Balanced forces

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The girl on the left exerts

an anti-clockwise moment,

which equals .

her weight x her distance

from pivot

her weight x her distance

from pivot

Slide 15

Principle of moments

When something is balanced about a pivot:

total clockwise moment = total anticlockwise moment

If the anticlockwise moment and clockwise moment are equal then the see-saw is balanced. This is known as the principle of moments.

Slide 16

Principle of moments – calculation

Two girls are sitting on opposite sides of on a see-saw. One girl weighs 200 N and is 1.5 m from the pivot. Where must her 150 N friend sit if the seesaw is to balance?

When the see-saw is balanced:

total clockwise moment = total anticlockwise moment

200 N x 1.5 m = 150 N x distance

200 x 1.5 = distance

150

distance of second girl = 2 m

Slide 17

Anagrams

Slide 18

Why don’t cranes fall over?

Tower cranes are essential at any major construction site.

Concrete counterweights are fitted to the crane’s short arm. Why are these needed for lifting heavy loads?

Slide 19

Using the principle of moments, when is the crane balanced?

moment of = moment of

load counterweight

If a 10,000 N counterweight is three metres from the tower, what weight can be lifted when the loading platform is six metres from the tower?

?

Why don’t cranes fall over?

Slide 20

Why don’t cranes fall over?

Slide 21

Crane operator activity

Where should the loading platform be on the loading arm to carry each load safely?