Do now Can you read the hydraulics pages of your books (p157)?
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Transcript of Do now Can you read the hydraulics pages of your books (p157)?
Do now Can you read the hydraulics pages of your books (p157)?
Today’s 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 principle 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
Do you push near the hinges?
I can’t do it!
Do you push far from the hinges?
That’s easier!
Moments – Turning forces
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
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
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
What do you do if the nut won’t move and you can’t push harder?!
nut
spanner (wrench)
50 N
0.15 m
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
Do you think we’ll be safe in this power point?
It’s not looking good!
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=
Rotational Equilibrium
When there is no resultant moment on an object (when anticlockwise moments = clockwise moments) we say the object is in rotational equilibrium.
YouTube - Alan Partridge's Apache office
Whew! We seemed to have
survived the power point
pivot
Don’t speak too
soon!
pivot
BO
MB
pivot
BO
MB
pivot
BO
MB
pivotBO
MB
pivot
pivot
Nice one!
pivot
You’ll have to do some questions
now, or he’ll kill me too!
pivot
Read pages 162 and 163 and make notes. Then try the questions.