I figured out the time, but I can\'t get the average velocitycorrect. I calculat

Question

I figured out the time, but I can't get the average velocitycorrect. I calculated the velocity for each of the threesteps in the trucks movement and averaged them together and got itwrong. I can't figure out how I'm doing this wrong.

A truck on a straight road starts from rest and accelerates at2.0 m/s2 until it reaches aspeed of 20 m/s. Then the trucktravels for 20 s at constant speeduntil the brakes are applied, stopping the truck in a uniformmanner in an additional 5.0 s. (a) How long is the truck in motion?
1 s

(b) What is the average velocity of the truck during the motiondescribed?
2 m/s (a) How long is the truck in motion?
1 s

(b) What is the average velocity of the truck during the motiondescribed?
2 m/s

Explanation / Answer

phase 1 ------------- u=0 a=2m/sec2 t=10sec       because 20=0+2*t s1=0+1/2*2*100=100m phase2 ------------ u=20m/sec t=20sec so s2= 20*20=400m phase3 ---------- u=20m/sec t=5sec a=0-20/5=-4 s=20*5-1/2*4*25=50m hence 10+20+5=35sec is the time for the truck is in motion average velocity=550m/35=15.71m/sec

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