Forces Study Guide

Weight is a fundamental force experienced by all objects with mass when they are in a gravitational field. Unlike mass, which is a measure of the amount of matter in an object and remains constant regardless of location, weight is the force of gravity acting on an object and therefore depends on the strength of the gravitational field. The unit of weight is the Newton (N), as it is a force. This force always acts downwards towards the centre of the gravitating body, such as the Earth.

The strength of a gravitational field is referred to as the gravitational field strength (g). It is defined as the force of gravity exerted per unit mass. On Earth, the approximate gravitational field strength is 9.8 N/kg. This value means that for every kilogram of mass an object possesses, the Earth exerts a gravitational force (weight) of 9.8 Newtons on it. This value can vary slightly depending on altitude and geological features, but 9.8 N/kg is a standard approximation.

The relationship between an object's weight (W), its mass (m), and the gravitational field strength (g) is given by the formula: Weight = mass × gravitational field strength, or W = m × g. For example, an object with a mass of 10 kg on Earth would have a weight of 10 kg × 9.8 N/kg = 98 N.

Weight is typically measured using a spring balance, also known as a Newton meter. This device works by measuring the extension of a spring which is directly proportional to the force applied to it. When an object is hung from the spring balance, the spring stretches due to the object's weight, and a scale indicates the force in Newtons.

When an object moves in a circle at a constant speed, it might seem counter-intuitive that its velocity is continually changing. This is a fundamental concept in physics, especially within circular motion. Velocity is a vector quantity, meaning it has both magnitude (speed) and direction. While the magnitude of the velocity (the speed) might remain constant, the direction of travel is continuously altering as the object traces its circular path. At any point on the circle, the object's instantaneous velocity vector is tangential to the circle at that point. As the object moves, this tangent direction changes, even if the speed along the path is constant. Because the direction component of the velocity vector is different at every instant, the overall velocity vector is continuously changing. This continuous change in velocity, even at constant speed, implies an acceleration (specifically, centripetal acceleration), which is directed towards the center of the circle. This acceleration is what causes the object to change direction and stay on its circular path, rather than flying off in a straight line.