Science · Physics
Chapter 1: Measurement and Motion in One Dimension
Some quantities need a direction to mean anything.
Ana's watch logged 7.0 km on a hike: 3.0 km north to a lookout, then 4.0 km south to a lake. Her map app said she finished 1.0 km from where she started.
Which of those two numbers is the answer, and to what question?
Make your guess to turn the page.
Still to come: 10 pages · Lab · Words to Know · What We Learned · 4 questions · Explain It in Writing · Open Questions
The idea this lesson is working on — Students read a minus sign as "less", so a negative velocity sounds like slowing down. The book sets that against the lab, where a braking car slows with a positive velocity throughout, and a reversing car holds a steady −12 m/s. Ask: which way is positive here, and is the size of the velocity changing? The idea to reach is that the sign is a direction and slowing is about magnitude.
Units check the physics — An equation whose two sides have different units is wrong, whatever the algebra, and dimensional checking catches it before any number is substituted. It cannot catch a wrong dimensionless factor. The Mars Climate Orbiter loss (1999) came from impulse data in pound-force seconds read as newton-seconds; the mishap board also blamed the checks that should have caught it.