# Help to understand Newtons Law

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1. Jan 1, 2015

### urbano

1. The problem statement, all variables and given/known data
I am trying to understand a diagram I have of a gymnast hanging from a bar. She weighs 50kg and is not swinging, just hanging. In the diagram it has her weight as 500N (using 10/m/s/s for gravity * 50kg) but the bar she is hanging from has an upward arrow with 550 N next to it. I interpret this to mean that the bar is exerting 550N of upward force against the gymnast. However I thought for every action there is an opposite reaction. Wouldn't this mean the bar is only exerting 500N up from the gymnast given here downward force is 500N ?

The object of the diagram is for people to interpret the net force on the gymnast.

2. Relevant equations
550N - 500N = 50N

or 500N - 500N = 0N

3. The attempt at a solution

2. Jan 1, 2015

### haruspex

Two possibilities:

The arrow is indicating an upward force on the bar from its supports. That would imply the bar itself weighs 50N.

The gymnast, though not swinging, is bouncing vertically. At the instant shown, the bar is deflected downwards and the gymnast is accelerating upwards.