Physics Chapter - Turning Effect Of Forces

Pure Physics -Turning Effects of Forces (Moments)

Definitions


• Moment Force Perpendicular Distance -The moment of a force is the turning effect of a force, or the ability of the force to make something turn.
• Moment of a force (M) about a point O is the product of the force (F) and the perpendicular distance (D) from the point to the line of action of the force.
• SI unit: Newton (N)
• Moments can be clockwise or anticlockwise.
• The turning effect of a force depends on:
• Location of applied force
• Perpendicular distance between the point of application of the force and the pivot.

Principle of Moments

The principle of moments state that:
When a body is in equilibrium, the sum of clockwise moments about the balanced point is equal to the sum of anticlockwise moments about the same point (pivot).Total clockwise moment = Total anticlockwise moment.


• When the clockwise moment is not equal to the anticlockwise moment, there is a resultant moment. The object will rotate in the direction of resultant moment.
• Therefore, if there is no resultant moment, the object is balanced!

Center of Gravity (c.g.)


• Definition: The center of gravity (CG) of a body is an imaginary point where the whole weight of the body seems to act in any orientation.
• The CG of a regular object is at the center.
• The CG of an irregular object is determined using a plumb line.
• If a body is hanging freely at rest, its center of gravity is always vertically below the pivot, thus the plumb line method works. It can only be used for flat, irregular objects.

Stability


• Stability is a measure of the body’s ability to maintain its original position.
• There are 3 types of stability:

Stability Type Effect Explanation- Stable Equilibrium Object will return into original position after slight disturbance. Weight will generate an anticlockwise moment by bringing the cone back to its original position (done by the restoring moment).These types of objects usually have low CG and big/heavier bases.

Unstable Equilibrium Object will topple/fall after slight disturbance .The weight of the cone will generate a clockwise moment outside the base area of the cone, thus there is a resultant moment and the object will fall.

Neutral Equilibrium Object remains in new position after slight disturbance .The center of gravity neither rises nor falls, it remains at the same level. The lines of action of the 2 forces always coincide and there is no moment provided by weight to turn the cone.

• Ways to improve stability of an object:
• Lowering the CG (A lower CG will allow the line of action to act within the base area of an object)
• Area of its base should be as wide as possible (allow line of action to act within base area)