03 Moments
- Revision: key ideas in turning effects of forces; also refer to FAQs
- turning effect of forces mindmap.pdf
- Worksheets: refer solutions posted in iTunes U.
- Assignment:
- AS 3.1 AS3.1 - Moments.pdf
- Hints: 1(b) 1150 Nm, (c) 3840 N,
- 2(b)(ii) 0.333 m, (iii) compare moments, determine net moment direction.
- AS 3.2 AS3.2 - CoG & Stability.pdf
- Hints: Whenever conditions change, sketch and label clearly separate freebody diagrams (use pencil) for easy analysis!
- 1(a) only 3 forces, (b) 7690 N, , calculate and state whole number of children, (d) 4000 N, (e) 800 N.
- 2(b)(i) describe clearly the direction of any net moment produced, and subsequent motion of the toy; (b)(ii) make reference to Fig. 2.1 and the conditions for equilibrium.
- 3(a) state in terms of area of base and the height of the centre of gravity above the base, (b) draw diagram showing maximum angle of tilt (refer to WS3.3 example of rectangular block).
- Practice Quiz S3 Quiz 6C with answers.pdf
Post your questions/comments on worksheet, assignments below.
Comments (9)
johnlittlephysics said
at 3:38 pm on Jun 23, 2013
305 Celeste: I don't understand the solution to example 1a, of WS 3.3 for physics. (the one about mike and Pete) could you explain it to me please?
johnlittlephysics said
at 3:49 pm on Jun 23, 2013
WS3.3 Example 1(a)
Problem solving method: Identify pivot P, identify the forces and corresponding moments produced about P, apply principle of moments to form an equation. Refer diagram showing the forces that produced moments about P
https://www.dropbox.com/s/6yc92fqqzka5rw0/Photo%2023-6-13%203%2033%2053%20PM.png
When each block starts to tilt, the block would lose contact with the ground except for the pivot P.
For Mike: Taking moments about P, (clockwise moments caused by Fm = anticlockwise moments caused by weight 500 N)
Fm x 1.5 = 500 x (1.10/2), hence force Fm can be found.
Similarly, Fp can be calculated.
johnlittlephysics said
at 9:30 pm on Jul 30, 2013
302 Li June: May I check if the principle of movements apply to all equilibrium,such as the unstable and neutral equilibrium?
johnlittlephysics said
at 9:31 pm on Jul 30, 2013
1. No. As the principle states, it is applicable only when an object is at equilibrium (at rest or moving at constant velocity).
Common cases are objects at rest or just about to rotate (or topple).
2. Stable, unstable and neutral equilibrium are terms used to compare the stability of objects in different positions, when given a small displacement.
johnlittlephysics said
at 9:33 pm on Jul 30, 2013
302 Wey Hsuan: How do you know whether an object will remain in its new position after being displaced or tilted?
johnlittlephysics said
at 9:39 pm on Jul 30, 2013
If an object does not remain in its new position, there must be a turning moment that causes it to rotate about a suitable pivot.
Refer to the mindmap of Moments in finding the turning moment of a force:
1. Locate the possible pivot P
2. Draw the force F on the object or its weight W
3. Check if the line of action of the force (or weight) passes through P.
4. If it does, no turning moment. If it doesn't, can determine d, the direction of moment, and hence direction of rotation.
johnlittlephysics said
at 6:04 pm on Sep 28, 2013
302 Celeste: How could I go about determining the position of the CG of an irregular lamina? Could you kindly use the question in EOY paper 2009 Q12 MCQ to explain it to me please?
johnlittlephysics said
at 6:12 pm on Sep 28, 2013
Generally, based on a given diagram,
1. use common lines of symmetry (vertical or horizontal line) to determine which half of the diagram the object has greater mass, so that the CG would lie in that half.
In EOY 2009 P1 Q12, CG lie in top left quadrant.
2. if the object can approximate as two separate parts, the CG may lie along the line joining the CGs of these two parts.
In this question, the parts are a left rectangle and top right small rectangle. The CG approximately lies nearer to CG1 along the line joining CG1 and CG2.
johnlittlephysics said
at 6:13 pm on Sep 28, 2013
See diagram https://www.dropbox.com/s/4file6jjd828esa/Photo%2028-9-13%205%2045%2005%20pm.jpg
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