Motion and forces Study Guide

When an object moves in a circular path, its velocity vector is continuously changing direction. Although the speed might remain constant, the change in direction means there is an acceleration. This acceleration, by definition, must be caused by a resultant force. For circular motion, this resultant force is directed towards the centre of the circle and is known as the centripetal force. Without this continuous inward force, the object would not follow a curved path but would instead move in a straight line, tangential to the circle at the point where the force ceased to act. Examples include the tension in a string holding a whirling object, the gravitational force keeping a satellite in orbit, or the frictional force between a car's tyres and the road when cornering.

Newton's third law of motion states that for every action, there is an equal and opposite reaction. This means that whenever one object exerts a force on a second object, the second object simultaneously exerts an equal and opposite force back on the first object. These forces are always in pairs, act on different objects, and are of the same type (e.g., both gravitational or both electromagnetic). It's important to understand that these forces do not cancel each other out because they act on different objects. When an object is in equilibrium, the net force acting on it is zero. This equilibrium can be static (at rest) or dynamic (moving at a constant velocity). While Newton's third law describes interaction pairs, equilibrium is about the balance of all forces acting on a single object. For example, a book resting on a table experiences a downward gravitational force from Earth and an upward normal force from the table. The gravitational force on the book and the normal force from the table are not a Newton's third law pair; rather, they are two forces acting on the book that are balanced, leading to equilibrium. The Newton's third law pair for the gravitational force on the book is the gravitational force the book exerts on the Earth. The Newton's third law pair for the normal force from the table on the book is the force the book exerts downwards on the table.