Equilibrium
Scalars & Vectors - OCR A-Level Physics
Key Definition
Equilibrium
An object is in equilibrium when the resultant force on it is zero. The object is either stationary or moving at constant velocity.
An object is in equilibrium when the resultant force on it is zero. The object is either stationary or moving at constant velocity.
- For an object in 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.. under three coplanar forces, the forces form a closed triangle.
- If the three force arrows are drawn nose-to-tail and the triangle closes (returns to the start), the resultant is zero.
- An open triangle means there IS a net force -- the object is NOT in 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...
- Alternatively, resolve all forces into horizontal and vertical components. For 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..: sum of horizontal $components = 0 AND sum$ of vertical $components = 0$.
Common Mistake
MEDIUM
Wrong: Saying equilibrium means the object is stationary.
Right: Equilibrium means the resultant force is zero. The object can be stationary OR moving at constant velocity. A skydiver at terminal velocityThe constant velocity reached when the driving force on an object is exactly balanced by resistive forces, so the resultant force is zero. is in equilibrium.
Right: Equilibrium means the resultant force is zero. The object can be stationary OR moving at constant velocity. A skydiver at terminal velocityThe constant velocity reached when the driving force on an object is exactly balanced by resistive forces, so the resultant force is zero. is in equilibrium.
Examiner Tips and Tricks
- When three forces act on an object in equilibrium, you can solve the problem EITHER by drawing a closed triangle of forces (scale drawing) OR by resolving horizontally and vertically and setting each sum to zero.
- The resolutionThe smallest change in a quantity that an instrument can detect. For example, a ruler has a resolution of 1 mm. method is more accurate in calculations.