Motion and forces Study Guide

Weight is the force exerted on an object due to gravity. It is directly proportional to the object's mass and the gravitational field strength (g) at its location. Gravitational field strength is defined as the force per unit mass exerted by a gravitational field. On Earth's surface, the gravitational field strength is approximately 9.8 N/kg, meaning a 1 kg object experiences a force of 9.8 Newtons. This relationship is expressed by the formula: Weight (N) = Mass (kg) × Gravitational field strength (N/kg). Consequently, an object's weight can change if its location changes to a region with a different gravitational field strength, such as on the Moon, where 'g' is much lower, or on a more massive planet where 'g' is significantly higher. Mass, however, is a measure of an object's inertia and the amount of matter it contains, and it remains constant regardless of location.

When an object moves in a circular orbit at a constant speed, its velocity is continuously changing. This might seem counter-intuitive because 'speed' is constant, but velocity is a vector quantity, meaning it has both magnitude (speed) and direction. In circular motion, even if the magnitude of the velocity (speed) remains the same, the direction of motion is constantly altering as the object travels around the curve. At any point in its orbit, the object's direction of travel is tangential to the circle. As this tangent direction is always shifting, the velocity vector is also always shifting, hence the velocity itself is not constant. This change in velocity implies an acceleration, even at constant speed, directed towards the centre of the circle.