Question

A rock is dropped from a bridge 320 feet above the river. The pathway that the rock takes can be modeled by the equation h= -16t2+320. How long will it take the rock to reach the river?

A
2.5 seconds

B
3.5 seconds

C
3.8 seconds

D
4.5 seconds

Answer 1

The rock will take about 4.5 seconds to reach the river

Explanation:

* lets study the situation of the rock

- The rock is dropped means the initial velocity is zero

- The motion is free fall under earth gravity

- The rock dropped from a bridge 320 feet above the river

- The equation of the pathway is h = -16t² + 320

- When the rock reach to the ground the height will be zero

* Now lets substitute h by zero in the equation to find t

∵ h = -16t² + 320

∵ h = 0

∴ 0 = -16t² + 320 ⇒ add 16t² to both sides

∴ 16t² = 320 ⇒ divide both sides by 16

∴ t² = 320/16 = 20

∴ t² = 20 ⇒ take √ for both sides

∴ t = √20 = 2√5 ≅ 4.5 seconds

* The rock will take about 4.5 seconds to reach the river