A.2
Forces and Momentum
Newton's laws, free-body diagrams, linear momentum, impulse, collisions and circular motion.
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Key Concepts, Forces and Momentum
Newton's Three Laws of Motion
A force is a push or a pull that acts upon an object as a result of its interaction with another object. Newton's First Law states that an object will remain at rest or continue to move with a constant velocity unless acted upon by a resultant external force. Note the second part carefully: it also applies to objects already moving at constant velocity, not only objects at rest. Newton's Second Law states that when forces are unbalanced, the resultant force causes acceleration. With constant mass, F = ma, where F is the resultant force in Newtons, m is mass in kg, and a is acceleration in m/s². Newton's Third Law states that if body A exerts a force on body B, then body B exerts an equal and opposite force on body A. The two forces in a Newton's Third Law pair must be of the same type, equal in magnitude, opposite in direction, and act on different objects.
Free-Body Diagrams and Contact Forces
A free-body diagram shows all forces acting on a single object, drawn as arrows from the centre of mass (or point of application). Contact forces you must know include: the normal force (perpendicular to a surface), friction (opposing relative motion), elastic (restoring force in springs), viscous drag (opposing motion through a fluid), and buoyancy (upward force from a fluid). Field forces act without contact: gravitational, electric and magnetic forces. In translational equilibrium, the vector sum of all forces equals zero and the object is at rest or moving at constant velocity, which is an application of Newton's First Law.
Linear Momentum and Impulse
Linear momentum is defined as p = mv, where m is mass in kg and v is velocity in m/s. Momentum is a vector quantity measured in kg m/s or N s. The law of conservation of momentum states that the total momentum of a system remains constant provided no resultant external force acts on it. Impulse is defined as the product of the resultant force and the time over which it acts: J = FΔt. Impulse equals the change in momentum of the system: FΔt = Δp = mv - mu. On a force-time graph, the impulse is the area under the curve. Newton's Second Law can be derived directly from this: F = Δp/Δt, which reduces to F = ma when mass is constant.
Collisions and Explosions
In all collisions and explosions, momentum is always conserved (provided no external resultant force acts). In an elastic collision, kinetic energy is also conserved. In an inelastic collision, kinetic energy is not conserved (it is converted to heat, sound or deformation energy), but momentum still is. A perfectly inelastic collision is one where the objects stick together after colliding. In an explosion (like a gun firing a bullet), both objects start at rest so total initial momentum is zero. After the explosion, the two objects move in opposite directions with equal and opposite momenta, so total momentum remains zero. For HL, two-dimensional collisions are also assessed, requiring vector addition of momenta in both the x and y directions.
Circular Motion
An object in circular motion travels at constant speed but is continuously changing direction, so it is accelerating. This centripetal acceleration is always directed towards the centre of the circle and has magnitude a = v²/r, which can also be written as a = ω²r. By Newton's Second Law, there must be a net force directed towards the centre, called the centripetal force: F = mv²/r = mω²r. This force is not a new type of force; it is provided by whatever physical force is directed towards the centre (gravity for an orbiting satellite, tension for a ball on a string, friction for a car rounding a bend). Angular velocity ω is the rate of change of angular displacement and is measured in rad/s. Linear speed and angular velocity are related by v = ωr.
Key Quantities and Equations for Circular Motion
Angular displacement θ is the angle in radians through which a point has rotated, defined as arc length divided by radius. Angular velocity ω = θ/t in rad/s. Linear (tangential) velocity v = 2πr/T in m/s, where T is the period. The link between linear and angular quantities is v = ωr. Centripetal acceleration a = v²/r = ω²r in m/s². Centripetal force F = mv²/r = mω²r in Newtons. Remember: all points on a rotating object share the same angular velocity but not the same linear velocity (points further from the centre move faster).
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Frequently Asked Questions, IB Physics Forces and Momentum
What is Forces and Momentum in IB Physics? ↓
Newton's laws, free-body diagrams, linear momentum, impulse, collisions and circular motion. This topic is part of Theme A (Space, Time & Motion) in the current IB Physics syllabus.
Is Forces and Momentum SL or HL in IB Physics? ↓
Forces and Momentum is covered by both SL and HL students in the current IB Physics syllabus. HL students study additional depth and extension content beyond the SL core.
What equations do I need for IB Physics Forces and Momentum? ↓
The key equations for Forces and Momentum are covered in the concept tutorial above. For a structured set of notes with all equations, conditions and worked examples, the GradePod Exam Pack includes a revision note template for every topic.
What are common exam mistakes in IB Physics Forces and Momentum? ↓
Common mistakes are covered in detail in the exam technique video above. The GradePod Exam Pack also includes exam-style questions with mark schemes so you can see exactly how marks are awarded and where students typically drop them.
How do I revise Forces and Momentum for the IB Physics exam? ↓
Follow the GradePod three-step method. First, watch the concept tutorial and tick off each learning objective on the checklist above as you go. Second, watch the exam technique video to see how IB-style questions are answered under exam conditions. Third, use the Exam Pack to practise independently with knowledge questions, exam questions and mark schemes. That's it. It works. I promise.