A balloon is floating in air. Which of the following describes the microscopic cause of the downward force exerted on the air by the balloon?

a) Electrostatic attraction between the nuclei in the molecules in the air and the electrons in the molecules of the balloon
b) Electrostatic repulsion between the nuclei in the molecules in the air and the nuclei in the molecules of the balloon
c) Electrostatic repulsion between the electrons in the molecules in the air and the electrons in the molecules of the balloon
d) Gravitational attraction between the molecules of the air and the molecules of the balloon

The base of a 50-meter tower is at the origin; the base of a 50-meter tree is at (0, 50, 0). the ground is flat and the z-axis points upward. the following parametric equations describe the motion of six projectiles each launched at time t = 0 in seconds. (i) r (t) = (50 + t2)k (ii) r (t) = 2t2 j + 2t2k (iii) r (t) = 50 i + 50 j + (50 − t2)k (iv) r (t) = 2t j + (50 − t2)k (v) r (t) = (50 − 2t) i + 2t j + (50 − t)k (vi) r (t) = t i + t j + tk (a) which projectile is launched from the top of the tower and goes downward? at time t = , the projectile hits the ground at point (x, y, z) = . (b) which projectile hits the top of the tree?