Calculating Frictional Force on a Bullet Using the Momentum Formula

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Homework Help Overview

The discussion revolves around calculating the average frictional force acting on a bullet that comes to rest after penetrating a block of wood. The problem involves concepts from mechanics, specifically momentum and work-energy principles.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • Participants explore the use of momentum and work formulas to determine the frictional force. Questions arise regarding the calculation of time and the work done by friction. There is a discussion on the signs of work and the implications of the bullet coming to rest.

Discussion Status

Participants have engaged in a back-and-forth regarding the appropriate formulas to use, with some guidance provided on considering work done by friction. The discussion has progressed towards calculating work and its relationship to force, though no consensus on the final answer has been reached.

Contextual Notes

Participants are navigating through the constraints of the problem, including the need to account for the bullet's initial and final velocities, as well as the depth of penetration in the wood. There is an emphasis on understanding the negative work done by friction.

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a rifle fires a(n) 6g bullet with a muzzle velocity of 390m/s into a block of wood. the bullet comes to rest after it has made a(n) 18cm deep hole in the wood. what is the magnitude of the average frictional force slowed the bullet?

can i use this formula?
J=FT=pf-pi=mv
 
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You could, but you'd first have to figure out the time. Instead, consider the work done by the frictional force on the bullet.
 
What would you use for the time?

I suggest asking yourself how much work the frictional force does.
 
is it W=Kf-Ko ?
 
yes, but make sure not to forget that the work done by friction will be negative.
 
so...W=1/2mVf^2-1/2mVo^2 does Vf cancel since it comes to rest?
 
yes, it does
 
the answer is W=456.3 is that all?
 
you've found the work, but the question wants to know the force. so what is the equation for work?
 
  • #10
is it W=Fd
 
  • #11
yes, that's right
 
  • #12
Thank You So Much!
 

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