How to Calculate Frictional Force and Time for a Bullet Penetrating Wood?

AI Thread Summary
To calculate the frictional force acting on a bullet penetrating wood, the work-energy theorem can be applied, considering the bullet's initial velocity of 400 m/s and its final velocity of 0 m/s after penetrating 12 cm. The discussion highlights confusion regarding the time it takes for the bullet to come to rest, with an initial calculation yielding 12 seconds, which is inconsistent with the expected answer of 600 microseconds. Participants suggest determining the acceleration induced by the frictional force, assuming it remains constant throughout the bullet's penetration. This approach will help clarify the time calculation. Accurate application of physics principles is crucial for resolving these calculations effectively.
brick06
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A 10 gram bullet with a horizontal speed of 400m/s hits a block of wood and penetrates 12 cm.
a. what is the frictional force?
b.how long does it take the bullet to come to rest

I have the answers but cannot figure out how to get there. I set my initial velocity to 400 m./s, making my final velocity 0... any help would be great, I am going nuts.
 
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I've given you a hint in your first thread.
 
okay i used the work energy theorem to find the friction force,and my other thread has disappeared. So what should i use now to find the time it takes to stop, when i tried i got 12 seconds, but that does not seem right whatsoever
 
btw the back of the book says the answer should be 600 mu seconds
 
Well, assuming the forceyou've found is constant over time, what acceleration does it induce?
 
Thread 'Variable mass system : water sprayed into a moving container'

Thread 'Variable mass system : water sprayed into a moving container'

Starting with the mass considerations #m(t)# is mass of water #M_{c}# mass of container and #M(t)# mass of total system $$M(t) = M_{C} + m(t)$$ $$\Rightarrow \frac{dM(t)}{dt} = \frac{dm(t)}{dt}$$ $$P_i = Mv + u \, dm$$ $$P_f = (M + dm)(v + dv)$$ $$\Delta P = M \, dv + (v - u) \, dm$$ $$F = \frac{dP}{dt} = M \frac{dv}{dt} + (v - u) \frac{dm}{dt}$$ $$F = u \frac{dm}{dt} = \rho A u^2$$ from conservation of momentum , the cannon recoils with the same force which it applies. $$\quad \frac{dm}{dt}...
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