a bullet fired into a large block of wood of uniform resistance hits the block with a velocity 48m/s. it penetrates a distance of 60cm and the velocity is then reduced to 24m/s .it penetrates a further distance of ( in cm)?

Question

a bullet fired into a large block of wood of uniform resistance hits the block with a velocity 48m/s. it  penetrates a distance of 60cm and the velocity is then reduced to 24m/s .it penetrates a further distance of ( in cm)?

wasiqss · Answer

well first calculare the deaacceleration
that is 
120a=(48)^2-(24)^2

wasiqss · Answer

assuming uniform deacceleration

anonymous · Answer

can we make that assumption? does uniform resistance mean uniform decelerating force?

wasiqss · Answer

actually its not 120a
indeed its 1.2 a

anonymous · Answer

im not saying it doesnt, i dont actually know

anonymous · Answer

yes it is uniform decceleration

wasiqss · Answer

1.2a=24^2-48^2

wasiqss · Answer

shameer i guess now you can do it

egenriether · Answer

you can find a ratio of kinetic energies, to see how much energy it took to get 60 cm, even though you don't have the mass (it will divide out)  then see how much further you can get with the energy you have left using the conversion factor.

anonymous · Answer

we dont need to find energy it is kinematics problem (motion)

anonymous · Answer

wasiqss can u give the equation plzz

wasiqss · Answer

shameer did you find acceleration

egenriether · Answer

you're right...I have it now, thanks!

anonymous · Answer

20 cm..i think..

wasiqss · Answer

after finding acceleration, use the same equation
2as=v^2-u^2

wasiqss · Answer

where u=0, v=24

mos1635 · Answer

in deaccelaration movement:
S=Vo*t-1/2*a*t^2 (1)
and
Vf=Vo-a*t (2)
if you solve (2) to t and repalce that to (1)
you will have S=(Vf^2-Vo^2)/2*a

anonymous · Answer

hey i get a=1400

anonymous · Answer

can anyone give a solved equation plzzz

anonymous · Answer

is my ans correct...i used energy conservation principle..

anonymous · Answer

yes the answer is 20cm i dont know the steps we have to use kinematics equation

anonymous · Answer

energy conservation is much simler...just equate the ratio of kinetic energy lost between v=[24,48] and[0,24] with the ratio of work done by block to decrease this energies...since KE loss ratio from 48 to 24 is 1/3 therefore the work done ratio must also be 1/3...work done=F.d
f=const so ratio of distances is also 1/3...
so 60/3 =20 is the ans..

anonymous · Answer

|dw:1336400358275:dw|here KE loss ratio =