Answer:
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