Rotational balance

Balanced Forces

Levers

Q. Which statements

are true?

a. The longer the lever, the smaller the moment of

the force that is needed to move an object.

b. Bones are examples of levers.

c. The shorter the lever, the bigger the force that is

needed to move an object.

d. Joints are examples of pivots.

e. It is easier to close a door if you push the door

close to the hinge (axis).

Answer: b, c, & d.

Joints are examples of pivots.

Bones are examples of levers.

Q. Choose the correct description

for each of the following terms:

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.

2. Unbalanced

system

Learning ObjectiveTo investigate, through practical

examples, 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

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, a.c.w.

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 centimeters (N.cm) or

Newton meters (N.m).

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

* 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 rotationally balanced. This is

known as the principle of moments.

Two girls are sitting on opposite sides of on a see-saw.

One girl weighs 200N and is 1.5m from the pivot. Where

must her 150N friend sit if the seesaw is to balance?

When the see-saw is balanced:

Principle of moments –

calculation

total clockwise moment = total anticlockwise moment

200N x 1.5m = 150N x distance

200 x 1.5 = distance

150

distance of second girl = 2m

Tower cranes are essential at any major construction site.

load armtrolley

loading platform

tower

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

Answer: 2000N @ 15m, 3000N @ 10m, 6000N @ 5m