# Forces: hand holding a tray

• B
• Richie Smash

#### Richie Smash

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?

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... 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.
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?

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.

Richie Smash
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'.