Physics / Lesson 02
Motion and interaction
Describe motion first, then explain changes in motion through interactions.
orient
Why this idea had to exist
Kinematics describes how motion changes; dynamics asks why. Keeping those questions separate prevents a common confusion between velocity, acceleration, and force.
intuition
Build a picture you can reason with
Position is the frame on a film strip, velocity is how the frame-to-frame displacement changes, and acceleration is how that velocity itself changes. Net force points with acceleration, not necessarily with motion.
formalize
Give the intuition a precise edge
Velocity is the time derivative of position and acceleration the derivative of velocity. Newton's second law states that net external force equals the time rate of change of momentum; for constant mass, F_net=ma.
work through
Follow the decisions, not just the symbols
A puck sliding right at constant speed has rightward velocity but zero acceleration, so the net force is zero. If friction slows it, acceleration and net force point left even while the puck still moves right.
experiment
Change one thing and watch the model answer
Adjust force and mass for a cart. Predict the acceleration vector before running each case, then compare position, velocity, and acceleration graphs over time.
retrieve
Close the page and reconstruct it
Answer before opening the explanation. Retrieval is evidence only when the answer is produced without a hint.
Can velocity point right while acceleration points left?
Yes. The object then moves right while its rightward velocity decreases.
transfer
Move the idea into a new setting
Analyze the first second after a ball is thrown upward: state the directions of velocity, acceleration, and net force before and after the highest point.
reflect
Leave with a diagnostic habit
Draw separate arrows for velocity, acceleration, and each interaction. Do not let one arrow silently stand for all three.
Source record
Follow the idea back.
- Reference
- University Physics
- Publisher
- OpenStax
- License
- CC BY 4.0
- Accessed
- 2026-08-12
- URL
- https://openstax.org/details/books/university-physics-volume-1