How Long Does It Take for Two Rocks Thrown from a Bridge to Hit the Water?

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SUMMARY

The discussion focuses on calculating the time difference between two rocks thrown from a bridge 46 meters above a river, one thrown downward at 30 m/s and the other thrown upward at the same speed. The key to solving this problem lies in applying the kinematic equations of motion under constant gravitational acceleration. The relevant equation is derived from the initial velocity, displacement, and acceleration, allowing for the determination of the time it takes for each rock to reach the water. The final solution reveals the elapsed time between the splashes of the two rocks.

PREREQUISITES
  • Understanding of kinematic equations of motion
  • Knowledge of gravitational acceleration (9.81 m/s²)
  • Familiarity with initial velocity concepts
  • Basic algebra for solving equations
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  • Study the kinematic equation: \(d = v_i t + \frac{1}{2} a t^2\)
  • Learn how to calculate time of flight for projectile motion
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  • Investigate the impact of air resistance on falling objects
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Homework Statement



Heather and Jerry are standing on a bridge 46 m above a river. Heather throws a rock straight down with a speed of 30 m/s. Jerry, at exactly the same instant of time, throws a rock straight up with the same speed. Ignore air resistance. How much time elapses between the first splash and the second splash?

Homework Equations



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The Attempt at a Solution

 
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Solve for the time it takes both rocks to hit the water under a constant gravitational acceleration. You know initial velocity, displacement, and acceleration. All that you need is time. So find the best kinematic equation for your problem.
 
The book claims the answer is that all the magnitudes are the same because "the gravitational force on the penguin is the same". I'm having trouble understanding this. I thought the buoyant force was equal to the weight of the fluid displaced. Weight depends on mass which depends on density. Therefore, due to the differing densities the buoyant force will be different in each case? Is this incorrect?

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