Free gift for 9th graders 2014 ;)

1. Balanced Forces

2. Levers

3. 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.

4. Joints are examples of pivots.
Bones are examples of levers.

5. Q. Choose the correct description
for each of the following terms:
2. Unbalanced
system
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.

7. Learning Objective
To investigate, through practical
examples, the principle of moments.
Recording your results
• What do we need to
record?
• How many columns will
we need in our table?

8. Recording your results

9. Moment calculation:
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.
0.5m
500N pivot

10. Moment equation
The moment of a force is given by the equation:
moment = force (N) x distance from pivot (cm or m)
moment
f x d
Moments are measured in Newton centimeters (N.cm) or
Newton meters (N.m).

11. 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

12. * If the anticlockwise moment and clockwise moment are
equal then the see-saw is rotationally balanced. This is
known as the principle of moments.
* When something is balanced about a pivot:
total clockwise moment = total anticlockwise moment

13. Principle of moments –
calculation
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 150 N friend sit if the seesaw is to balance?
When the see-saw is balanced:
total clockwise moment = total anticlockwise moment
200N x 1.5m = 150N x distance
200 x 1.5 = distance
150
distance of second girl = 2m

14. Why don’t cranes fall over?
Tower cranes are essential at any major construction site.
load arm
trolley
loading platform
counterweight
tower
Concrete counterweights are fitted to the crane’s short arm.
Why are these needed for lifting heavy loads?

15. Why don’t cranes fall over?
Using the principle of moments, when is the crane balanced?
6m
3m
? 10,000N
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?

16. Why don’t cranes fall over?
moment of
counterweight
load x distance of load from tower
distance of counterweight
from tower
=
counterweight x
= 10,000 x 3
= 30,000 Nm
moment of
load
=
= ? x 6
moment of load = moment of counterweight
? x 6 = 30,000
? = 3,000
6
? = 5,000 N

17. Crane operator activity
Where should the loading platform be on the loading arm
to carry each load safely?
Answer: 2000N @ 15m, 3000N @ 10m, 6000N @ 5m