- Textbook or Notebook (one per student)
- Piece of paper cut to fit a size smaller than the surface area of the book or notebook
- Lead students through a series of four mini-experiments. Have students make a hypothesis as to what will happen before each experiment and have the students explain to each other why they think the way they do. It is very important for students to explain their thinking and debate about it in order to correct it.
- Experiment 1: Have the students hold the book and the piece of paper side by side and simultaneously drop the book and the piece of paper as shown in the diagram below.
- Ask the students, which will fall at a faster rate, the book or the paper? Have them write a hypothesis, share their ideas, and then try the experiment. Ask them why they think the book fell faster. Most of them will say that the book is heavier and heavier things fall faster, which is incorrect, but do not correct their thinking just yet.
- Experiment 2: Now tell the student they are going to put the piece of paper underneath the book and have the students answer the question which will fall at a faster rate, the book or the paper? Again, have them write down, explain, and share their ideas before they try the experiment. Many will think that the paper, being lighter, will fly up from underneath the book fall at a slower rate. This is incorrect, but do not correct them.
- Have the students try their experiment and then explain their results. The paper will stay under the book. Many students will still hold on to the miss conception that the paper must fall slower because it has less mass and will explain the phenomenon by saying that the book pushed the piece of paper down. This is incorrect. At this point you will have some students that are starting to get the idea and will argue that mass does not affect the rate at which something falls to the ground. Let the students intellectually debate this.
- Experiment 3: Have the students place the piece of paper on top of the book and again have the students predict which will fall at a faster rate, the book or the paper. Set up experiment as shown in the diagram below.
- The paper will fall at the same rate as the book and appear to stick to the top of the book. Some students will explain this as some sort of suction, in fact students might believe there is something sticky on the top of the book. Have them check to make sure there isn’t anything sticky.
- Experiment 4: Have the students drop the book and the piece of paper again side by side, this time crumple up the paper into a very tight ball. Be sure to discuss that the mass of the paper does not change by crumpling it up, it still has the same “heaviness”. Again let the student predict what will happen by answering the question which will fall at a faster rate, the book or the paper. Have the students explain their reasoning.
- The book and the piece of paper will both hit the ground at the same time this time. Have the students debate why the book and paper fall at the same time.
- What has more mass, the book or the paper?
- Does the mass effect the rate at which something falls? Use the information you collected from the demonstrations to back up your answer. Be sure to write in complete sentences.
- What is air resistance?
- Does air resistance affect the rate at which things fall? Us the information you collected from the demonstrations to back up your answer. Be sure to write in complete sentences.
Mass does not effect the rate at which something falls to the ground. Assuming no air resistance, all objects accelerate towards the ground at a rate of 9.8 m/s2. The reason the paper fell slower the first time is because air resistance was pushing up on the paper resisting its motion. Since the paper has less gravitational force acting on it than the book, the upward air friction force effects the acceleration due to gravity the greatest, causing the paper to accelerate slower than the book.
In the second and third experiments, the air resistance of the paper is blocked by the book. The piece of paper on or under the book acts the same as all the pieces of paper inside the book. They fall at the same rate and act as one object.
In the last experiment, the paper is crumpled into a tight ball. Decreasing the surface area of the paper will decrease the air resistance and the paper is now allowed to fall at nearly the same rate as the book.
Have students accurately measure the acceleration due to gravity using photogates, spark timers, or electric timers for greater accuracy. A list of products available to do just this is provided below.
For students who need more reinforcement of the above activity have the students play with the Galileo’s Experiment Apparatus. This apparatus simultaneously releases two different massed balls and show that they both hit the ground at the same time. Quick and easy to use, this device also allows students to explore size and density variations in fall objects as well.
Eliminate the effect of air resistance all together by dropping a penny and a feather in a vacuum tube. Also an excellent conceptual knowledge check to the above activity, have the students predict and explain what happens when two objects fall on earth without air resistance acting on them.