Acceleration 1d kinematics

1. Oct 11, 2006

Kildars

Starting from rest, a speedboat reaches a speed of 3.4 m/s in 1.8 s. What is the boat's speed after an additional 2.6 s has elapsed, assuming the boat's acceleration remains the same?

I have tried finding the velocity for the first 1.8 seconds

$$v = v_0 + a t$$

3.4 = 0 + a(1.8)

3.4 / 1.8

a = 1.889

Then tried finding the final velocity assuming it was the same, but using 3.4 as the initial velocity..

v = 3.4 + (1.8)(3.6)

Didn't work.

I also tried v = (1.8)(4.4) adding the total time together, and using v0 as 0.

2. Oct 11, 2006

radou

It's v = 3.4 + 1.8*2.6

3. Oct 11, 2006

Kildars

Yeah that's a typo on my part, I meant to type 2.6

The velocity still comes out to 8.08 and that is incorrect.

4. Oct 11, 2006

civil_dude

Well, I get 8.31 s, and if that is not the answer, then Houston, we have a problem....

5. Oct 11, 2006

Kildars

How do you get 8.31?

6. Oct 11, 2006

civil_dude

I always use 3 significant figures, 3.4/1.8 = a =1.889, not 1.8, then round to 2 at the end.

So, (1.8 + 2.6)s is the elapsed time which equals 4.40s. Then, you multiply that by 1.889m/s/s to get 8.31m/s.

Last edited: Oct 11, 2006
7. Oct 11, 2006

Kildars

I guess that was my problem, wasnt doing enough figures.

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