CIE AS & A Level Physics 4.2 Equilibrium of forces Study Notes- 2025-2027 Syllabus
CIE AS & A Level Physics 4.2 Equilibrium of forces Study Notes – New Syllabus
CIE AS & A Level Physics 4.2 Equilibrium of forces Study Notes at IITian Academy focus on specific topic and type of questions asked in actual exam. Study Notes focus on AS/A Level Physics Study Notes syllabus with Candidates should be able to:
- state and apply the principle of moments
- understand that, when there is no resultant force and no resultant torque, a system is in equilibrium
- use a vector triangle to represent coplanar forces in equilibrium
4.2.1 state and apply the principle of moments
The principle of moments, also known as Varignon’s Theorem, states that the sum of all the moments acting on an object along a given axis is equal to the net moment about that axis.
If a body is in equilibrium the sum of the clockwise moments is equal to the sum of the anticlockwise moments.
All anti-clockwise moments can be taken to be positive and clockwise moments to be negative or vice versa.
Principle of moments for a body in equilibrium: An object is in equilibrium if the sum of all anticlockwise moments about the pivot is equal to the sum of all clockwise moments about the same pivot.
4.2.2 understand that, when there is no resultant force and no resultant torque, a system is in equilibrium
A system is in equilibrium
• The state of a body or physical system that is at rest or in constant and unchanging motion.
• If a system is in static equilibrium, there are no net forces and no net torque in the system.
• If a system is in stable equilibrium, small disturbances to the system cause only a temporary change before it returns to its original state.
Two conditions must be satisfied for static equilibrium to take place for a rigid body.
a) Resultant force = 0.
The vector sum of all external rorces acting on a rigid body must be zero i.e. no net/resultant force
∑ F=0
b) Resultant torque = 0. .
The vector sum of all external torques acting on a rigid body must be zero i.e. No net/resultant torque, OR
Mathematically,
∑ τ = 0
4.2.3 use a vector triangle to represent coplanar forces in equilibrium
(1) Two forces in equilibrium
Consider two forces acting on a crate is in equilibrium,
Conclusion:
They must act along the same line of action (collinear), be equal in magnitude and opposite in direction.
$
\begin{array}{ll}
\Sigma F_x=0 ; & F_1-F_2=0 \\
& F_1=F_2 \text { (no resultant force) }
\end{array}
$
(2) Three forces in equilibrium
Consider three forces acting on a crate is in equilibrium,
Conclusion:
1) They must act on the same plane (coplanar)
2) They form a closed triangle.
3) They must all act through the same point (concurrent),
(3) Four or more forces in equilibrium
Consider four or more forces acting on a crate is in equilibrium,
Conclusion:
1) They must act on the same plane (coplanar)
2) They form a closed polygon
3) They must all act through the same point.
