Models and Where They Stop: teacher key

Pushes and Pulls, grade 10 edition, written for 10th grade. Newton's laws describe motion well only for objects much slower than light and larger than most molecules. The kilogram is the unit of mass, and the newton is the unit of force.

Read the book. Print this page for the key.

Questions and answers

1. What does the usual model of friction say friction is proportional to?
   • The speed of the object
   • The area of the surfaces
   • The normal force pressing the surfaces together (answer)
2. What can happen to two flat metal pieces made smoother and smoother in a vacuum with no contaminants?
   • They can stick together in a cold weld (answer)
   • They slide with no friction
   • They push each other apart
3. When a car turns a corner, what makes it turn?
   • Your body pushing outward
   • A centripetal force, such as friction on the tires (answer)
   • The speed of the car alone
4. NASA's goal was to change Dimorphos' orbit by at least 73 seconds. How did the measured change compare?
   • It was much larger than the goal (answer)
   • It fell short of the goal
   • It matched the goal exactly
5. In the passage on DART, what does recoiled mean for debris blasted off the asteroid?
   • Burned up
   • Stuck to the surface
   • Moved back and away (answer)
6. For which objects do Newton's laws describe motion well?
   • Objects near the speed of light
   • Objects much slower than light and larger than most molecules (answer)
   • Only objects at rest

Words

• kilogram: unit of mass
• coefficient: ratio number
• centripetal: center-seeking
• recoiled: sprang back
• relativity: Einstein's physics