Make sure that you remember that acceleration measures how quickly the velocity changes, that is, the rate of change of velocity. In physics, acceleration does not mean ‘gets faster’. Neither does it measure how much the velocity changes.
QUESTIONS 1. Define acceleration. 2. State an everyday name for negative acceleration. 3. As a stone falls, it accelerates from 0 m/s to 30 m/s in 3 seconds. Calculate the acceleration of the stone and give the units.
4. A racing car slows down from 45 m/s to 0 m/s in 3 s. Calculate the acceleration of the racing car and give the units.
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velocity
USING VELOCITY– TIME GRAPHS A velocity–time graph provides information on velocity, acceleration and distance travelled. In the graph, the object is already moving when the graph begins. If the object starts with a velocity of zero, then the line starts from the origin. Note that the object may not move to begin with. In this case the line will start by going along the time axis, showing that the velocity stays at zero for a while.
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time
REMEMBER
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For any graph of motion, always check carefully to see if it is a distance– time graph or a velocity–time graph. Although the graphs may look similar, you need to interpret them very differently. For instance, the slope of a distance–time graph gives the velocity (since velocity = distance/time) but the slope of a velocity–time graph gives the acceleration (since acceleration = change of velocity/time).
MOVEMENT AND POSITION
∆ Fig. 1.10 Steady speed is shown by a horizontal line. Steady acceleration is shown by a straight line sloping up.
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