Copyright © 2007 Dorling Kindersley
Dynamics is the study of how objects move when forces act on them. Normally objects stay still or move along at a steady pace. They resist changes in their motion because of their INERTIA. Once they start moving, they tend to carry on doing so because of their MOMENTUM. Most types of everyday movement can be explained by just three simple LAWS OF MOTION. These were originally worked out by English physicist Sir Isaac Newton.
Newton’s three laws of motion (often called Newton’s laws) explain how forces make objects move. When the forces that are acting on an object are balanced, there is no change in the way it moves. When the forces are unbalanced, there is an overall force in one direction. This changes the object’s speed or the direction in which it is moving. Physicists call a change in speed or direction an acceleration.
An object will stay still or move along at a steady pace unless a force acts on it. For example, a rocket on a launchpad remains in place because there is no force acting on it to make it move.
When a force acts on an object, it makes the object change speed or move in a different direction. When the rocket’s engines fire, the force they produce lifts the rocket up off the launchpad and into the air.
When a force acts on an object, the object pulls or pushes back. This reaction is equal to the original force but in the opposite direction. As the hot gases shoot down from the engines, an equal force pushes the rocket up.
Newton’s three laws of motion enabled him to produce a complete theory of gravity, the force that dominates our Universe, and to explain why the Moon circles round Earth. Newton also made major discoveries about optics (the theory of light) and explained how white light is composed of many colours.
Newton’s first law explains that objects remain where they are or move along at a steady speed unless a force acts on them. This idea is known as inertia. The greater the weight (or mass) of an object, the more inertia it has. Heavy objects are harder to move than light ones because they have more inertia. Inertia also makes it harder to stop heavy things once they are moving.
As a car accelerates, passengers are thrown backwards; when a car brakes or crashes, passengers are thrown forwards. In both cases, this is because the inertia caused by their mass resists the change in movement. During crash-tests, dummies that weigh the same as a human body are used to help test safety belts and airbags.
Moving objects carry on moving because they have momentum. The momentum of a moving object increases with its mass and its speed. The heavier the object and the faster it is moving, the greater its momentum and the harder it is to stop. If a truck and a car are travelling at the same speed, it takes more force to stop the truck because its greater mass gives it more momentum.
A foal is smaller and has less mass than a horse. When a foal and a horse gallop along together at the same speed, the horse has more momentum because of its greater mass. This means that it is easier for the foal to start moving, stop moving, and change direction than the horse. The momentum of a moving object is equal to its mass times its velocity.