Question 1

What is the difference between Centre of Gravity and Centre of Mass?

Accepted Answer

The Centre of Mass (CoM) is the point where an object's entire mass is considered to be concentrated, while the Centre of Gravity (CoG) is the point where its entire weight acts. In a uniform gravitational field (like on Earth), these two points coincide and are often used interchangeably in A-Level Physics.

Question 2

How do I know which direction a moment acts?

Accepted Answer

Moments are typically classified as either clockwise or anticlockwise, depending on the rotation they would cause about the pivot point. It's crucial to consistently define one direction as positive (e.g., clockwise) and the other as negative (anticlockwise) when solving problems.

Question 3

Can a single force create a couple?

Accepted Answer

No, a single force cannot create a couple. A couple specifically requires *two* forces that are equal in magnitude, opposite in direction, parallel, and act along different lines of action. A single force creates a moment, but not a pure couple.

Question 4

How does an object's stability relate to its Centre of Gravity?

Accepted Answer

An object is more stable if its Centre of Gravity (CoG) is lower and its base of support is wider. When an object is tilted, its stability depends on whether the vertical line passing through its CoG falls inside or outside its base of support. If it falls inside, it is stable and will return to its original position. If it falls outside, it is unstable and will topple.

Turning effects of forces

What you need to know

Turning effects of forces