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Principle of moments Questions
1
A force of 20 N acts at a distance of 20 cm from a pivot.
What is the moment of this force about the pivot? Include the unit in your answer.
...............20 x 20 = 400Ncm.......................................................................................
.....................................................................................................................................(1)
2
What weight W must be hung on the ruler to balance it?
........... Principle of moments so clockwise moments = anticlockwise moments ...........
............So 100 x 30 = W x 20 .................................................................................
.............W = 3000/20.................................................................................................
.............W= 150 N................................................................................................(2)
3
The diagram shows a 1 m ruler, pivoted at its midpoint.
At what distance x from the left-hand end of the ruler must the 10 N force be applied
to balance the ruler?
....... Principle of moments so clockwise moments = anticlockwise moments ...............
........So 20 x 20 = 10 x (50 – x) .......................................................................
.........400 = 500 – 10 x ...........................................................................................
..........100 = 10x
...........x = 10cm ...........................................................................................(3)
4
A ruler is placed on a pivot; three forces act on it, as shown.
a
Which force has no moment about the pivot? Explain your answer.
................20 N as it is 0cm from the pivot ................................................................
.....................................................................................................................................(2)
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b
Calculate the moments of the other two forces about the pivot.
.........6 x 70 = 420 Ncm .............................................................................................
.........14 x 30 = 420 Ncm............................................................................................
..........................................................................................................................................
.....................................................................................................................................(3)
c
Is the ruler turning? Explain.
...........No – moments are equal........................................................................................
.....................................................................................................................................(2)
5
The diagram shows a horizontal beam, pivoted at its midpoint, with two
downward forces acting on it.
a Which force is tending to turn the beam clockwise?
Calculate the moment of this force.
.............20 N ..............................................................................................................
..............1.2 x 20 = 24Nm.......................................................................................(2)
b Which force is tending to turn the beam anticlockwise?
Calculate the moment of this force.
..........30 N................................................................................................................
...........30 x 0.8 = 24Nm.......................................................................................(2)
c Are these moments balanced, or will the beam start to turn?
...........Balanced..............................................................................................................
.....................................................................................................................................(1)
6
The diagram shows how a heavy slab of stone can be raised using a lever.
Calculate the force F needed to lift the stone.
........1000x0.3=1xF..........................................................................................................
.........F=300N....................................................................................................................
..........................................................................................................................................
.....................................................................................................................................(3)