Intro
This week builds circular-motion intuition before the full dynamics machinery arrives. The key idea is that velocity can change even when speed stays constant, because direction is part of velocity.
Core Lesson
In circular motion, the velocity vector is always tangent to the path. If the object keeps turning, the velocity direction keeps changing. A changing velocity means there must be acceleration, even when the speed is steady.
That acceleration points inward toward the center of the circle. It is not a “forward” acceleration. The inward acceleration changes direction of motion, not necessarily speed.
Students often confuse cause with path. The object is not pushed outward by a real force simply because it wants to keep moving straight. If the path curves inward, some interaction must produce an inward net force.
AP Lift
The AP exam repeatedly pressures students on the difference between tangential velocity and inward acceleration. Strong responses explain what changes physically and which force direction causes the curved path.
Must-Master Objectives
- Explain why circular motion can involve acceleration at constant speed.
- Distinguish inward acceleration from tangential velocity.
- Describe the force logic required for circular motion.
- Reject fake outward-force explanations in inertial-frame reasoning.
Problem Set Prompts
- A car travels around a circular track at constant speed. Is it accelerating? Why?
- In uniform circular motion, what direction is the velocity vector pointing?
- In uniform circular motion, what direction is the acceleration vector pointing?
- Why is an outward “centrifugal force” not part of a standard free-body diagram in an inertial frame?
- If the speed of an object in circular motion doubles, what happens qualitatively to the inward acceleration?
- A ball on a string moves in a horizontal circle. Which force or forces point inward?
- Why can a car moving at constant speed still require a nonzero net force?
- Stretch: Describe what happens to circular-motion acceleration if the radius gets larger while speed stays fixed.
- Stretch: Compare circular motion at constant speed with straight-line motion at constant speed.
Reflection Prompt
- Which feels more natural right now: thinking about what an object is doing or why the path is changing?
- Do you still instinctively imagine a force in the direction of motion?