What is the average force on Superman's chest when hit by 3g bullets at 500 m/s?

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Homework Help Overview

The problem involves calculating the average force exerted on Superman's chest when impacted by bullets of mass 3 grams each, traveling at a speed of 500 m/s, with a specific rate of fire. The context includes considerations of momentum and force in a physics framework.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss the calculation of impulse and the relationship between mass, velocity, and force. There are questions about determining the time interval for the calculations and how to handle the rebound of the bullets. Some participants explore unit conversions and the implications of using different equations.

Discussion Status

The discussion is ongoing, with participants sharing their interpretations and calculations. Some have provided insights into unit analysis and the importance of understanding the relationships between the variables involved. There is no explicit consensus on the approach to take, but various lines of reasoning are being explored.

Contextual Notes

Participants are navigating the complexities of the problem, including the assumptions about the bullets' behavior upon impact and the need for clarity in the time variable used in calculations. There is an emphasis on understanding the underlying physics rather than solely applying formulas.

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Homework Statement


Suppose a gangster sprays Superman's chest with 3 g bullets at the rate of 100 bullets/min, and the speed of each bullet is 500 m/s. Suppose too that the bullets rebound straight back with no change in speed. What is the magnitude of the average force on Superman's chest?

Homework Equations


J = FavgT
p = mv
J = pf - pi

The Attempt at a Solution


I don't really understand how to get the delta T. I got J = -3 Ns from equation 3, don't know where to go from there.
 
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You have 300 grams of bullets a minute hitting at 5 grams a sec on average.

Taking rebound into account you have a change in velocity of 1000 m/s then don't you have 5k grams/s*m/s? That looks like Newtons to me.
 
LowlyPion said:
You have 300 grams of bullets a minute hitting at 5 grams a sec on average.

Taking rebound into account you have a change in velocity of 1000 m/s then don't you have 5k grams/s*m/s? That looks like Newtons to me.

hmm interesting. So how would I know to play with the units rather than using typical linear momentum equations?
 
Shatzkinator said:
hmm interesting. So how would I know to play with the units rather than using typical linear momentum equations?

Insight. Regardless of the equations you must end with the same units they are asking. Units are often a guide to the answer. But it's not like I'm suggesting you not use the equations, so much as suggesting you understand what the equations are relating to begin with.

F = m*a = m*Δv/Δt = Δmv/Δt = Δp/Δt
 

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