Are you doing work when you hold a heavy bag still?
Short answer: W = F·d. No displacement, no work on the bag (your muscles burn energy internally).
The trap
The wrong idea
“Holding a heavy bag still means you are doing work on it.”
It sounds reasonable, and a lot of JEE/NEET aspirants carry it into the exam. Here's a one-question check, then the explanation that makes it click.
Quick check
Try it before you peek
You hold a 10 kg bag still for 5 minutes. Work done on the bag by you is:
- Positive
- Zero
- Negative
Show the answer
B. Zero
W = F·d. No displacement, no work on the bag (your muscles burn energy internally).
The click
In physics, the work done by a force is W = Fd cos θ, where d is the displacement of the point where the force acts. If the bag doesn't move, d = 0, so the work you do on the bag is zero, however heavy the bag is and however long you hold it.
Check with energy: work is energy transferred to an object by a force. The bag's kinetic energy doesn't change (it stays at rest) and its gravitational potential energy doesn't change (same height). No energy went into the bag, so no work was done on it.
So why do you get tired? To hold the load, your muscle fibres keep contracting and relaxing, a process that uses chemical energy from ATP and releases it as heat inside your body. That is physiological energy use, not mechanical work on the bag. A table holding the same bag still can do it forever without tiring, and it does zero work too.
Also note: if you carry the bag horizontally at constant velocity, your upward force is perpendicular to the displacement, so θ = 90° and the work you do on the bag is still zero (ignoring the brief moments of speeding up and slowing down). Work needs a component of force along the displacement.
Got this one? The Knot Scan finds the misconception you don't know you have, in 8 questions.