B What is the Effort in a Hand Holding a Tray?

  • B
  • Thread starter Thread starter Richie Smash
  • Start date Start date
  • Tags Tags
    Forces hand
AI Thread Summary
In the discussion about a waiter holding a tray, a rough diagram illustrates the forces acting on the tray, with a total downward force of 30N and an upward force of 30N from the middle finger pivot. The key question revolves around identifying which part, the middle finger or the thumb, constitutes the effort when lifting the tray. It is clarified that when the tray moves upward, the upward forces perform positive work, while the downward forces do negative work. The conversation emphasizes the importance of choosing an appropriate pivot point for calculating moments, suggesting that using the thumb or middle finger simplifies the analysis. Understanding these concepts is essential for accurately calculating work in similar scenarios.
Richie Smash
Messages
293
Reaction score
15
HI, I've attached a rough diagram(emphasis on rough) of a waiter holding a tray, there is a downward force of 5N acting on the clockwise side, and a downwards force of 25N on the anticlockwise side of the pivot, which is his middle finger.

The total force acting down is 30N, so therefore the middle finger or pivot is giving an upwards force of 30 N.

I just want to know, if the waiter moves the tray up, is the middle finger the effort or is the thumb the effort?
 

Attachments

  • Untitled.png
    Untitled.png
    11.8 KB · Views: 875
Physics news on Phys.org
Richie Smash said:
... is the middle finger the effort or is the thumb the effort?
Why does it matter?
 
So I can know in situations like this for example a question I'm doing, how to calculate the work if something is being lifted.
A.T. said:
Why does it matter?
 
True, but i think I better way to rephrase my question would be, If the waiter moves the tray up, would it be the downward forces doing the work, or the upward force of the pivot finger?
 
Richie Smash said:
True, but i think I better way to rephrase my question would be, If the waiter moves the tray up, would it be the downward forces doing the work, or the upward force of the pivot finger?
Just apply the definition. If the tray moves upward, the upward forces do positive work while the downward forces do negative work.
 
  • Like
Likes Richie Smash
Richie Smash said:
is the middle finger the effort or is the thumb the effort?
In problems like this one, it is possible to 'take moments' about any point but it is convenient to choose either the thumb or finger. If you choose an arbitrary point along the tray, the equation you come up with has more parts to it so it's a waste of time and, once you get used to this sort of thing, you can often spot a quick way of approaching the problem.
Draw the forces on the diagram and equate the forces and the moments. The answer 'falls out'.
 
Back
Top