Moments – Turning forces

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					Do now!
           Can you write down on the
           piece of paper 3 things you
          learnt last lesson (there is a
              clue in the picture!)?




                          WITHOUT
                        looking in your
                            books!
Do now!
           Can you answer the 5
          questions on the piece of
               paper please?




                      WITHOUT
                    looking in your
                        books!
              Last lesson
• Know what the turning effect of a force is
• Know that the moment of a force depends
  on force and the distance from the pivot
• Know the principal of moments
     The turning effect of a force
       depends on two things;
The size of the force
        Obviously!
      The turning effect of a force
        depends on two things;
The distance from the pivot (axis of rotation)
       Not quite
      so obvious!




                       Axis of rotation
        Turning effect of a force
   The turning effect of a force is called the
   moment of the force

   Moment (Nm) = Force (N) x distance from pivot (m)




Note the unit is Nm, not N/m!
       A simple example!


50 N


         0.15 m

                        nut




                  spanner
                  (wrench)
       A simple example!

         Moment = Force x distance from pivot
         Moment = 50 N x 0.15 m
50 N     Moment = 7.5 Nm


         0.15 m

                                  nut




                            spanner
                            (wrench)
 What do you do if the nut won’t
move and you can’t push harder?!


   50 N


          0.15 m

                         nut




                   spanner
                   (wrench)
       Get a longer spanner!

                 Moment = Force x distance from pivot
                 Moment = 50 N x 0.25 m
50 N             Moment = 12.5 Nm


        0.25 m


                                          nut




                                    spanner
                                    (wrench)
        More than one force
Take an uneven see-saw for an example
                                Only
          I thought he didn’t   dead
               like dogs?       ones!




                      pivot
If the see-saw is balanced, what must be the
         weight of the dog on the left?



          1.2 m           2.2 m




                  pivot




     ?N                             110 N
The force on the left is trying to turn the see-
     saw anticlockwise about the pivot



           1.2 m            2.2 m




                   pivot




     ?N                                110 N
The force on the right is trying to turn the
  see-saw clockwise about the pivot



         1.2 m            2.2 m




                 pivot




   ?N                                110 N
  If the see-saw balances, the turning effect
  anticlockwise must equal the turning effect
                  clockwise



           1.2 m           2.2 m




                   pivot




      ?N                              110 N

Anticlockwise moment = clockwise moment
 Anticlockwise moment = clockwise moment
             ? X 1.2 = 110 x 2.2
                ? X 1.2 = 242
                 ? = 242/1.2
                 ? = 201.7 N
          1.2 m           2.2 m




                  pivot




     ?N                           110 N

Anticlockwise moment = clockwise moment
Principle of moments

           An object is in
           equilibrium when
           anticlockwise
           moments = clockwise
           moments




        YouTube - Alan Partridge's
        Apache office
30 second silent thinking time
               Do you have any
               questions?
    What do these have in common?
•   YouTube - Soccer-world cup 1986 by Diego Maradona(Argentina vs England
           Today’s lesson
• Centre of gravity
• Forces on beam supports (start thinking
  about!)
Centre of gravity?
                      He can be
                     really boring
                     sometimes.
         Centre of gravity
The centre of gravity of an object is the
point where the object’s weight seems to
act.
                                    I think he
                                    wants you
                                   to copy this
         Centre of Gravity “Circus”




 Sketch and number
each object and put a
cross where you think
the Centre of Gravity
         is.
Complex shapes
         How do you find the
         centre of gravity of
          complex shapes?




    Complex shape man
Position of centre of gravity
     Draw the sentence

1.     2.     3.     4.




5.     6.     7.     8.
     Write the drawing?

1.     2.     3.      4.




5.     6.     7.      8.
               Self-assessment
i. Place a compass or needle through any part of
      the card.
ii. Make sure that the card “hangs loose”.
iii. Hang a plumb line on the needle.
iv. After it has stopped moving, carefully draw a
      line where the plumb line is.
v. Place the needle in any other part of the card.
vi. Repeat steps ii to iv.
vii. Where the two drawn lines cross is where the
      centre of mass is.
viii. Physics is the most interesting subject.
The ruler trick!
Let’s sit on a plank!
The ruler trick!
 Your homework is to explain how the ruler trick
   works in language simple enough that your
    imaginary 8 year old brother Jack could
  understand. Minimum 100 words, maximum
  500 words. Due Friday 12th November 2010.
     Let’s try some questions
Page 57 Questions 1, 2
       and 3

				
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