05 5 Bodies connected by a string over a smooth pulley
Two vertical masses
- When the pulley is small and smooth and the string is light and inextensible, the tension is the same on both sides.
- Let . Equations of motion: and .
- The pressure on the pulley axis is .
- If the masses start level, after time each has moved and the vertical distance between them is .
Special cases
- Equal masses: and the system stays at rest or moves with uniform velocity.
- Extra mass added to one side: and .
- Extra mass removed after seconds: the system keeps moving in the same direction with the velocity gained, but with the new (smaller) acceleration, which may be a deceleration.
- String cut after seconds: each body continues in its direction with the velocity gained and then moves under gravity alone ().
- If the heavier side is unknown and the system returns to its start, ; if it moves uniformly, .
✍️ Worked examples
Example 14: Two masses start level and after s the vertical distance between them is cm. Find ().
Solution:
- Each moved cm: , so cm/s.
- , so .
- , so .
- .
Example 15:
- Masses kg and kg hang level at cm above the ground.
- Find , and the pressure on the pulley.
- The string is cut after s; find the speed of each mass on reaching the ground.
Solution:
- m/s.
- N, so the pressure on the pulley is N.
- At the cut: m/s and each has moved m.
- The kg mass is m above the ground moving down: , so m/s.
- The kg mass is m above the ground moving up: , so m/s on reaching the ground.