Acceleration

Key Formulas

 a average acceleration m/s2 Δv change in velocity m/s Δt elapsed time s
 a average acceleration m/s2 vf final velocity m/s vi 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 vi or vf is implicity given to be zero. For example, “comes to a stop” indicates that vf is zero. Similarly, “a ball is dropped” (rather than thrown) usually indicates that vi is zero.
• Objects whose velocity is decreasing have negative acceleration. Therefore, words such as “a car skids, decelerating at 6 m/s2” often mean that the acceleration is actually -6 m/s2. 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/s2, since the acceleration unit comes from velocity units (m/s) divided by time units (s). While most physics texts use m/s2, math texts often use m/s/s.