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.