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Can Someone Help Me With My Homework Perlease

Ash-Kennard

By Ash-Kennard
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Ash-Kennard Post Whore

Ash-Kennard

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Right for coursework at uni this lecturer has given us a load of problems to do on an on-line test. He has taught us very little about the subject. Showed my dad the notes we were given and he said they were piss poor. And what is worse the questions are on different subjects of engineering.

So basically I was wondering if anyone could help me. Either on here or on msn or something. I've been trying to do these questions for a week now, and i have come up with 2 answers total, and they could definitely be wrong lol.

This is the first one:

The problem statement, all variables and given/known data

A bullet of mass 0.04kg is fired horizontally into a wooden block of mass 0.09kg. the wooden block is suspended from the ceiling by a long string. after an impact the bullet is embedded in the block and bullet and the block swing together until the block is 0.10m above its initial position.

A) Find the velocity of the bullet and block just after impact.

B) Find the initial velocity of the bullet

7-p-052.gif

Relevant equations

these may not be the right equations but i am out of ideas

mu + MU = ( m + M ) v

( 1/ 2) (m + M ) v ^ 2 = ( m + M ) gh

The attempt at a solution

This has taken question has taken me a long time and the method i used has given me a very unexpected answer...

I found some similar working online, but I'm not convinced.

mass of bullet m = 0.04 kg

mass of block M = 0.09 kg

maximum height reachs by block h = 0.1 m

from law of conservation of momentum , mu + MU = ( m + M ) v

mu + 0 = ( m + M ) v

where U = initial velocity of the block = 0

u = the initial speed of the bullet = ?

u = ( m+ M ) v / m

= 3.25 v ---( 1)

from law of conservation of energy ,

Kinetic energy of the bullet block system just after collision = potentia energy at maximumheight h

( 1/ 2) (m + M ) v ^ 2 = ( m + M ) gh

from this speed of the system just after collision v = √[ 2gh ]

= 1.4 m / s

plug this in eq ( 1) we get initial velocity of the bullet u = 3.25*1.4

= 4.55 m / s

is this right?

Edited by Ash-Kennard
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PeanuckleJive Trials Elite

PeanuckleJive

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Ok This means nothing to me, but to me sounds like its time for a 'piratical' :D. Do it then surely you have you answer if u have camera's metering tape and things?

I facepalmed so hard at that. I won't even begin to explain how stupid a suggestion it is >.<

That question does sound a bit dumb, and very ambiguous

it appears to me that it's saying "the block swings and when it stops, it's 10cm higher than when it started" but surely when it stops, it will be at the starting position (lowest point)?

Or does it mean the highest point of the swing is 10cm? it's hard to tell.

I'm afraid I can't help but i did notice the working you put on the end has a flaw in it

the question says the block ends 0.10m above inital starting position, in your equation you've put the value in as 0.01m above, try adjusting that if you haven't already

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Luke Rainbird Post Whore

Luke Rainbird

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Ok This means nothing to me, but to me sounds like its time for a 'piratical' :D. Do it then surely you have you answer if u have camera's metering tape and things?

Are you honsestly that stupid or are you attempting to be funny?

Ash - I'll have a look after food if I get a chance and nobody else has hooked you up mate (Y)

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Ash-Kennard Post Whore

Ash-Kennard

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I facepalmed so hard at that. I won't even begin to explain how stupid a suggestion it is >.<

That question does sound a bit dumb, and very ambiguous

it appears to me that it's saying "the block swings and when it stops, it's 10cm higher than when it started" but surely when it stops, it will be at the starting position (lowest point)?

Or does it mean the highest point of the swing is 10cm? it's hard to tell.

I'm afraid I can't help but i did notice the working you put on the end has a flaw in it

the question says the block ends 0.10m above inital starting position, in your equation you've put the value in as 0.01m above, try adjusting that if you haven't already

ahh thank you i wrote it down wrong. it is meant to be 0.1 m. and im assuming it means the highest point. cheers for that ill change it now

EDIT: YAY LUKEYYY :)

Edited by Ash-Kennard
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PeanuckleJive Trials Elite

PeanuckleJive

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ahh thank you i wrote it down wrong. it is meant to be 0.1 m. and im assuming it means the highest point. cheers for that ill change it now

awesomesauce, hopefully that'll sort you out

thing is, I bet they'll try to put you off by having an answer totally non-typical of a real bullet velocity

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Matt Vandart Too Much Spare Time

Matt Vandart

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It's very early in the morning, but I think it would be handy to know the length of the string.

I'm not sure if this is necessary as I have:

A: Not woken up yet

B: Not had a cup of tea

C: Not had my nicotine

E: Not eaten all the kids easter eggs

I can and will look at it again, later after the above, you should get hold of Cormac (Psycholist) for sure.

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