Kids Research Express

. Acceleration often involves both a change in speed and a change in direction. Changing both components of velocity results in a curved path of motion. In these cases, the acceleration vector is the sum of two parts (components). One part, the tangential acceleration, acts along the direction of motion, parallel to the velocity, resulting in a change of speed. The other part, the radial acceleration, acts perpendicular to the direction of motion, resulting in a change of direction. In order to change the speed of an object moving in a circle, for example, one needs some acceleration along the direction of motion, in addition to the component of acceleration in the radial direction (pointing to the center) that keeps the object moving in a circle. In the case of a space shuttle in orbit, the radial acceleration is the force of gravity pulling the shuttle toward Earth, while a tangential acceleration is achieved by firing rockets along the direction of motion.

. 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 Ear...

. 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 dr...

. Acceleration (velocity), in physics, the rate of change of velocity over time . An accelerating object is speeding up, slowing down, or changing the direction in which it is moving. Acceleration is a vector quantity—that is, it has both a magnitude and a direction. Acceleration describes both the magnitude of an object’s change in velocity, and the direction in which it is accelerating. Acceleration can thus involve changes of speed, changes of direction, or both. As acceleration is a rate of change of velocity over time and velocity is measured in meters per second (m/s), the units of measurement of acceleration are meters per second per second (m/s2). Objects do not speed up, slow down, or change direction unless they are pushed in some way. Newton’s Second Law sums up this idea, stating that the acceleration of an object results from the application of a force. The acceleration (a) of an object with mass (m) produced by a given force (F) may be calculated using the equation F = ...

. Gravitation plays a crucial role in most processes on the earth. The ocean tides are caused by the gravitational attraction of the moon and the sun on the earth and its oceans. Gravitation drives weather patterns by making cold air sink and displace less dense warm air, forcing the warm air to rise. The gravitational pull of the earth on all objects holds the objects to the surface of the earth. Without it, the spin of the earth would send them floating off into space. The gravitational attraction of every bit of matter in the earth for every other bit of matter amounts to an inward pull that holds the earth together against the pressure forces tending to push it outward. Similarly, the inward pull of gravitation holds stars together. When a star's fuel nears depletion, the processes producing the outward pressure weaken and the inward pull of gravitation eventually compresses the star to a very compact size (see Star , Black Hole ). Acceleration If an object held near the surfac...