3.3 Projectile Motion Under the influence of gravity alone, an object near the surface of the Earth will accelerate downwards at 9.80m/s 2.
Example A ball thrown horizontally from a 13m high building strikes the ground 5.0m from the building. With what velocity was the ball thrown?
Solution By definition, we have y = v oy t – ½ gt 2 t 2 = -2y/g = - [2×-13]/9.8 = 2.65s 2 t = 1.63s Also, by definition we have x = v ox t + ½ a x t 2 v ox = x/t = 5/1.63 = 3.07m/s
Problems to be solved 3.12, 3.24, 3.41, 3.45, 3.51, 3.72 B3.1 A snowball is thrown from 2m above the ground at a velocity of 10m/s directed at 30° above the horizontal. (a)How long will the snowball be in the air? (b) Where will it land?
B3.2 Some books advice serving a tennis ball at an angle below the horizontal direction. To see if this is sound advice, suppose a ball is struck at an angle of 5° downward at a height of 2.4m with the relatively high speed of 30m/s. How high will it be when it reaches the net 12m away? (The net is 0.9m high. Neglect air resistance.)
B3.3 A tennis ball is served 2.5m above the ground at an angle of 5° above the horizontal direction with an initial speed of 30m/s. (a) When will it hit the ground? (b) How far will it travel?