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

[myusername]

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].

 

[LowlyPion]

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.