Pairs, Squares and Trade-offs: teacher key

Pushes and Pulls, grade 9 edition, written for 9th grade. Forces in a third-law pair are equal and opposite but act on different objects, so they do not cancel. Drag grows with the square of speed, and a lever trades force for distance.

Read the book. Print this page for the key.

Questions and answers

1. Why do the two forces in a third-law pair not cancel?
   • They are unequal in size
   • They act on different objects (answer)
   • They happen at different times
2. How does a rocket speed up in space?
   • It pushes off the air
   • It pushes off distant planets
   • It expels gas backward, and the gas pushes it forward (answer)
3. A cyclist doubles her speed in air. Roughly what happens to the air pushing back on her?
   • It roughly quadruples (answer)
   • It roughly doubles
   • It stays the same
4. A car of 1,400 kilograms moves 15 meters per second. What is its momentum?
   • 93 kg·m/s
   • 1,415 kg·m/s
   • 21,000 kg·m/s (answer)
5. In the passage on levers, what is a fulcrum?
   • The load being raised
   • The fixed point where the bar pivots (answer)
   • The force you put in
6. Why does a cannonball dropped from a tower land at the tower's foot, according to Galileo?
   • Earth is standing still
   • Air pushes the ball back
   • The ball keeps moving along with the tower (answer)

Words

• vacuum: empty space
• relative: compared with
• molecules: tiny particles
• fulcrum: pivot point
• momentum: mass times velocity