Förberedande Mekanik

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Rad 33:		Rad 33:
	[[Image:ex13.1whole.GIF]]		[[Image:ex13.1whole.GIF]]
-	Find the moment of each force shown below about the point O.
-	[[Image:ex13.1.GIF]]
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-	Solution
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-	(a)
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-	<math>20\times 0.8=16</math>
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-	Moment is -16 Nm
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-	(b)
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-	<math>12\times 1.2=14.4</math>
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-	Moment is 14.4 Nm
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Rad 59:		Rad 41:
	[[Image:ex13.2.GIF]]		[[Image:ex13.2.GIF]]
-	Find the moment of each force shown below about the point O.	+	Example 13.3
-	(a) (b)	+
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-	Solution
-	(a) (b)
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-	<math>40\times 3\sin 60{}^\circ =104</math>
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-	<math>100\times 2\sin 20{}^\circ =68.4</math>
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-	Moment is 104 Nm (to 3 sf) Moment is 68.4 Nm (to 3 sf)
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-	Example 13.3
	For the rectangular lamina shown below, find the total moment of the forces acting, about the corner marked O.		For the rectangular lamina shown below, find the total moment of the forces acting, about the corner marked O.
-		+	[[Image:ex13.3.GIF]]
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	Solution		Solution

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Versionen från 17 september 2009 kl. 19.22

Theory

Exercises

Key Points

The moment of the force about the point O is the product of the force and the perpendicular distance to the line of action of the force from O.

\displaystyle \text{Moment }=Fd

\displaystyle \text{Moment }=Fd\sin \theta

Clockwise moments are negative. Anti-clockwise moments are positive.

Example 13.1

Example 13.2

Example 13.3

For the rectangular lamina shown below, find the total moment of the forces acting, about the corner marked O.

Solution

Force Moment 5N at O \displaystyle 5\times 0=0

8 N \displaystyle -8\times 1.2=-9.6

7 N \displaystyle 7\times 0=0

6 N \displaystyle -6\times 0.5=-3

5 N \displaystyle 5\times 1.2=6

4 N \displaystyle 4\times 0.5=2

Total Moment \displaystyle 0-9.6+0-3+6+2=-4.6\text{ Nm}