Hi there, Sorry for so many questions, but I feel frankly stupid for getting these wrong, and yet I have no clue why they're so off. Any ideas of why the answers are what they are? Thanks so much. 15. A truck starts from rest and accelerates at 1.5 m/s2for 20 s. It then travels at constant speed for 20 s and decelerates at –1.5 m/s2until it stops. How far did the truck travel? a. 300 m b. 600 m *c. 1200 m d. 1500 m I got 600 because I used x = 1/2at^2 + v0t + x0 with v0 = 0, a = 1.5 m/s^2, t = 20 s, and x = ? 17. A ball is dropped from the top of a tall building. Just before it strikes the ground it is moving at 34.3 m/s. How tall is the building? a. 60 m *b. 131 m c. 250 m d. 125 m I got 60 m using v^2 = 2ax with v0 = 0, vf = 34.3 m/s, a = -9.8 m/s^2, and x = ? 18. A bus leaves a terminal en-route to a destination 200 km away. During the trip, the bus stopped for a 45 minute lunch. The bus arrived at its destination four hours after it departed. The average velocity was: a. 0 km/hr b. 61 km/hr c. 30 km/hr *d. 50 km/hr I used v = (delta)x / (delta)t with delta x = 200 km, and delta t = 3.25 hr 19. The position of a particle is given by x ( t ) =24 t – 4 t 2. At what time is the particle at rest? *a. 3s b. 8s c. 10s d. 2s I got 6, Wolfram got 6, I'm very confused. 20. The position of a particle as a function of time is given by 4 t +3 t 2–2 t 3. What is the average acceleration over the time interval from 0 to 4s? a. 90 m/s2 *b. –18m/s2 c. –102 m/s2 d. 102 m/s2 Since acceleration is the second-derivative of position, I got 6 - 12t. Which apparently is wrong. Thanks again.