Copyright © 2007 Dorling Kindersley
From molecules in this sheet of paper, to planets in orbits, the objects around us are in constant motion. The simplest motion is in a straight line at constant speed. When the speed or direction change, scientists say that motion is accelerated.
When you drop a stone, it starts from rest (speed equals zero), then speeds up as it falls. The stone is accelerating. A force is always needed to produce acceleration—in this case, it is the force of gravity. Acceleration is slowed by FRICTION—in this case, air resistance.
Two measurements are needed to find speed—the distance moved, and the time taken. Speed is calculated by dividing the distance by the time. If a runner covers five meters (about 5 yards) in one second, his or her speed is five meters per second. A car that travels 100 miles in two hours has an average speed of 50 miles per hour.
The force of friction opposes motion when one surface slides, or tries to slide, over another. You feel friction as you drag your hand across a table. Friction is produced by forces between the molecules in the surfaces. Drag is the friction between a solid object and the fluid it is traveling through.
Friction is not always a problem—sometimes we use it to prevent or slow down motion. Without friction, your shoes would not grip the ground and you would slip and fall, and a car’s wheels would spin and skid. Friction is increased by making shoe soles and tires from soft, “sticky” materials such as rubber.
Friction between parts of machines can damage them by wear and tear. Friction also wastes energy as heat instead of movement. Friction can be reduced by using oil as a lubricant to make a slippery film between surfaces. Machines built with a streamlined shape reduce drag. Airplanes are designed to let air flow over them smoothly with the least resistance. The study of air flow is called aerodynamics.