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3 Velocity Motion Motion Acceleration described? speedingupandslowingdown How are Velocity andAcceleration uphill anddownhill. abicycle asitgoes whathappensto thespeedof Describe but oneacceleration ispositive andoneisnegative. thecarisaccelerating, In bothcases, negative acceleration. thespeeddecreases. slows down, When thecar thecarhaspositive acceleration. increasing, ischanging. itsspeed because also accelerating, It is isdecreasing down Acarthat isslowing itsspeed. speed. Remember thatacceleration isachange in is alsoaccelerating. Butdown thatisslowing anobject up. itasspeeding think of position. change of of Speedistherate carmoves The faster andfaster. moving. thedriver presses thegaspedalandcarstarts green, turns hne tsed t ieto,or both. itsdirection, changes itspeed, Acceleration meansthatanobject direction itismoving. a change inhow orachange fastsomethingismoving in the achange invelocity canbeeither this, of Because and direction. is accelerating. theobject changes, anobject When thevelocity of velocity. mgn a en rvndw od fthespeedis If acarbeingdrivenImagine aroad. down you probably somethingaccelerating, When you thinkof When thelight isnotmoving. A carsittingatastoplight Remember thatvelocity isameasure thatincludesbothspeed Read toLearn Before YouBefore Read A c c e le r at io The decreasing speed is called n stert fchange of of is therate D
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Acceleration has direction, just like velocity. In the figure below, both cars are accelerating because their speeds are changing. When a car’s acceleration and velocity are in the same direction, the speed increases and the acceleration is pos- itive. Car A has positive acceleration. When a car is slowing down, the acceleration and velocity are in opposite directions. The acceleration is negative. Car B has negative acceleration. Car A Car B Picture This Acceleration Acceleration 1. Describe the acceleration of the cars in each figure. Velocity Velocity
Does changing direction affect acceleration? A change in velocity is either a change in an object’s speed or its direction. When a moving object changes direction, its velocity changes and it is accelerating. The speed of a horse moving around on a carousel remains constant, but it is constantly changing direction. So, the horse is accelerating. How do you calculate acceleration? Acceleration is the rate of change in velocity. To calculate 2. Use Variables Write what acceleration, you first find the change in velocity. To find vf and vi mean. change in velocity subtract the beginning velocity of an object from the velocity at the end of its movement. Beginning
velocity is called the initial velocity, or vi. Velocity at the end is called the final velocity, or vf. change in velocity final velocity initial velocity vf vi
If motion is in a straight line, the change in speed can be Applying Math used to calculate the change in velocity. The change in speed is the final speed minus the initial speed. 3. Calculate Suppose a bird To find acceleration, divide the change in velocity by the takes off from a tree and flies in a straight line. length of time during which the velocity changed. It reaches a speed of 10 m/s. change in velocity (v v ) What is the change in the (a) f i acceleration time (t) or a t bird’s velocity? Copyright of a division © Glencoe/McGraw-Hill, The McGraw-Hill Inc. Companies, The SI unit for velocity is meters per second (m/s). To find acceleration, velocity is divided by the time in seconds (s). So, the unit for acceleration is m/s2.
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