• You may want to try the activity ahead of time. The experiment can take some fine-tuning. Make sure that the books are light enough to allow the side with 2 books to move in the latter part of the experiment. If the books are too heavy, the stack of two books will not move at all.
• Gather materials for student groups.

1. Using four paper clips and two 6" rubber bands (or a chain of smaller rubber bands 6-10" long), attach a rubber band/paper clip sling to each of two books (see Figure 1 for example of how to configure sling). Bend the free end of each paper slip to make a right-angled hook. Insert each hook into the book's spine, or using masking tape, tape the hooked paper clip to the outside bookend. The object is to have the rubber bands pull the books toward each other.

Figure 1. A textbook attached to rubber bands. click for copyright

2. Place the experimental setup on a smooth surface, such as a wood table or laminate (or other) countertop.
3. Lay down the yardstick in front of the books (see Figure 2) so you can see where the books are located in relation to one another; they should be 6-10" apart.

4. Pull the books apart a few inches (see Figure 3). Read the numbers on the yardstick that correspond to the inside edges of each book (the area spanned by the rubber band slings). Have the students record this number on their activity worksheet.
5. Now let the books go. They should move the same distance towards each other.
6. Record the position on the worksheet where the books collide or come to rest.
7. Repeat steps 4-6 three trials (i.e., three times).
8. Stack two books on top of one of the books being measured, and run the experiment three more times. If there are twice as many books on one side, the lighter side should move twice as far as the heavier side.
9. For each experiment, have the students subtract the initial position of each book relative to the final position of the books. The distances should be nearly proportional to the weight of each stack of books. A stack of two books will move only1/2 the distance of the single book.

Tell students that this activity demonstrates Newton's 2nd Law of Motion: the force required to move an object is proportional to the mass of the object. In the activity, the stretched rubber bands pull on both books equally, and both books move towards each other equally (assuming they are the same weight) when the rubber bands are released. When one book is stacked higher (with two books total), the pulling force only moves the stack half as far as the single book. Large airplanes weigh a lot and, therefore, require a large force to move them.

Voting: Ask a true/false question and have students vote by holding thumbs up for true and thumbs down for false. Count the number of true and false, and write the number on the board. Give the right answer.

• True or False: Most airplanes are made from steel. (False, aluminum)
• True or False: Newton's 2nd Law of Motion states that a heavy object requires a greater force to move than a lighter object. (True)

Worksheet: Have the students record measurements and answer questions on their Physics Tug of War worksheet. After students have finished their worksheet, have them compare answers with their peers. Discuss as a class.

Student-Generated Questions: Have each student come up with one question of their own to ask the rest of the class. Be prepared to help some students form a question. Have students take turns asking their questions to the class.

Roundtable: Have students form teams of 3-5. Ask the class a question with several possible answers. Students on a team make a list, each one writing an answer and passing the paper on to the next person. Have the teams share their responses with the class

• Have the students come up with a class list of situations that demonstrate Newton's 2nd Law. (The force needed to move an object is proportional to the object's mass.) Can they think of any examples that show this Law at home? In the playground? At School?