# Force on a hinge when a rod is being released

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1. Dec 25, 2016

### randomgamernerd

1. The problem statement, all variables and given/known data: A uniform rod of weight W is free to rotate about a hinge at one end. The hinge is fixed to a wall. If the rod is released from a horizontal position, what force will it exert on the hinge just after being released?
a)W
b)W/2
c)W/3
d)W/4
choose the correct option

2.The attempt at a solution: Since the rod is just released, the only force that it will exert at the hinge is its own weight which is W. So I think the answer should be W. But I'm unable to use the fact that the hinge is fixed to a wall and I think thats the reason I'm getting my answer wrong.

Thanks for helping.

2. Dec 25, 2016

### TSny

Hello.
You're jumping to a conclusion here that is not justified.
Draw a free body diagram and apply the appropriate dynamical laws.

3. Dec 26, 2016

### randomgamernerd

The only forces I could identify was the weight of the rod, the reaction force from the hinge on the rod and the reaction force from the wall on the hinge...and then?I cant figure out how to proceed...I would be grateful if you could give me a bit more hint...and then I could proceed well...I'm not requesting for a solution but just a bit more hint.

4. Dec 26, 2016

### TSny

Can you either post a figure showing your free body diagram for the rod or describe the free body diagram in words?

What equations can you set up using your free body diagram?

5. Dec 26, 2016

### haruspex

That is not useful. In the free body diagram for a rigid body, only include the forces which act directly on it. How the wall and hinge interact is their own affair.

6. Dec 26, 2016

### Cutter Ketch

Ok, one hint. The rod isn't static.

7. Dec 27, 2016

### randomgamernerd

okay...you mean a centrpetal force is acting...right?

8. Dec 27, 2016

### randomgamernerd

This means N= W-F

where
N=reaction from hinge
W= weight of rod
F= Centripetal force

9. Dec 27, 2016

### haruspex

No. Centripetal force requires some speed. At the moment of release, there is acceleration but no speed yet.
Just write out the usual equations relating force to acceleration and/or torque to rotational acceleration. (Which looks to be the more useful here?)

10. Dec 27, 2016

### Cutter Ketch

No, I mean the forces and torques don't add up to zero. What do they add up to?

11. Dec 27, 2016

### randomgamernerd

okay....I'm so sorry everyone....I missed out the word "horizontal"....From the very beginning I thought that the rod was released from a vertical position.....Thats the reason I was so hell confused....Anyways...thanks for the hints...and making me reread the question(this time carefully)....I would be able to work it out now...
Sorry for the inconvenience and THANKS for the help...

12. Dec 27, 2016

### lychette

Welcome to physic forums....did you get an answer