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!Can you answer the 5

questions on the piece of paper please?

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

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

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

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

30 second silent thinking time

Do you have any questions?

What do these have in common?• YouTube - Soccer-world cup 1986 by Diego Maradona(Argentina vs England

Today’s lesson

• Centre of gravity

• Forces on beam supports (start thinking about!)

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