The Tug of War
Two teams pull a rope as hard as they can. The flag in the middle does not move at all.
Both teams are pulling hard. Why does nothing move?
Page 1
Science · Grade 3 Science
Chapter 1: Forces and Motion
It is the difference that moves things.
Two teams pull a rope as hard as they can. The flag in the middle does not move at all.
Both teams are pulling hard. Why does nothing move?
Make your guess to turn the page.
Still to come: 7 pages · Words to Know · What We Learned · 4 questions · Explain It in Writing · Ask and Wonder
The idea this lesson is working on — Children expect the bigger force to win, so a big pull should always move something. The story puts two big pulls of 9 against each other and the flag moves 0 cm; then one side drops, and the rope slides. Ask: both teams were pulling hard the whole time, so what changed? The answer to reach is the difference between the pulls, not their size.
Still does not mean no forces — A book on a table has gravity pulling it down and the table pushing it up. Both forces are real and they balance, so the book stays put. Being motionless is not the same as being force-free — take the table away and you find out at once.
About the numbers — The meters read in plain numbers rather than newtons, so the child can do the subtraction that matters. The flag distances grow with the leftover force (0, 1 and 4 give 0, 15 and 60 cm), which is what a real rope does when the leftover force is what speeds it up.