Physics Puzzle: What Will the Scale Read?
June 19th, 2009
| Categories: Mechanics, Physics Puzzles
Level of Difficulty: Highschool
A 10-kg block is hung from a rope. The rope is connected to a (massless) digital scale which displays the attached weight, as shown in (A).
Next, a pulley is attached to the block via a piece of rope, and a second 10-kg block is hung from the pulley, as shown in (B). The rope’s other end is attached to the (immobile) floor. What will the scale read now?
All ropes are massless, as is the pulley. Neglect friction.
EDITED, 19/June/2009: a solution has been posted in the comments section by our reader DU; I’ll be posting a more detailed solution later tomorrow.
EDITED, 20/June/2009: click here for a detailed solution.
The scale says 30. Ignore the top weight for a minute and draw a force diagram for the pulley. There’s a weight on the left of 10. The tension in the rope on the other side is also 10 (because the pulley isn’t turning) and points in the same direction. Therefore there’s a force of 20 (well, 20*g) pulling down on the pulley. There has to be a 20 pulling upwards also.
Add that 20 to the 10 that’s already up there and we get 30.
D’oh, that’s my answer too. Too slow!