Date – Friday 02nd October, 2020.
Class – Form 3.1 Integrated Science
Lesson 3 – principle of moments and levers
Moments of a force is the turning effect of a force. For a force to cause an object
to move, it depends on the size of the force and the distance from the pivot. So
ideally, a large/heavy object can be moved from a very far distance by a very
small force.
Let us examine a see-saw. A see-saw has to force acting on it at the same time
about a pivot.
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A see-saw has one person on the left and another on the right, there is a
pivot at the center that allows motion.
One person creates an anticlockwise moment while the other creates a
clockwise moment.
If the moments are not equal, the see-saw will not be balanced and one
side will go up while the other side stays down.
In this example, both children are sitting at the same distance from the pivot.
There are two ways in which this seesaw can be balanced so that it is
horizontal.
1. The lighter child can be replaced by a child equal in weight to the heavier
child
2. The heavier child could move closer to the pivot.
The principle of equilibrium states that when a system is in equilibrium the
anticlockwise moments are equal to the clockwise moments.
A system at equilibrium is stable. The turning effects of the forces cancel each
other out.
Examples:
1. This seesaw is balanced:
-The clockwise force and
anticlockwise forces are the same
(50N)
-The distance from the pivot is the
same (10ft)
-The moments are equal on both
sides - 50x10=500
-No overall turning effect, so the
seesaw is balanced
2. This system is balanced:
30
N
-The Force acting clockwise is
larger than the anticlockwise force,
but the distance from the pivot is
shorter.
-calculate the clockwise moments
= Force (30) X Distance (8m)
= 30 X 8 = 240 Nm
-calculate the anticlockwise
moments = force (20) X distance
(12) = 20 X12 = 240 Nm
Anticlockwise moments
Clockwise moments
- The moments are equal on both
sides so the system is stable.
3. This system is unbalanced
-Both objects are the same
distance from the fulcrum (1m)
-The Elephant has a much larger
force than the rock
-Therefore the clockwise moments
are much larger than the
anticlockwise moments
2
N
200
N
Clockwise moments = 200N X 1m
= 200 Nm
Anticlockwise moments = 2N X 1m
= 2 Nm
Now you can use the equations of moments of a force and the principle of
moments.
Practice question:
1. What force F is needed so that the seesaw below is balanced?
1.25m
1.5m
F
200 N
Solution
Clockwise moments = 200N x 1.5m = 300 Nm
Anticlockwise moments = F x 1.25m
When the seesaw is balanced:
Clockwise moments = anticlockwise moments
300 Nm = F x 1.25m
F = 300/1.25
F = 240N