Question

1. If you drop a rock from a height of 21 meters, how fast is the rock moving when it hits the ground?? Hint 60

Answer 1

Answer:

1) 20.49 ms⁻¹

2) ↓5 ms⁻¹ or -5 ms⁻¹

3) 300 ft/s

4)31.25 m

5) 17.32 ms⁻¹

Explanation:

1)v² = u² + 2as

v² = 0 + 2g(21)

v = 20.49 ms⁻¹

u = 0 as the object is dropped. let g = 10 ms⁻²

2)a = (v-u)/t ⇒↑ v = u + at    (a = g)

                            = 30 - 10×3.5 = ↓5 ms⁻¹ or -5 ms⁻¹

3) By the conservation of Energy principle,

Energy of the arrow just after launch = energy of the arrow just before hit ground

$\frac{1}{2} mu^{2} = \frac{1}{2} mv^{2}\\v = u$

So velocity just before hitting ground is 300 ft/s

4) s = ut + (1/2)at²

      = 0 + 0.5×10×(2.5)² =  31.25 m

5)v² = u² + 2as

v² = 0 + 2g(15)

 v = 17.32 ms⁻¹