Torque-direction of rotation

1. Jul 3, 2015

goldfish9776

1. The problem statement, all variables and given/known data
Taking moment about A , the mass of 2.0g and 0.50g is rotated in the clockwise direction is because they are both directed in downwards position ? while R2 is acted in upward direction , so it is rotated in counter clockwise direction ?

Taking moment about G , why R1 is rotated in the clockwise direction ? while 2.0g and R2 is rotated in anticlockwise direction ?

How to determine the direction of rotation ?

2. Relevant equations

3. The attempt at a solution

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2. Jul 3, 2015

SteamKing

Staff Emeritus
It depends 1.) on the direction of the force and 2.) the point of application of the force relative to the reference point about which the moments are taken.

Compare the direction of the moment of a force taken about A to the direction of the moment for the same force taken about G.

If it helps, imagine a see-saw.

3. Jul 4, 2015

goldfish9776

For taking moment about G , I tried determine the direction of rotation using the thumb rule , I found the ans is correct . For the forces on the left G , I use LEFT HAND THUMB RULE , I found it should rotate in clockwise because of the R1 acted in upwards direction .

My question is can I determine the direction of rotation in this way ? is LEFT HAND THUMB RULE exist in physics?

4. Jul 4, 2015

haruspex

You can certainly use a thumb rule to figure it out, though I don't understand why you need it here. Isn't it obvious which way each force tends to turn about the chosen axis?
Whether it's LH or RH rule depends on how you assign your fingers to the different vectors.

5. Jul 4, 2015

goldfish9776

Why you said it is obvious to figure out the force turn about the chosen axis ? I have problem of finding out the direction of the force....Can you give me an example and explain on it ?

6. Jul 4, 2015

SteamKing

Staff Emeritus
The force points in the direction of the arrow head in the diagram. That's how vectors like forces are generally represented graphically.

7. Jul 4, 2015

haruspex

Take this example:
Consider G as a pivot. R1 is an upward force at A, to the left of the pivot. Ignore all other forces for the moment. If you were to push up on the rod at A, which way would you expect the rod to turn?

8. Jul 4, 2015

goldfish9776

clockwise . Lol , the principle is easy to understand , why didn't I think of this ? *facepalm*

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