How Do You Calculate the Change in Velocity of a Flying Saucer?

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A flying saucer decelerates from 20 m/s east to 50 m/s west over 9 seconds, requiring the calculation of change in velocity. The initial attempt incorrectly calculated velocity as 270 m/s without considering direction. The correct approach involves determining the distance traveled while decelerating eastward and then moving westward. The final velocity should be 15 m/s west, highlighting the importance of understanding velocity as a vector quantity. Proper unit analysis reveals the error in the initial calculation.
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Homework Statement


A flying saucer decelerates uniformly from 20 m/s [E] to 50 m/s [W] in 9 seconds. Find the velocity for the 9.0 interval.


Homework Equations


Velocity = acceleration x time
or Velocity = displacement/time


The Attempt at a Solution


Velocity1 = 20 m/s [E]
Velocity2 = 50 m/s [W]
Time = 9 s
Velocity = 30 x 9
= 270

However, the answer should be 15 m/s [W].
 
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You need to consider what velocity represents.

V = Δx/Δt

Your spaceship has been traveling East at 20m/s and reverses to 50m/s West.

First figure how far it went in stopping going East and then figure how far it went West. Then you can develop the Δx that it traveled in the 9 sec.

The units of your attempt should have told you there was a problem, because you had just the units of displacement in your answer.
 

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