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Balanced Forces
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
