This chapter investigates the cause of changes in motion — force — and presents Newton’s three laws. A force can start, stop, speed up, slow down or turn a moving object, or change its shape; forces are measured in newtons (N) with a spring balance. Several forces can act together, and their net (resultant) force decides the outcome; balanced forces cancel out while an unbalanced force changes motion. Friction is a contact force that opposes relative motion. Newton’s first law (law of inertia) says an object stays at rest or in uniform straight-line motion unless an unbalanced force acts on it; inertia is the tendency to resist change in the state of motion and increases with mass. Momentum is \(p = mv\), the product of mass and velocity. Newton’s second law states that the rate of change of momentum is proportional to the applied force and is in its direction, giving the central relation \(F = ma\); a given force produces less acceleration on a heavier body. Newton’s third law states that to every action there is an equal and opposite reaction, the two forces acting on different bodies — explaining how a canoe moves forward when water is pushed back, gun recoil, and walking. The chapter also introduces conservation of momentum: in the absence of an external force the total momentum of a system stays constant, as seen in collisions and recoil.
Force, net force and frictionInertia and Newton’s first lawMomentum and Newton’s second law (\(F = ma\))Action and reaction: Newton’s third lawConservation of momentum
Key terms
Force
A push or pull that can change the speed, direction or shape of an object; measured in newtons (N).
Inertia
The natural tendency of an object to resist any change in its state of rest or uniform motion; it increases with mass.
Momentum
The product of an object’s mass and velocity, \(p = mv\); a vector pointing in the direction of motion.
Newton’s second law
The rate of change of momentum is proportional to the applied force, leading to \(F = ma\).
Newton’s third law
For every action there is an equal and opposite reaction, the two forces acting on different bodies.
Conservation of momentum
In the absence of an external unbalanced force, the total momentum of a system remains constant.
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