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 Direction

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Acceleration can also involve a change in the direction an object is moving. A ball on the end of a string being whirled overhead at a constant speed is an example of this type of acceleration. Since velocity is a vector quantity like acceleration, velocity has a speed component (magnitude) and a direction component. At every instant in its motion overhead, the ball’s velocity is changing because the velocity’s direction is different at every point on the circular path. Changing velocity is acceleration. The acceleration of the object is directed toward the center of the circle, and is of constant magnitude a=v2/r, where r is the radius of the circle and v is the speed of the object (with mass m). This type of acceleration is called radial or centripetal acceleration. Radial acceleration results from the action of the force generated by the string that pulls the ball toward the center of the circle. In the case of a satellite in orbit, the force causing the radial acceleration is Earth’s gravity pulling the satellite toward the center of the planet.