|Jun1-12, 11:06 AM||#1|
A few quick problems with kinematics
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?
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
c. –102 m/s2
d. 102 m/s2
Since acceleration is the second-derivative of position, I got 6 - 12t. Which apparently is wrong.
|Jun1-12, 12:01 PM||#2|
Blog Entries: 1
|Jun1-12, 07:38 PM||#3|
|kinematics, kinematics free fall|
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