Centre of mass
Forces in Action - OCR A-Level Physics
Key Definition
Centre of mass
The single point where the entire mass of an object can be considered to act. For a uniform object, it is at the geometric centre.
The single point where the entire mass of an object can be considered to act. For a uniform object, it is at the geometric centre.
Key Definition
Centre of gravity
The single point where the entire weight of an object can be considered to act. In a uniform gravitational field, it coincides with the centre of mass.
The single point where the entire weight of an object can be considered to act. In a uniform gravitational field, it coincides with the centre of mass.
- The conditions for static equilibriumAn object is in equilibrium when the resultant force on it is zero. The object is either stationary or moving at constant velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹..: (1) the resultant force in any direction is zero, AND (2) the resultant moment about any point is zero
- An object topples when the line of action of its weight falls outside its base of support
- A lower centre of gravityThe single point at which the entire weight of an object can be considered to act. and wider base make an object more stable
- To find the centre of gravityThe single point at which the entire weight of an object can be considered to act. of an irregular lamina: suspend it from two or more points, draw plumb lines, and find where they intersect
Examiner Tips and Tricks
- When solving equilibriumAn object is in equilibrium when the resultant force on it is zero. The object is either stationary or moving at constant velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹.. problems, you can take moments about ANY point.
- Choosing the point where an unknown force acts eliminates that force from the moment equation, simplifying the calculation.