12. The reaction to the air resistance on the rock is: a. The rock\'s weight; b.
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12. The reaction to the air resistance on the rock is: a. The rock's weight; b. The rock pushing down on the air; c. The rock pushing up on the air; d. The rock pulling up on Earth. A soccer player kicks a ball from the origin with an initial speed of 45 m/s at an angle of 40.0 above the horizontal. The ball lands in a hole in the ground that is 10.0 m below the level where the ball was kicked. Neglect air resistance. Questions 13-17 refer to this situation. 13. How long is the ball in the air? a. Less than 3.0 sec.. b. Between 3.0 and 6.0 sec. c. Between 6.0 and 9.0 sec. d. More than 9.0 sec. 14. How far has the ball traveled in the horizontal direction when it lands? a. Less than 50 m; b. Between 50 and 100 m; c. Between 100 and 150 m; d. More than 150 m. IS What is the speed of the ball when it lands? a. Less than 45 m/s; b. Between 45 and 50 m/s; c Between 50 and 55 m/s d. More than 58 m/s. 16 At the highest point of its path, the ball's speed is: Zero b Equal to 45 cos40°; c. Equal to 45 sin 40°; d Exactly 45 m/s. 17. At the highest point of its path, the net force acting on the ball is: a Zero b. Pointing in horizontal direction: c. Pointing straight down. d. None of the above 18. Ignoring air resistance, a ball that is hit at an angle of 400 will have the same range as a ball hit at an angle of: a. 200; b. 45 c. 500; d. None of the above.Explanation / Answer
13.
consider the vertical motion
Voy = initial velocity in vertical direction = 45 Sin40 = 28.93 m/s
Y = displacement = - 10 m
a = acceleration = - 9.8 m/s2
t = time
using the equation
Y = Voy t + (0.5) a t2
- 10 = 28.93 t + (0.5) (-9.8) t2
t = 6.23 sec
c. between 6 and 9 sec
14
X = Vox t = 45 Cos40 (6.23) = 214.8 m
d. more than 150 m
15.
Vfy = Voy + at = 28.93 + (-9.8) (6.23) = - 32.12 m/s
Vfx = Vox = 45 Cos40 = 34.5 m/s
speed = sqrt(V2fx + V2fy ) = sqrt((34.5)2 + (-32.12)2) = 47.14 m/s
b) between 45 and 50
16.
b 45 Cos40
17.
c. pointing straight down
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