Sec 3 Physics - Turning Effect of Forces

What is the moment of a force/torque?

It is the product of the force F and the perpendicular distance d from the pivot to the line of action of the force.

M = Fd

Define the Principle of Moments.

It states that when a body is in equilibrium, the sum of clockwise moments about a pivot is equal to the sum of anticlockwise moments about the same pivot.

What are the conditions for equilibrium?

Zero rotational or translational motion, which means:

• Resultant moment on body is zero

• Resultant force on body is zero

Define the centre of gravity of an object.

It is an imaginary point where the entire weight of the object seems to act.

What effect can the centre of gravity affect the stability of a body and how?

A body can be more stable by the following factors:

• Lowering the centre of gravity

• Increasing the base area

After pushing an object slightly to the right, the object returns to its original position. Why is that so?

The centre of gravity that is acting vertically downwards is within the block’s base. As a result, the block’s weight creates an anti-clockwise moment about the pivot that helps the block return to its original position.

Due to stormy weather, many tall waves are produced from the high windspeeds. A ship had encountered a particularly massive one and rolls over to the left side of point P (assuming we are looking at it from the front). After awhile, the ship is seen rolling back to its original position. Explain the phenomenon.

The centre of gravity shifts to the right side of Point P. This causes a clockwise moment about point P, which cause the ship to roll back to its original position and stabilise.

How to make objects (or in this case, ships) more resistant to turning over during rough seas and capsize?

They would need to lower the centre of gravity, which can be achieved by two ways:

1. Move heavier cargo/objects to the bottom part of the ship

2. Make the top of the ship lighter by removing heavier objects