Page 5 - CLASS VII Lesson Notes Forces and Energy

SUBTOPIC: 3.4 Turning Forces

Key Words: lever, pivot, moment, newton metres, turn
Turning Effects of Forces:
• When you apply a force to something like a door handle, a bicycle pedal, or a spanner, it can cause
the object to turn.
• A lever is an object that turns when a force is applied.
• The point around which the lever turns is called the pivot.

Examples:
• A bicycle pedal turns around a pivot when you push it.
• Your arm acts as a lever and your elbow are the pivot when you bend it.
Moments:
• A moment is the turning effect of a force around a pivot.
• The size of the moment depends on:
o The size of the force
o The distance from the pivot to where the force is applied
Calculating Moment:
Moment = Force × distance from pivot
• Force is measured in newtons (N)
• Distance is measured in metres (m)
• Moment is measured in newton metres (N m)
Important: Write moment units with a capital N and lowercase m: Nm
Worked Example 1:
A force of 35 N is used to open a door. The distance from the handle to the pivot is 0.8 m.
Solution:
Moment = Force × distance from pivot
= 35×0.8=28 Nm
Worked Example 2:
A weight of 20 N hangs 0.25 m from the pivot.
Moment = Force × distance from pivot
=20×0.25=5 Nm
Balancing Moments:
• A seesaw is an example of a lever.
• A seesaw is balanced when the moments on both sides are equal
and opposite.
Worked Example 3:
Marcus (600 N) sits 2 m from the pivot.
Moment = 600×2 = 1200 Nm
To balance this, Arun (800 N) must produce the same moment:
distance= Moment/weight of Arun
= 1200/800=1.5 m

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