Laws of Motion

The One Thing

Forces determine how objects move: a non-zero resultant force produces acceleration, whereas balanced forces leave an object’s motion unchanged. Newton’s laws describe these effects, while inertia and conservation of momentum explain resistance to change and interactions within systems.

Reactions, Processes and Experiments

Key Terms

• Force: A push or pull that may change the motion, direction, speed, or shape of an object.
• SI unit of force: The SI unit of force is the newton (N). One newton produces an acceleration of 1 m/s² in a mass of 1 kg.
• Balanced forces: Forces equal in magnitude and opposite in direction, giving a zero resultant force and no change in motion.
• Unbalanced forces: Forces whose resultant is not zero and which cause acceleration or a change in motion.
• Resultant force: The vector sum of all forces acting on an object.
• Inertia: The property of an object by which it resists changes in its state of rest or uniform motion.
• Inertia of rest: The tendency of a resting object to remain at rest, such as dust falling from a carpet when it is beaten.
• Inertia of motion: The tendency of a moving object to continue moving, such as passengers moving forward when a bus stops suddenly.
• Inertia of direction: The tendency of an object to continue moving in the same direction, such as water leaving a rotating umbrella tangentially.
• Newton's First Law: An object remains at rest or continues in uniform straight-line motion unless acted upon by an unbalanced external force.
• Newton's Second Law: The acceleration of an object is directly proportional to the applied force and inversely proportional to its mass, in the direction of the force.
• Newton's Third Law: For every action, there is an equal and opposite reaction; the two forces act on different objects.
• Momentum: The quantity of motion of an object, equal to the product of its mass and velocity.
• Acceleration: The rate of change of velocity with time.
• Mass: The amount of matter in an object and a measure of its inertia.
• Weight: The gravitational force acting on an object, given by W = mg.
• Impulse: The product of force and the time for which it acts; it equals the change in momentum.
• Vector quantity: A physical quantity with both magnitude and direction.
• Conservation of momentum: The principle that the total momentum of an isolated system remains constant when no external force acts on it.

Easily Confused

• Balanced forces and unbalanced forces: Balanced forces have a zero resultant and do not change motion; unbalanced forces have a non-zero resultant and cause acceleration or another change in motion.
• Mass and weight: Mass is constant for an object and measures its inertia; weight is gravitational force and can vary from place to place.
• Inertia of rest and inertia of motion: Inertia of rest resists starting motion, whereas inertia of motion resists stopping or changing existing motion.
• Inertia of motion and inertia of direction: Inertia of motion describes the tendency to continue moving; inertia of direction describes the tendency to continue in the same direction.
• Newton's Second Law and Newton's Third Law: The Second Law relates force, mass, and acceleration for an object; the Third Law concerns equal and opposite forces acting on different objects.
• Action-reaction forces and balanced forces: Action-reaction forces act on different bodies and therefore do not cancel each other on one object; balanced forces act on the same object and give a zero resultant.
• Force and momentum: Force changes motion and is related to the rate of change of momentum; momentum is the product of mass and velocity.
• Force and impulse: Force is an interaction; impulse is the product of force and the time for which it acts and equals the change in momentum.

What Gets Asked

• Definitions of Newton’s laws: State that Newton’s First Law is also called the law of inertia, that the Second Law gives F = ma, and that the Third Law involves equal and opposite forces acting on different bodies.
• Distinguishing force types: Identify whether forces are balanced or unbalanced by considering the resultant force and whether the motion changes. The common error is claiming that balanced forces mean the object is necessarily at rest.
• Inertia examples: Classify dust falling from a carpet, passengers moving forward when a bus stops suddenly, and water leaving a rotating umbrella as inertia of rest, motion, and direction respectively.
• Numerical applications: Use F = ma, p = mv, W = mg, and Ft = mv - mu with the stated SI units. Marks are lost by confusing mass with weight or omitting the direction associated with a vector quantity.
• Newton’s Third Law explanations: Identify the two interacting bodies and state that the forces are simultaneous, equal, opposite, and of the same type. The forces must not be treated as cancelling on one object.
• Momentum conservation: Apply conservation of momentum only when no external force acts on the system, or when external forces are absent or balanced.