Key relationship
Ground frame: N − mg = ma. Lift frame: N − mg − ma = 0. Thus N = m(g + a).
Physics · Class 11
Apparent weight in a lift
A scale measures the normal support force. Upward acceleration requires N greater than mg; downward acceleration requires less. During free fall both lift and person accelerate at −g, so the support force vanishes. In the accelerating lift frame a pseudo-force −ma restores the equilibrium-form equation.
Open the interactive experiment →Acceleration changes the scale reading.
எடை அளவுகோல் தாங்கும் விசையை அளக்கும். மேல்நோக்கிய முடுக்கத்தில் வாசிப்பு அதிகரிக்கும்; கீழ்நோக்கிய முடுக்கத்தில் குறையும். தடையற்ற வீழ்ச்சியில் வாசிப்பு பூஜ்யம்.
Key notes, relationships and model limits
Remember
A scale measures the normal support force. Upward acceleration requires N greater than mg; downward acceleration requires less. During free fall both lift and person accelerate at −g, so the support force vanishes. In the accelerating lift frame a pseudo-force −ma restores the equilibrium-form equation.
Common trap
A downward-moving lift can have a higher reading if its acceleration is upward.
Model limits
Person remains in contact with a rigid scale; uniform lift acceleration starts from rest. Input restricted to a ≥ −g; at free fall N = 0. Pseudo-force is a frame correction, not an extra interaction.
What you will learn
Calculate apparent weight and compare inertial and accelerated frame equations.
Try the experiment
Compare upward, downward and free-fall presets. Set acceleration to zero to recover mg.
Think of it this way
The heavy feeling at a lift’s start and light feeling at its slowdown come from acceleration.
Worked example
For m = 60 kg and a = 2 m/s² upward, N = 60(9.81+2) = 708.6 N, while mg = 588.6 N.
Why this matters
Separating velocity direction from acceleration direction makes lift problems much clearer.
Before you start
Vector components · velocity · SI units
Check your understanding
A lift moves downward while slowing down. Its apparent weight is…
- Always below mg
- Above mg because acceleration is upward
- Zero because motion is downward
Read the explanation
Slowing downward motion means upward acceleration, giving N = m(g+a) greater than mg.
Course connection: CBSE XI Chapter 4; NEET/Main Unit 3; JEE Advanced Mechanics. Apply each exam’s stated assumptions.