CIE A Level Physics复习笔记4.1.4 Conditions for Equilibrium
Equilibrium
- A system is in equilibrium when all the forces are balanced. This means:
- There is no resultant force
- There is no resultant torque
- An object in equilibrium will therefore remain at rest, or at a constant velocity, and not rotate
- The system is in an equilibrium state when applying the principle of moments (see The Principle of Moments)
Worked Example
Four beams of the same length each have three forces acting on them.Which beam has both zero resultant force and zero resultant torque acting?
Coplanar Forces in Equilibrium
- Coplanar forces can be represented by vector triangles
- Forces are in equilibrium if an object is either
- At rest
- Moving at constant velocity
- In equilibrium, coplanar forces are represented by closed vector triangles
- The vectors, when joined together, form a closed path
- The most common forces on objects are
- Weight
- Normal reaction force
- Tension (from cords and strings)
- Friction
- The forces on a body in equilibrium are demonstrated below:
Three forces on an object in equilibrium form a closed vector triangle
Worked Example
A weight hangs in equilibrium from a cable at point X. The tensions in the cables are T1 and T2 as shown.
Which diagram correctly represents the forces acting at point X?
Exam Tip
The diagrams in exam questions about this topic tend to be drawn to scale, so make sure you have a ruler handy!
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