Solve 500g Falling Rock Problem: Initial Speed

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Homework Help Overview

The problem involves a 500g rock thrown straight down from a bridge, impacting the water 5.2 m below at a speed of 12.5 m/s. Participants are exploring the initial speed of the rock using kinematic and momentum concepts.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss the use of kinematic equations to relate initial velocity, final velocity, displacement, and acceleration. There is some uncertainty about the appropriate equations to apply, with one participant questioning the relevance of momentum equations.

Discussion Status

The discussion is ongoing, with participants offering different approaches to the problem. Some guidance has been provided regarding kinematic equations, and there is an exploration of the relationship between momentum and the problem context.

Contextual Notes

There is a noted confusion regarding the classification of the problem as a momentum assignment, with participants expressing differing opinions on the appropriate methods to use.

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Homework Statement



A 500g rock is thrown straight down from a bridge and hits the water 5.2 m below. If the rock strikes the water at a speed of 12.5 m/s, what was the initial speed of the rock?

Homework Equations



I'm not totally sure...

The Attempt at a Solution



I have no idea how to even start this. Could someone help me start to solve it, please?
 
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Can you find a kinematic equation which relates the initial velocity, final velocity, displacement and the acceleration?
 
It's apparently supposed to be a momentum equation...but I have to disagree with them there...

Well, I suppose vf^2 = vi^2 + 2ad is worth a shot.

Which gives me...

Vi^2 = vf^2 - 2ad

That works out to the right answer...but it doesn't look like it belongs on a momentum assignment.

Thanks!
 
OK.
Find the final momentum and time to travel a distance 5.2 m. Force acting on the rock is known. Using Newton's second law of motion, find the initial momentum. From that you can find the initial velocity.
 

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