Inertia and Inertial Frames
In an inertial frame, sum F_ext = 0 implies a = 0. Earth-fixed frames are approximate when rotational effects are negligible.
Why this shows up in the exam
Testing frames · Explaining inertia demonstrations · Recognizing force-free motion
Learn the idea
Zero net external force means constant velocity, with rest as its zero-velocity case. Objects resist velocity changes; they do not need a continuing force merely to keep moving.
🧠 Memory hook: No net force means no velocity change.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- sum F_ext = 0 => a = 0 — in an inertial frame
- v = constant — consequence of zero acceleration
How to approach it
- 1Choose the frame
- 2Sum external forces
- 3Keep the vector velocity constant if the sum is zero
Common slip-ups that cost marks
- •Motion needs force
- •Zero net force means no forces
- •Every Earth frame is exact
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
A 2 kg block moves on a horizontal rough surface with coefficient of kinetic friction 0.2. A horizontal force of 10 N acts on it. Take g = 10 m/s^2. What is its acceleration?
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Newton’s Second Law in Vector Form
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