Kinetic/Potential Energy Problem

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Homework Help Overview

The discussion revolves around a problem involving kinetic and potential energy related to a cliff diver's jump. The original poster presents a scenario where a diver with a mass of 90 kg jumps from a height of 20 m, seeking to find his potential and kinetic energy at the top of the cliff, the point at which his kinetic energy is 450 J, and his speed upon entering the water.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants discuss the calculation of potential energy (PE) and kinetic energy (KE) at the top of the cliff, with one participant stating a value for PE and questioning the relationship between PE and KE. There are inquiries about the conditions affecting KE during the dive, particularly the assumption of starting from rest and the implications of energy conservation principles.

Discussion Status

The discussion is ongoing, with participants exploring the relationship between potential and kinetic energy, particularly in the context of free fall. Some guidance has been offered regarding the initial conditions of the dive and the nature of energy transformations, but no consensus has been reached on the specific calculations for the point where KE equals 450 J.

Contextual Notes

Participants note the absence of information regarding any initial velocity at the start of the dive, which is critical for determining the kinetic energy at various points during the fall.

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Homework Statement



Given
mass = 90kg
distance = 20m above sea level.

Find his potential and kinetic energy at the top of the cliff, at what point in his leap is his kinetic energy 450 J and at what speed does he enter the water?

Homework Equations





The Attempt at a Solution



I can solve for PE and KE at the top of the cliff. Have trouble calculating when his KE = 450 J and his velocity entering the water.
 
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Hello, mathpat. Could you please state the problem exactly as given to you? It's not clear what's going on at the top of the cliff initially.
 
sure it's as follows:

Sherlock, a cliff diver, is preparing for his final cliff jump into the ocean. He has a mass of 90 kg and stands on a cliff 20m above the sea. What is his PE and KE at the top of the cliff? At what point in his leap is his KE 450J? At what speed does he enter the water?
 
OK. What did you get for the initial KE and PE at the top of the cliff?

At any point during the fall, what quantity is the same as at the top of the cliff?
 
I got 17640J for PE. And that would also equal the KE at the top of the cliff?
 
OK for the PE. Why would the KE equal the PE?
 
Due to no resistance or friction from a free-fall position
 
mathpat said:
Due to no resistance or friction from a free-fall position

I don't follow. What determines the KE of an object?
 
whether the particle or object is moving. Wow ok lol. So the KE = 0 J at the top of the cliff.
 
  • #10
mathpat said:
So the KE = 0 J at the top of the cliff.

Yes. Since the problem didn't state whether or not Sherlock jumps from the cliff with an initial velocity, I think you are going to have to assume that he doesn't jump. He just falls from rest.

So, good. His initial KE is zero.

Can you think of a way to find the point where the KE is 450 J?
 
  • #11
I'm seriously stumped when it comes to that part. I know I can't plug in that value in KE = 1/2 mv^2 because that would not give me a point.
 
  • #12
mathpat said:
I know I can't plug in that value in KE = 1/2 mv^2 because that would not give me a point.

Right, you already know that KE is 450 J at the point you are interested in. So, you don't need to calculate KE there. You are dealing with energy concepts here. Energy is really important due to a very basic principle that you have studied. Can you think of what that principle is?
 

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