Normal Reaction and Apparent Weight
With upward positive, N-mg=ma; contact requires N >= 0.
Why this shows up in the exam
Elevators · Floor reactions · Contact-loss checks
Learn the idea
A scale reads normal force, which equals mg only at zero vertical acceleration. Supports push harder for upward acceleration and less for downward acceleration.
🧠 Memory hook: Scale reading is N, not automatically mg.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- N = m(g+a) — upward acceleration
- N = m(g-a_d) — downward acceleration
- N = 0 — free fall
How to approach it
- 1Choose upward positive
- 2Write N-mg=ma
- 3Check N is nonnegative
Common slip-ups that cost marks
- •Changing g with travel direction
- •Calling weightlessness no gravity
- •Omitting supported loads
🌟 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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