Forces in Equilibrium — A-Level Physics Revision
Revise Forces in Equilibrium for A-Level Physics. Step-by-step explanation, worked examples, common mistakes and exam-style practice aligned to AQA, Edexcel and OCR.
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This topic deals with situations where the net force and net torque on an object are zero, meaning the object is not accelerating and is in a state of equilibrium. This can mean the object is stationary or moving at a constant velocity. You will learn to solve problems involving concurrent and coplanar forces by resolving forces into components and by using the triangle of forces method for three-force systems.
Board notes: This is a fundamental mechanics topic covered by all A-Level boards (AQA, Edexcel, OCR). The complexity of the force systems and the mathematical techniques required (e.g., resolving forces vs. sine/cosine rule with force triangles) can vary. AQA and Edexcel often integrate these concepts with moments in more complex problem-solving scenarios.
Step-by-step explanationWorked example
A 10 kg mass is suspended by two ropes, one at 30° to the vertical and the other at 45° to the vertical. To find the tension in each rope, we resolve the forces horizontally and vertically. Let the tensions be T1 and T2. Horizontally: T1sin(30°) = T2sin(45°). Vertically: T1cos(30°) + T2cos(45°) = 10g = 98.1 N. Solving these simultaneous equations gives T1 ≈ 71.7 N and T2 ≈ 50.7 N.
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Common mistakes
- 1Forgetting that equilibrium means zero *net* force, not the absence of forces. An object in equilibrium can have multiple forces acting on it, but they must all cancel each other out.
- 2Mixing up vector components when resolving forces. A common error is to use sine instead of cosine (or vice versa) for the horizontal and vertical components of a force acting at an angle.
- 3Drawing the triangle of forces incorrectly. The forces must be drawn tip-to-tail in a closed loop for the object to be in equilibrium. If the triangle does not close, there is a resultant force.
Forces in Equilibrium exam questions
Exam-style questions for Forces in Equilibrium with mark-scheme style solutions and timing practice. Aligned to AQA, Edexcel and OCR specifications.
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Step-by-step method
Step-by-step explanation
4 steps · Worked method for Forces in Equilibrium
Core concept
This topic deals with situations where the net force and net torque on an object are zero, meaning the object is not accelerating and is in a state of equilibrium. This can mean the object is stationa…
Frequently asked questions
What is the difference between concurrent and coplanar forces?
Concurrent forces are forces whose lines of action all intersect at a single point. Coplanar forces are forces that all lie within the same two-dimensional plane.
What are the conditions for an object to be in equilibrium?
For an object to be in equilibrium, two conditions must be met: 1) The vector sum of all forces acting on the object must be zero (no translational acceleration). 2) The sum of the moments about any point must be zero (no rotational acceleration).