5. Principle Of Moments Answers r481y

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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)