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03_faq

Page history last edited by johnlittlephysics 10 years, 9 months ago Saved with comment

03 Moments FAQ


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Comments (8)

johnlittlephysics said

at 10:55 pm on Jun 13, 2013

302 Li June: How do we determine if a moment is clockwise or anti clockwise? Is there like fixed rules?

johnlittlephysics said

at 10:56 pm on Jun 13, 2013

Refer example of a diagram of a black box that is tilted at https://www.dropbox.com/s/orus1onp0r5wst0/Photo%2010-6-13%205%2053%2044%20PM.jpg
What is direction of moment caused by a force?:
1. Locate the pivot P (turning point about which a body is going to rotate)
2. Locate and draw the force producing the moment (e.g. weight which acts vertically at the CG of body)
3. Determine the direction of the turning effect of this force (how it is going to rotate the body)
E.g. The weight would cause the black box to rotate in an anti-clockwise direction,
so the moment (due to this force) is anti-clockwise.

johnlittlephysics said

at 10:57 pm on Jun 13, 2013

302 Ying Ting: I don't really understand the topic for principle of moments whereby an object is in equilibrium when upward forces=downward forces. When two equal forces act upwards and downwards respectively, but are at different distances from the pivot, wouldn't a turning moment be induced, yet upward forces=downward forces? Then how do we determine when upward forces=downward forces or when do we look at resultant moment or resultant force.

johnlittlephysics said

at 11:10 pm on Jun 13, 2013

If it is given an object is in equilibrium, then both conditions need to be satisfied.
You may use either the 1st condition or the 2nd condition or both conditions to form the necessary number of equations to solve the same number of unknown quantities that are required by the question.

E.g. if there are 2 unknown forces to be determined, you need to form 2 equations.
Condition 1 can provide 1 equation, and condition 2 can provide another equation; OR both equations can be formed using condition 2 (net moment = 0) using two different pivots.
Note that where possible, it is usually good to form equation with condition 1 (net force = 0) as the equation formed is simpler to manipulate and solve.

johnlittlephysics said

at 11:11 pm on Jun 13, 2013

There are 2 conditions to be satisfied for a body to be in equilibrium (v = 0 or constant v) as stated in WS 3.2:
1. the net force acting on the body must be zero.
2. the net moment acting on the body must be zero.

If the net force is zero, e.g. upward forces = downward forces, the body would not experience a net force or acceleration in any particular direction, so its C.G. would remain at rest.
If these are equal forces (one upwards and one downwards) acting at different distances from the C.G. or a fixed pivot, there would be a net moment, so the object would rotate in the same direction as the net moment.
So in the example you describe, the net force on the body is zero but the net moment is not. Since not both conditions for equilibrium are satisfied, this body is not in equilibrium (it rotates, not at rest).

Goh Ying Ting said

at 10:42 pm on Jun 15, 2013

Hi Mr Ang, I would like to ask how does the size of a base area of an object affect its stability? Is it due to normal contact force? Thanks!

johnlittlephysics said

at 1:20 pm on Jun 16, 2013

Consider the example of the box in https://www.dropbox.com/s/orus1onp0r5wst0/Photo%2010-6-13%205%2053%2044%20PM.jpg lying on its longer side at rest.
A larger base area can mean a longer distance d from the line of action of the weight from the pivot (on the right corner). This means that to rotate the box, a larger clockwise turning moment about the pivot is needed to overcome the anti-clockwise moment due to its weight. Hence the box with larger base area is more stable.

Goh Ying Ting said

at 10:22 pm on Jun 22, 2013

Thanks!:)

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