Unit 4 - Turning Effects - PHYSICS S3B T2

What is moment?

The turning effect of a force is measured by a physical quantity called the moment of a force, measured as Force x Perpendicular Distance from Pivot. Its unit is Nm.

→ Moment is affected by distance, LARGER distance = LARGER moment

→ Moment is also affected by force, LARGER force = LARGER moment

→ Moments have DIRECTION. They can be clockwise (CW) or anticlockwise (ACW).

→ Moment is a vector quantity, it has distance, force and direction.

Explain how applying a turning force on an object creates a turning effect.

Objects turn about a fixed location, called the pivot.

The pivot is the point that doesn’t move when a force is applied. In a turning force diagram, there is typically a load (the item affected by the moment), a pivot (the unmoving point the object turns around about), and a force (depicted with a clear direction).

What is equilibrium?

When bodies or objects are balanced, they are in equilibrium, this means that there is no resultant force acting on the object. Moments are balanced when the anticlockwise moment is equal to the clockwise moment. We say that the system is in equilibrium.

PRINCIPLE OF MOMENTS: If an object is balanced, the anticlockwise moment about a pivot is equal to the clockwise moment about a pivot.

→ In equilibrium, the TURNING FORCE of an object has a UPWARD CONTACT FORCE acting through the pivot keeping it in equilibrium. The total weight of the rod/beam/object balanced on a pivot is the sum of ALL THE FORCES acting on it.

When an object is in equilibrium:

• the forces on it must be balanced (no resultant force)

• the turning effects of the forces on it must also be balanced (no resultant turning effect.)

What is gravity and its effect on bodies?

Attraction between masses, every object with mass attracts every other object. Nearby objects experience a greater gravitational pull. Gravitational forces pull at every part of an object. The resultant force of this is its weight (?) CHECK THIS.

Explain the characteristics of the Centre of Gravity

The centre of gravity is where all the mass of an object could be located and the object would behave the same (ignoring any spin).

Regular shaped objects have a centre of gravity located at its midpoint.

The centre of gravity of a symmetrical object is along the axis of symmetry.

The position of the centre of gravity affects the object’s stability.

An object is stable when its centre of gravity is above its base.

→ An object is more stable when it has a wider base and a lower height.
This is because an object with a wider base has more room for its line of action of weight to swing around before it can actually GO OUTSIDE of the Base of Support, causing the object to topple. As for the height, a shorter object usually has a centre of gravity CLOSER TO THE GROUND, this means the object requires a much larger tilt angle before it can topple as compared to a tall object.