# Simple kinematic problem

1. Nov 21, 2011

### hotshot1kille

1. The problem statement, all variables and given/known data

An airplane maintains a constant acceleration of 4.0m/s^2 as it speeds up from 16m/s [E] to 28 m/s [E].

A what is the average velocity?

b) how long does the airplane accelerate?

c) when is the instantaneous velocity equal to the average velocity for the interval?

a= 4.0 m/s^2

vi= 16m/s [E]
vf= 28m/s[E]

2. Relevant equations

3. The attempt at a solution

i found the answer to A and B

Average velocity = 22 m/s. Total time = 3.0 s

I don't know how instantaneous velocity is related to average velocity for the interval. Or how to find the answer. Someone help me?

Last edited: Nov 21, 2011
2. Nov 21, 2011

### Zula110100100

If you found the average velocity to be 22m/s it is asking when the instantaneous velocity is 22m/s which should just be V = vi + at

3. Nov 21, 2011

### hotshot1kille

Ok got it thanks :)

4. Nov 21, 2011

### PeterO

When are the two numerically equal?

Suppose you accelerated away from traffic lights for 10 seconds. After 10 seconds you may have reached quite a high speed, but your average speed will be less than that. At some time during the 10 seconds, the speedo will have shown a speed that just happens to equal your average speed [you won't have realised at the time, since you need the full trip to calculate your average speed]. When was that time?

5. Nov 21, 2011

### hotshot1kille

ahh ok thank you so much!

6. Nov 21, 2011

### PeterO

btw; Part C of this problem is a learning exercise, so that in future you will know the bit I highlighted red.

7. Nov 21, 2011

ok thank you