Physics Question: Average Speed

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Date Posted: 2/8/2008 6:48:50 AM Status: Closed

Average Speed

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Question Details:

Hi

Can anyone advise please?

Question:
A vehicle runs a straight line with the following kinematic parameters.
Starting at t (time) = 0 the acceleration is constant up to a speed of 185ms^-1, which is reached at 2.8s. The velocity is constant until 4.4 seconds after starting.

What is the average speed for the first 3 seconds?

A 98.7 ms^-1
B 113.5 ms^-1
C 130.5 ms^-1
D 150.1 ms^-1
E 172.6 ms^-1
F 198.5 ms^-1
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Actually, this question is long and I just quoted the important part related to the question of the average speed for the first 3 seconds. So let me know if you need the whole question then I will post. : )

I already calculated the acceleration from t = 0 to 2.8s which is 66.1ms^-2.
So I thought the average was just the (sum of velocity at t=0,1,2,3) / 3.
But my answer doesn't match to the choices of answer.

My calculation:
I know the acceleration from t = 0 to 2.8s is constant, 66.1ms^-2.
For the constant acceleration, use the constant acceleration model equation,
v = u + kt (where u = initial velocity, k = constant acceleration)

So,

(at t= 0) v = u + kt
                 = 0 + (66.1)(0)
                 = 0

(at t = 1)    = 0 + 66.1 * 1
                  = 66.1

(at t = 2) = 0 + 66.1 * 2
               = 132.3

(at t = 3) = 185..... because "velocity is constant from 2.8s to 4.4s with 185ms^-1

So, (0 + 66.1 + 132.3 + 185) / 4 = 95.8ms^-1..... this is my answer but it's wrong.

Can anyone advise?
Thank you!!

Average Speed

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