Copyright © 2007 Dorling Kindersley
Everything in the world is moving. Even things that seem still are in motion, because the atoms inside them are vibrating. An object moves from one place to another when forces act on it and those forces are not balanced. When a force in one direction changes the SPEED or VELOCITY of an object, or the way it moves, this is known as ACCELERATION.
A roller coaster’s carriages accelerate (gather speed) when the force of gravity pulls them down a steep incline. The speed and weight of the carriages then keeps them moving, even when they continue in a straight line or climb upwards.
When we think of speed, we think of cars, jet planes, anything that moves quickly. To scientists, however, speed means things moving fast or slow. Speed is defined as the distance an object travels in a certain amount of time. Fast cars travel at higher speed than slow cars, so they can go further in the same time.
Velocity is the speed of an object moving in a particular direction. Two cars driving at the same speed have different velocities if one of them goes north and the other goes south. Velocity is measured in metres per second (mps), which divides the distance travelled by the time taken, in a specific direction.
When we talk of things accelerating, we usually mean they are speeding up. In science, however, acceleration means any change in an object’s velocity, whether it goes faster, slower, or changes direction. According to Newton’s second law of motion, a force is always needed to produce an acceleration. The bigger the force, the faster the change in velocity.