"an ice skater applies a horizontal force to a
20.-kilogram block on frictionless, level ice,
causing the block to accelerate uniformly at
1.4 meters per second2 to the right. after the
skater stops pushing the block, it slides onto a
region of ice that is covered with a thin layer
of sand. the coefficient of kinetic friction
between the block and the sand-covered ice is
0.28.

calculate the magnitude of the force of friction
acting on the block as it slides over the
sand-covered ice. [show all work, including the
equation and substitution with units.]

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?