Solving Paratrooper's Fall: Estimating W, F, & Air Resistance

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Homework Help Overview

The discussion revolves around a physics problem involving the fall of a paratrooper, focusing on concepts of work, force, and air resistance. The problem presents specific parameters such as the paratrooper's mass, fall distance, and terminal velocity, and asks for estimates of work done by the snow, average force exerted by the snow, and work done by air resistance.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • Participants explore the relationship between gravitational force and the force required to stop the paratrooper, referencing Newton's third law. They discuss the definition of work and its calculation, questioning the validity of the provided answers and suggesting different methods to approach the problem.

Discussion Status

The discussion is active, with participants questioning the correctness of initial calculations and exploring various interpretations of work and energy principles. Some guidance has been offered regarding the relationship between work done by different forces and the change in kinetic energy, but no consensus has been reached on the correct approach or answers.

Contextual Notes

Participants note discrepancies between their calculations and the provided answers, indicating potential misunderstandings or misapplications of the concepts involved. There is also mention of conservation of energy equations and kinematic equations as possible frameworks for solving the problem.

cyspope
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Homework Statement



71. A paratrooper fell 370 m after jumping from an aircraft without his parachute opening. He landed in a snowbank, creating a crater 1.1 m deep. Assuming the paratrooper's mass was 80 kg and his terminal velocity was 30 m/s, estimate : a) the work done by the snow in bringing him to rest; b) the average force exerted on him by the snow to stop him; c) the work done on him by air resistance as he fell.

A. a) -36000 J , b) -3300 N , c) -250000 J

Homework Equations


W = mgh = Fdsin\theta= \frac{1}{2}mv^{2}


The Attempt at a Solution


a) I got a) by using .5mv^{2}. However, I don't know how to get b) and c)
 
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what's making the paratrooper fall? Force by gravity. So to stop him what force must be applied? Think of Newtons third law.

What is work? work is force times distance. The distance is the depth of the crater, what is the work?
 
Where are these answers coming from? None are correct...
 
Newton's third law is about action and reaction, isn't it?

this is what i did for b)
W = Fdsin\theta = 80 * 9.8 * 1.1 = 862.4 Joule

However, this is wrong because my answer is different from the given answers.
 
cyspope said:
Newton's third law is about action and reaction, isn't it?

this is what i did for b)
W = Fdsin\theta = 80 * 9.8 * 1.1 = 862.4 Joule

However, this is wrong because my answer is different from the given answers.

your answer and the given answers are incorrect. The net work done on the 'trooper in bringing him to rest is his change in KE. The net work includes the work done by the snow and the work done by gravity. Once you calculate the work fone by the snow, then the force of the snow on the trooper can be calculated using the definition of work. Are you familiar with conservation of energy equations? Otherwise, you'll have to use the kinematic equations and Newtons laws.
 
What is work?

Work=\int F ds

From this you can find that

Work=-\Delta U and Work=\Delta KE

Read your book about work and energy.
 

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