do now! can you write down on the piece of paper 3 things you learnt last lesson (there is a clue in...
TRANSCRIPT
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!
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 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
Note the unit is Nm, not N/m!
Moment (Nm) = Force (N) x distance from pivot (m)
A simple example!
nut
spanner (wrench)
50 N
0.15 m
Moment = Force x distance from pivot
Moment = 50 N x 0.15 m
Moment = 7.5 Nm
Get a longer spanner!
nut
spanner (wrench)
50 N
0.25 m
Moment = Force x distance from pivot
Moment = 50 N x 0.25 m
Moment = 12.5 Nm
More than one force
Take an uneven see-saw for an example
I thought he didn’t like dogs?
Only dead ones!
pivot
If the see-saw is balanced, what must be the weight of the dog on the left?
pivot
1.2 m 2.2 m
110 N? N
The force on the left is trying to turn the see-saw anticlockwise about the pivot
pivot
1.2 m 2.2 m
110 N? N
The force on the right is trying to turn the see-saw clockwise about the pivot
pivot
1.2 m 2.2 m
110 N? N
If the see-saw balances, the turning effect anticlockwise must equal the turning effect
clockwise
pivot
1.2 m 2.2 m
110 N? 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
pivot
1.2 m 2.2 m
110 N? N
Anticlockwise moment clockwise moment=
Principle of moments
An object is in equilibrium when anticlockwise moments = clockwise moments
YouTube - Alan Partridge's Apache office
What do these have in common?• YouTube - Soccer-world cup 1986 by Diego Maradona(Argentina vs England
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.
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!
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.