---
title: "Elevator Scale & Apparent Weight Interactive Lab | Sandbox Physics"
description: "When does a scale stop measuring your weight? Whenever the lift accelerates."
canonical_url: "https://sandboxphysics.com/labs/elevator-scale/"
language: "en"
page_type: "lab"
alternate_languages:
  zh-CN: "https://sandboxphysics.com/zh/labs/elevator-scale/"
last_modified: "2026-10-27"
---

M013 · Forces & contact

# Elevator Scale & Apparent Weight

**When does a scale stop measuring your weight?** Whenever the lift accelerates. A scale measures how hard it pushes up on you, not gravity itself. It reads more when the lift accelerates upward, less when it accelerates downward, and zero in free fall, when gravity still acts but nothing pushes back.

Program a lift and compare riding at constant speed with accelerating. Then release the drive and follow the passenger leaving the scale, the impacts on landing and the work done by the moving floor.

> Interactive simulation: open this page in a web browser to change the parameters and run the model.

The mechanics are ideal. The page states exactly how contact is handled and what the scale measures.

Physics tutorial

## Elevator Scale & Apparent Weight

**Background** While the passenger stays on the floor, the support force is whatever it takes to make them move with the floor. That holds only as long as contact is possible.

**Why it matters** Program the lift, compare constant speed with acceleration and then release the drive. You can follow the passenger leaving the scale, the impacts when they land again and the work done by the moving floor.

Start with the essentials

- **Focus question**: When does a scale stop measuring your weight?
- **One-sentence intuition**: A floor can push a passenger up, but it cannot pull a passenger down.

### Core mathematical model

#### Support force while in contact

$$
N-mg=ma_p,\qquad N\geq0
$$

The support takes whatever value the motion requires, but only while the passenger touches the floor.

#### Flight relative to the floor

$$
\begin{aligned}r&=y_p-y_f\geq0\\\ddot r&=-g-a_f\quad(N=0)\end{aligned}
$$

The accelerations are constant, so exact arcs give the moment the gap closes again.

#### Perfectly inelastic landing

$$
\begin{aligned}J&=m(v_f-v_p^-)\\Q&=\tfrac12m(v_p^--v_f)^2\end{aligned}
$$

The floor's velocity is prescribed from outside, and the impact takes zero time.

#### Energy balance with a moving floor

$$
\begin{aligned}\Delta(K+U)&=\int N\,\mathrm d y_f\\&\quad+\sum_i J_i v_{f,i}-\sum_i Q_i\end{aligned}
$$

When the floor moves, both the work done during impacts and the energy dissipated in them count.

### Common difficulties

#### Moving down is not the same as accelerating

**Typical misconception** Whenever the elevator goes down, I get lighter.

**Better mental model** At constant speed, up or down, the scale reads your ordinary weight. Only acceleration changes the reading. If the drive pulls the floor down faster than free fall, a gap opens. When you land again, the contact delivers an impulse, not a finite force peak.

### Run the experiment

1. 01

   #### Speed or acceleration?

   Compare Constant upward speed with Start / cruise / brake.

   **What to observe:** A scale that is moving can still read your ordinary weight.

2. 02

   #### Program the lift

   In Leave and return, drag one command below free-fall acceleration.

   **What to observe:** The passenger lifts off the scale instead of feeling a negative support force.

3. 03

   #### Look closely at the landing

   Drag the passenger around the return time listed under Acquisition and diagnostics.

   **What to observe:** The velocity jumps because of an impulse, while the positions stay continuous.

4. 04

   #### Record the scale data

   Open Acquisition and diagnostics to acquire or export the scale record.

   **What to observe:** The regular force samples and the instantaneous impact impulses are kept separate.

Physical conclusion

A scale measures the support force. In free fall the reading can drop to zero, but gravity still acts.

Motion & dynamics · Starter path

## From a falling ball to an orbit

Start by measuring gravity and splitting motion into components. Then add contact forces and collisions, and follow the same laws all the way to an orbit.

[Start here → **Free Fall & Measuring Gravity** Capture a few drops and fit the camera positions to estimate gravitational acceleration. Then add air damping and watch the fit drift from the truth. Observe · 14 min](https://sandboxphysics.com/labs/free-fall/)

1. [1 Free Fall & Measuring Gravity](https://sandboxphysics.com/labs/free-fall/)
2. [2 Projectile Flight Designer](https://sandboxphysics.com/labs/projectile-flight/)
3. [3 Inclined Plane & Friction](https://sandboxphysics.com/labs/inclined-plane/)
4. [4 One-Dimensional Collision Bench](https://sandboxphysics.com/labs/collision-bench/)
5. [5 Mass–Spring Oscillator](https://sandboxphysics.com/labs/mass-spring/)
6. [6 How Orbits Take Shape](https://sandboxphysics.com/labs/orbit/)

[View the full path →](https://sandboxphysics.com/topics/mechanics/#starter-path)