define uniform acceleration
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Uniform acceleration means that a body changes his velocity equally with equal interval of time.
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uniform acceleration in physics is synonymous with constant acceleration, and in first year physics courses the primary example is that of the acceleration near Earth's surface of an object due to the Earth's gravitational force on the object. You must remember that acceleration is a vector, so it can be constant only if both its magnitude and direction remain the same throughout the motion under consideration. Since the gravitational force Fg=mgFgmg of the Earth (near its surface) on an object of mass mm is always vertically downward, and of constant magnitude mgmg, where g=9.8g9.8 m/s22, Newton's second law relating this force to the acceleration it produces, Fg=ma=mgFgmamg, gives for the acceleration the constant vector a=gag.
A good rule of thumb to remember is this: if a particle or rigid body is observed to move in time along a curved path, it is being accelerated, hence some force is acting on it. This is due simply to the fact that the velocity vector is tangent to the path, and if the path curves, the direction of the velocity vector is changing with time. A changing velocity vector is, by definition, an acceleration. And acceleration happens only if a net force is acting.
In a comment you ask if a rock on a string being spun around your head is being uniformly accelerated. The answer is no. Not even for what is referred to as uniform circular motion, defined as motion restricted to a circle, and for which the angular velocity is constant (hence also the tangential speed...but not tangential velocity). In this case, although there is no tangential acceleration there is a radial acceleration vector always pointing toward the center of the circle (centripetal acceleration). Since the direction of this center-pointing vector is different at different points on the circle, the direction of the centripetal acceleration is changing (although its magnitude is constant), so the acceleration vector is not constant, hence this is not a case of uniform acceleration.
A good rule of thumb to remember is this: if a particle or rigid body is observed to move in time along a curved path, it is being accelerated, hence some force is acting on it. This is due simply to the fact that the velocity vector is tangent to the path, and if the path curves, the direction of the velocity vector is changing with time. A changing velocity vector is, by definition, an acceleration. And acceleration happens only if a net force is acting.
In a comment you ask if a rock on a string being spun around your head is being uniformly accelerated. The answer is no. Not even for what is referred to as uniform circular motion, defined as motion restricted to a circle, and for which the angular velocity is constant (hence also the tangential speed...but not tangential velocity). In this case, although there is no tangential acceleration there is a radial acceleration vector always pointing toward the center of the circle (centripetal acceleration). Since the direction of this center-pointing vector is different at different points on the circle, the direction of the centripetal acceleration is changing (although its magnitude is constant), so the acceleration vector is not constant, hence this is not a case of uniform acceleration.
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