Preview
IB DP Physics — Unit A: Space, Time and Motion | SL & HL | A.2 Forces and Momentum
📋 What You Will Learn
- Observe and describe everyday situations where forces cause objects to accelerate, decelerate, or remain in equilibrium
- Define force, net force, inertia, and momentum, and distinguish between mass and weight
- Construct accurate free-body diagrams (FBDs) showing all forces acting on a body with correct directions and labels
- State, apply, and derive consequences of Newton's three laws of motion in a variety of physical contexts
- Apply Newton's second law (\( F_{\text{net}} = ma \)) to single-body and multi-body systems, including inclined planes and connected objects
- Evaluate common misconceptions about force and motion, and connect Newton's laws to momentum, energy, and circular motion across the IB Physics course
🌍 Real-World Context When a rocket launches from Earth, its engines produce an enormous downward thrust of hot gas. In response, the rocket accelerates upward — yet nothing physically pushes it from below. When a car brakes suddenly, passengers lurch forward even though no force is pushing them forward. And when two ice skaters push off each other, both move apart even though only one initiated the push. These everyday observations seem puzzling until we understand the three fundamental rules that govern every force interaction in the universe — Newton's Laws of Motion.
🔍 Guiding Question How do forces acting on a body determine its state of motion, and how can we use free-body diagrams and Newton's laws to predict and explain the behaviour of objects in any mechanical situation?
Physical Setup: What Is a Force?
Before we can apply any laws, we need to understand what a force actually is. A force is a push or a pull — an interaction between two objects. When you push a shopping trolley, your hand exerts a force on the trolley. When Earth pulls you downward, gravity exerts a force on your body. Forces are not properties of a single object; they always arise from an interaction between two objects.
Forces have both magnitude and direction, which means they are vector quantities. The direction of a force matters enormously — a 10 N force pushing an object to the right is fundamentally different from a 10 N force pushing it to the left. When multiple forces act on the same object simultaneously, their combined effect is described by the net force (also called the resultant force), which is the vector sum of all individual forces.
In the diagram above, the book is stationary — it is in equilibrium. Two forces act on it: its weight pulling it downward due to gravity, and the normal reaction force from the table pushing it upward. These two forces are equal in magnitude and opposite in direction, so the net force is zero. This simple example already hints at Newton's First Law: when the net force is zero, the object does not accelerate.
Core Concepts and Definitions
To work with Newton's laws precisely, we need to define several key quantities carefully. These definitions are not arbitrary — each one captures a specific physical idea that will appear in equations and exam questions. We will build them up in the order we need them, starting with the most fundamental.
📖 Key Definition — Force A force is a vector quantity that describes an interaction between two objects. It can cause a change in the velocity (speed or direction) of an object, or a deformation of the object. The SI unit of force is the newton (N), where \( 1 \, \text{N} = 1 \, \text{kg m s}^{-2} \).
📖 Key Definition — Mass Mass is a scalar quantity that measures the amount of matter in an object. It is also a measure of an object's inertia — its resistance to changes in its state of motion. Mass is measured in kilograms (kg) and does not change with location.
📖 Key Definition — Weight Weight is the gravitational force acting on an object due to a gravitational field. It is a vector quantity directed toward the centre of the gravitational body (e.g. toward the centre of Earth). Weight \( W = mg \), where \( g \) is the gravitational field strength at that location (approximately \( 9.81 \, \text{m s}^{-2} \) near Earth's surface).