Motion Language

Intro

Before graphs and equations become useful, motion has to make sense in language. This week is about turning a motion story into position, displacement, velocity, and acceleration without immediately collapsing into algebra.

Core Lesson

Position tells you where an object is relative to a chosen origin. Displacement tells you how position changed. Velocity tells you how position changes with time and includes direction. Acceleration tells you how velocity changes with time.

Those distinctions are the difference between understanding a problem and guessing at it. A runner can finish exactly where they started and still travel a large distance. A car can move in the positive direction while slowing down, which means velocity and acceleration can have opposite signs.

Always attach a coordinate choice to your words. “Left,” “right,” “up,” and “down” only become physics once you define what counts as positive.

AP Lift

Modern AP Physics 1 constantly asks students to translate between representations. A short paragraph about motion should be enough to tell you the sign of velocity, the sign of acceleration, and whether the object is speeding up, slowing down, or changing direction.

Must-Master Objectives

• Define position, displacement, velocity, and acceleration in plain language.
• Distinguish distance traveled from displacement.
• Infer the sign of velocity and acceleration from a motion story.
• Explain how coordinate choices affect the meaning of signs.

Problem Set Prompts

1. A student walks 4 m east and then 4 m west. What are the distance traveled and displacement?
2. A train moves north while slowing down. If north is positive, what are the signs of velocity and acceleration?
3. An elevator speeds up downward. If upward is positive, what are the signs of velocity and acceleration?
4. A bike moves from x = -3 m to x = 5 m. What is the displacement?
5. At the top of a tossed ball’s path, what are the velocity and acceleration?
6. Write a short motion story for an object with negative velocity and positive acceleration.
7. Describe a trip with zero final displacement but nonzero distance.
8. Stretch: Give a real example of zero velocity with nonzero acceleration.
9. Stretch: Give a real example of nonzero velocity with zero acceleration.

Reflection Prompt

• Which pair is still easiest to mix up: distance vs displacement or velocity vs acceleration?
• What helps you notice direction before you start calculating?