more, possibly much more depending on how steep the angle is).
Build the balloon rocket. First attach one piece of fishing wire, so that it runs from high to low, on some kind of angle (the exact angle does not matter). Then attach another piece of string so that it runs parallel to the floor. Before tying off the ends of these string segments, make sure to run both of them through the straw. Chairs, for example, will make for sturdy supports. Now, blow up two balloons, and hold the air inside using elastic bands or twist ties. Make sure that the balloons have approximately the same amount of air. Now tape the balloons to the straws.
Release the tie, or elastic band, on the horizontal string. Make a note of how far it goes. Then release the balloon on the slanted string. Make a note of how far this balloon runs, compared with the horizontal rocket. After an initial run, try filling the balloons with different amounts of air and seeing how far they go.
- What is gravity?
- Which balloon is it acting on? (Trick question: It’s actually acting on both, but since the horizontal rocket is not trying to move upwards, it’s motion is not affected)
- What is causing the motion of the balloon rockets?
- Is this an impact force or a force at a distance?
- What kinds of things need to worry about overcoming gravity? (planes, space shuttles, high rise buildings)
Adapted from Teach Engineering