The percentage and force of Friction ( 9/28/09)

AI Thread Summary
The discussion revolves around a physics homework problem involving a ballistic pendulum, where a 10g bullet is fired into a 4kg wood block, causing a vertical displacement of 5 cm. The initial velocity of the bullet was calculated to be 19.8 m/s using the equation for kinetic energy and gravitational potential energy. The second question regarding the percentage of kinetic energy lost in the collision remains unanswered in the discussion. The force of friction acting on the bullet as it stops within the block was calculated to be 1.5E-3 N using the formula F=ma. Overall, the thread seeks assistance with the energy loss calculation and confirms the force of friction.
sumo87
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Homework Statement



Hi everyone,

I have 2 quick Questions, about my HW so if u think u can give me a hit please post.

Homework Equations



the Question was :-

10g bullet is fired into a ballistic pendulum made from a 4kg wood block and two light cords. The block's vertical displeacment is 5 cm.

1-

what is the inital velocity.

what percentage of the bullet's initial kinetic energy is lost in the collision

what is the force of friction acting on the bullet in the block assuming that it stops in 3 cm

The Attempt at a Solution

I found the initial velocity into the equation

1/2mv^2 = mgh

and i got 19.8 m/s

so please help me with 2nd and the 3rd Qs
 
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k i got for the 3ed Q

1.5E-3

I used the equation

F= ma
 
Thread 'Minimum mass of a block'

Thread 'Minimum mass of a block'

Here we know that if block B is going to move up or just be at the verge of moving up ##Mg \sin \theta ## will act downwards and maximum static friction will act downwards ## \mu Mg \cos \theta ## Now what im confused by is how will we know " how quickly" block B reaches its maximum static friction value without any numbers, the suggested solution says that when block A is at its maximum extension, then block B will start to move up but with a certain set of values couldn't block A reach...
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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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