# 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!
questions on the piece of

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
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-

1.2 m            2.2 m

pivot

?N                                110 N
The force on the right is trying to turn the

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

Apache office
30 second silent thinking time
Do you have any
questions?
What do these have in common?
Today’s lesson
• Centre of gravity
• Forces on beam supports (start thinking
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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