Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.

Acceleration: Changes in Speed

A car that starts at a standstill and then increases its speed along a straight road is subject to an acceleration. That acceleration is due to the application of a force originating in its engine. A car that reduces its speed, by application of a force generated by its brakes for example, is also subject to an acceleration. In such situations, where acceleration is in a direction opposite to velocity, the acceleration is often called deceleration.

A constant acceleration (a) over a given time interval (Δt), results in a change in velocity (Δv) that can be calculated using the equation Δv = aΔt m/s (the Δ symbol is often used in physics equations to indicate a change in the quantity that follows it.)

The force of gravity near Earth’s surface results in a very familiar form of straight-line acceleration. The strength of Earth’s gravitational field near the surface (g) is an acceleration equal to 9.8 m/s2. So every second that an object falls, its speed increases by 9.8 m/s. A ball dropped from a rooftop, for example, would start with 0 velocity. It would have a velocity of 9.8 m/s one second after it was dropped. After two seconds, it would be moving 2(9.8) = 19.6 m/s.