# Acceleration

## Key Formulas

a | average acceleration | m/s^{2} | |

Δv | change in velocity | m/s | |

Δt | elapsed time | s |

a | average acceleration | m/s^{2} | |

v_{f} | final velocity | m/s | |

v_{i} | initial velocity | m/s | |

Δt | elapsed time | s |

## Tips to Remember

- Use the second formula when you have the beginning and ending velocity. Use the first formula (with the Δv) when you care how much the velocity
*changes*, but you don’t know the starting or ending velocities. (e.g., “How much faster is the ball going after three seconds?”) - Watch for cases where v
_{i}or v_{f}is implicity given to be zero. For example, “comes to a stop” indicates that v_{f}is zero. Similarly, “a ball is dropped” (rather than thrown) usually indicates that v_{i}is zero. - Objects whose velocity is decreasing have negative acceleration. Therefore, words such as “a car skids, decelerating at 6 m/s
^{2}” often mean that the acceleration is actually -6 m/s^{2}. Just don’t assume that negative acceleration*always*means that something is slowing down. Changing from -5 m/s to -10 m/s is a negative acceleration since the velocity is decreasing (getting more negative), but the object is moving faster, not slower. - Some texts will use the units m/s/s for acceleration instead of m/s
^{2}, since the acceleration unit comes from velocity units (m/s) divided by time units (s). While most physics texts use m/s^{2}, math texts often use m/s/s.

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