A Monkey Climbing A Rope - What Happens?
A classic mechanics puzzle asks what happens when a monkey climbs a rope that passes over a pulley, with an equal counterweight on the other side.
Question
A monkey hangs on one end of a rope, and an equal weight hangs on the other end. When the monkey starts climbing, what happens to the weight?
Short Answer
The weight rises when the monkey pulls hard enough to accelerate upward. If the monkey climbs at constant speed, both sides can move together at that speed under nearly balanced forces.
Physics Explanation
1. Starting point: static balance
When everything is at rest and masses are equal, tension in the rope balances the weight on both sides.
2. Climbing changes rope tension
To accelerate upward, the monkey must pull down on the rope harder than in the resting case. That increases rope tension. The same tension acts on the counterweight side, so the counterweight gets a larger upward force than before.
3. What motion you see
- If monkey acceleration is upward, the counterweight accelerates upward.
- If monkey speed is constant, tension is close to weight and both sides can move steadily.
- If monkey slows or stops pulling, the system trends back toward balance.
The key point is that climbing is not just “moving up” visually. It is a force change in the rope, and that force is transmitted through the pulley.
Takeaway
This puzzle is a clean example of Newton’s laws: motion changes only when forces change. The monkey’s climb works by changing tension, and the counterweight responds immediately to that same tension.