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
What happens when you try to open a door with one finger?
Where do you push?
Hi, my name is Mr Stick
hinges
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
The moment is calculated by multiplying the force by the distance from the pivot
Turning effect of a force – moment of a force
Moment (Nm) = Force (N) x distance from pivot (m)
Note the unit is Nm, not N/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=
Centre of gravity
The centre of gravity of an object is the point where the objects weight seems to act.
With a uniform ruler, the C of G is at the mid-point.
I think he wants you
to copy this
Finding the centre of gravity
i. Place a compass or needle through any part of the shape.
ii. Make sure that the shape “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 science subject.
Stability
Stable objects have a wide base and a low centre of gravity.
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