Forces Study Guide

When a force causes an object to move some distance, 'work' is said to be done. We define work done as the amount of energy transferred from one form to another, or from one store to another. The key relationship between work done, force, and distance is: Work done = Force × Distance. The force must be applied in the same direction as the distance moved. For example, if you push a box horizontally across a floor, the force you apply (horizontally) causes it to move a horizontal distance. Therefore, work is done. If you lift an object, the upward force you exert against gravity causes an upward displacement, meaning work is done against gravity. The unit for work done is the joule (J), which is equivalent to one newton-metre (Nm). For energy transfers, such as lifting an object, chemical potential energy from your muscles is converted into gravitational potential energy of the object, and some energy is also transferred to the surroundings as heat due to friction or muscle movement. Whenever work is done, there is always an energy transfer involved. For instance, when a car engine applies a force to move the car, the chemical potential energy in the fuel is converted into kinetic energy of the car, with work being done by the engine's force over the distance travelled.

When an object is in free fall, it is moving solely under the influence of gravity. This means that if we ignore the effects of air resistance, all objects, regardless of their mass or shape, will accelerate downwards at the same rate. This constant acceleration is due to the force of gravity pulling objects towards the Earth's centre. On Earth, this acceleration due to gravity is approximately 9.8 m/s². For many GCSE calculations, this value is often rounded to 10 m/s² for simplicity, but it's important to remember the more precise value. This acceleration means that for every second an object is falling, its downward velocity increases by about 9.8 m/s. The concept of free fall is a simplification because in reality, air resistance always plays a role, especially for objects falling through the atmosphere. Air resistance is a force that opposes motion through the air, and its effect becomes more significant as an object's speed increases. In a vacuum, however, where there is no air, a feather and a bowling ball would indeed fall and hit the ground at the exact same time, both accelerating at 9.8 m/s².