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The problem is like this:

A bullet with mass 15g moving at a speed of 425 m/s is brought to a stop in 3.5 cm in a block of wood. Find the force on the bullet, assuming that it is constant.

So I know that: F = m*a

m = 15

g=9.81

a = v/t

v = 425 m/s

t = ?

My question is: how to calculate time? My guess is, if the velocity is 425 m/s, and the bullet stumble upon a block of wood in 3.5 cm, I could transform the 425 m/s to cm/s (42500 cm) and then get the time, but what I get is a very tiny number, so I think I am doing it wrong...

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The time should be a tiny number, and the acceleration a very big number. Also, its easier to use work, W = F*d, where W is the initial kinetic energy of the bullet, d is the distance traveled. – user1631 Dec 30 '11 at 21:40
F=W/d. Where W=1/2mv^2. W=1354687.5 N d=0.035m F=1354687.5/0.035=38705357,14N Just to be sure -- is this right? It's like, a huge number for me... – Ron Dec 30 '11 at 22:02

1 Answer

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Can I ask exactly why you're bringing the gravitational constant g into this discussion? I'm pretty sure the only "g" in this problem refers to "grams," NOT the gravitational constant.

That being said...if that's your stumbling block, I think this problem will start to make a lot more sense. HINT: Think about what equations/physical principles you know that relate velocity, mass, distance, and force without needing to know any other quantities...

Good luck!

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Thank you for your answer. Actually I don't know why I wrote that there, I was just doing other problem and mixed all the thing! Well, I did this, applying the formula that the guy from above suggested: F=W/d. Where W=1/2mv^2. W=1354687.5 N d=0.035m F=1354687.5/0.035=38705357,14N -- is this right, Steven? – Ron Dec 30 '11 at 23:54
Ron, I'm happy to answer general conceptual questions about physics or physics problems, but ultimately this isn't a site where people will do your homework or check your answers for you. The community would do more harm than good in helping you learn physics if we were to do that. – Steven D. Dec 31 '11 at 0:50
That being said: a good first check for any physics problem is, "Do I understand the physical significance of the formulae I used and my final answer?" A second easy check is, "Did I use the correct units at all stages of solving this problem?" This latter question may prove particularly helpful for you. – Steven D. Dec 31 '11 at 0:51
Oops, I just saw the issue there... I used g instead of kg. The final result is 38705.35714. Well, I am very sorry about violating the rules, I won't do it again... and thanks everybody for helping me to resolve my problem! :) – Ron Dec 31 '11 at 0:56

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