Balanced Forces

Levers

Write out the statements that are true.

• a The longer the lever, the bigger the force that is needed to move an object.

• b It is easier to close a door if you push the door close to the hinge

• c The shorter the lever, the bigger the force that is needed to move an object

• d Joints are examples of pivots.• e Bones are examples of levers.

C, D and E

Learning Objective To investigate, through

practical experimentation, the principle of moments.

• What do we need to record?• How many columns will we need

in our table?

Recording your results

Recording your results

Weight and Mass

• YouTube - Eureka! Episode 7 - Weight vs. Mass

YouTube - Eureka! Episode 6 - Gravity

Racing Balls

Write out each term along with its correct description

Descriptions• anticlockwise moments = clockwise

moments• two boys of different weights sit opposite

each other on a see saw, both the same distance from the pivot

• the turning effect of a force

moment balanced systemunbalanced system

Lever Principle

GCSE PHYSICS: Moments

Gina weighs 500 N and stands on one end of a seesaw. She is 0.5 m from the pivot.

What moment does she exert?

moment = 500 x 0.5 = 250 Nm

0.5 m

500 N

pivot

Moment calculation

moment = force (N) x distance from pivot (cm or m)

The moment of a force is given by the equation:

Moments are measured in Newton centimetres (Ncm) or Newton metres (Nm).

moment

f x d

Moment equation

Principle of moments

The girl on the right exerts a clockwise moment, which equals...

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

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.

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:

Principle of moments – calculation

total clockwise moment = total anticlockwise moment

200 N x 1.5 m = 150 N x distance200 x 1.5 = distance150

distance of second girl = 2 m

Anagrams

Tower cranes are essential at any major construction site.

load armtrolley

loading platformtower

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

counterweight

Why don’t cranes fall over?

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?

6 m

3 m

10,000 N?

Why don’t cranes fall over?

moment of counterweight

distance of counterweight from tower

=

= 10,000 x 3= 30,000 Nm

counterweight x

moment of load

=

= ? x 6

load x distance of load from tower

moment of load = moment of counterweight ? x 6 = 30,000

? = 3,000 6

? = 5,000 N

Why don’t cranes fall over?

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

Crane operator activity