Sorry your browser is not supported!

You are using an outdated browser that does not support modern web technologies, in order to use this site please update to a new browser.

Browsers supported include Chrome, FireFox, Safari, Opera, Internet Explorer 10+ or Microsoft Edge.

Geek Culture / Physics Question

Author
Message
Robin
21
Years of Service
User Offline
Joined: 22nd Feb 2003
Location: United Kingdom
Posted: 21st Oct 2004 03:48
Link
I was wondering if anyone here could just look over my answers to the question below and let me know if they're correct. I've got a feeling part A's wrong and if so, B,C & D will be wrong too :-( It's kinda important

A 50g bullet hits a 2Kg block of wood, suspended vertically from a string. The bullet penetrates 3cm into the wood and remains embedded as the block swings up through a vertical height of 30cm.

a) What was the speed of the bullet and block just after the collision?
b) What was the bullet's speed before the collision?
c) Estimate the average force exerted on the bullet by the block.
d) Calculate the kinetic energies before and just after the collision.
e) Was this collision elastic or inelastic?

My answers:

A) I got 2.4m/s (v=0m/s, a=-9.8m/s/s, s=0.3m, v^2 = u^2 + 2as)
B) Using the conservation of momentum rule:
m(1)u(1) + m(2)u(2) = m(3)v(3)
(0.05 x u) = (2.05 x 2.4)
u=98.4m/s
C) Using F=ma
v=0m/s u=98.4m/s s=0.03m
v^2 = u^2 + 2as
0 = (98.4)^2 + (2xax0.03)
a = 161376m/s/s
F = 0.05 x 161376 = 8068N
D) KE before collision:
KE = 0.5 x 0.05 x (98.4)^2 = 242J
KE after collision:
KE = 0.5 x 2.05 x (2.4)^2 = 5.9J
E) Inelastic, because the bullet gets embedded into the block of wood.

Thanks,

Robin

http://www.thegameszone.tk | robin@thegameszone.tk
Back to top
Profile PM Email Website
Neofish
20
Years of Service
User Offline
Joined: 7th Apr 2004
Location: A swimming pool of coke
Posted: 21st Oct 2004 04:19
Link
Quote: "A 50g bullet hits a 2Kg block of wood, suspended vertically from a string. The bullet penetrates 3cm into the wood"


Quote: "a) What was the speed of the bullet and block just after the collision?"


Err technically it depends on the wood, the velocity of the bullet, the width of the wood (maybe) and other stuff like that...Anywho im not very good at physics like that...

I just noticed the second question

[center]:: Life is a box-plot :: Want Gmail? Email me ::
[center]
Back to top
Profile PM Email Website
Robin
21
Years of Service
User Offline
Joined: 22nd Feb 2003
Location: United Kingdom
Posted: 21st Oct 2004 04:24 Edited at: 21st Oct 2004 04:25
Link
Yeah thats what I was thinking - if the question would have given you the time the bullet took to slow down or something else then it would be easier to work out - I'm assuming air resistance etc. can be ignored. I managed to get some answers but I don't know if they're right...

http://www.thegameszone.tk | robin@thegameszone.tk
Back to top
Profile PM Email Website
Ian T
22
Years of Service
User Offline
Joined: 12th Sep 2002
Location: Around
Posted: 21st Oct 2004 04:33 Edited at: 21st Oct 2004 04:33
Link
Holy shtik. I should enjoy the relative non-mathiness of Chemistry while it lasts


Preparing for a month of insanity, inspiration and insight... again!
Back to top
Profile PM
Wiggett
21
Years of Service
User Offline
Joined: 31st May 2003
Location: Australia
Posted: 22nd Oct 2004 21:01
Link
man physics sucks, why cant they just take it as it is, there is a bullet in the wood, ho wdid it get there? well obviously someone shot it out of a gun into the wood, how fast did it go? as fast as a speeding bullet. case closed. you think physics is hard try doing forensic physics, more bullets embedded in things like torso's and cars breaking through rails and flying over cliff faces. would be pretty fun if were not for the most boringest maths problems ever.


http://www.angelfire.com/wizard2/unpotkf
Back to top
Profile PM Website
Ilya
21
Years of Service
User Offline
Joined: 10th Aug 2003
Location:
Posted: 22nd Oct 2004 23:07
Link
I know simple physics from experience.

The default editor is fine.

Quote: "I've seen the word programming and I'm not sure what it means. Anybody please explain?"
Back to top
Profile PM Email Website
bitJericho
21
Years of Service
User Offline
Joined: 9th Oct 2002
Location: United States
Posted: 23rd Oct 2004 10:53
Link
eh.. robx... you're a friggin programmer^_^ This is stuff you *HAVE* to be able to do to make physics capable games

I loved my physics class And I hated chemistry Mostly I liked it because I could apply it to my programming..

Alas I have no answer for you, as you're missing some necessary details


No plot? No problem! Come write with the worst of us!
Back to top
Profile PM Email Website
Wiggett
21
Years of Service
User Offline
Joined: 31st May 2003
Location: Australia
Posted: 23rd Oct 2004 11:25
Link
hey if i whip out my work and formulae i could do this work, its jsut a matter of i don't want to so i will beg someon elike van b to do it for me


http://www.angelfire.com/wizard2/unpotkf
Back to top
Profile PM Website
pizzaman
20
Years of Service
User Offline
Joined: 18th Feb 2004
Location: Gateshead, UK
Posted: 23rd Oct 2004 12:35 Edited at: 23rd Oct 2004 12:38
Link
hi

My Answers (all to 3 significant figures)

A) 2.45 m/s

Gain in gravitational potental energy = Loss in kinetic energy
Therefore mgh = 0.5mv^2
v = sqrt(2gh)
v = sqrt(2 x 9.8 x 0.3)
v = 2.45

B) 101 m/s (Think your mistake was rounding error here)

m1.u1 + m2.u2 = m3.v
0.05 x u1 + 2 x 0 = 2.05 x 2.45
u1 = 5.03 / 0.05
u1 = 101

C) 81.8 N

a = ((v^2) - (u^2)) / (2s) Work out decceleration of bulletin wood
a = (0 - 101) / (2 x 0.03)
a = -1677

t = (v - u) / a Time it takes for bullet to deccelerate
t = (0 - 101) / 1677
t = 0.06

F = m(v - u) / t Average force produced by wood
F = 2(0 - 2.45) / 0.06
F = 81.8

D) 253 J (before) 6.03 J (After)

E = 0.5mv^2 + 0.5mv^2 E = mgh
E = 0.5 x 0.05 x 101^2 E = 2 x 9.8 x 0.3
E = 253 E = 6.03

E) Inelastic, because the bullet gets embedded into the block of wood.

Note my answers could be wrong, its been nearly a year and half since I've done physics, so done blame me if they are

Enjoy
pizzaman
Back to top
Profile PM Email
Robin
21
Years of Service
User Offline
Joined: 22nd Feb 2003
Location: United Kingdom
Posted: 23rd Oct 2004 23:49
Link
Thank you so much!
Robin

http://www.thegameszone.tk | robin@thegameszone.tk
Back to top
Profile PM Email Website
pizzaman
20
Years of Service
User Offline
Joined: 18th Feb 2004
Location: Gateshead, UK
Posted: 24th Oct 2004 04:45
Link
No probs, was kind of fun remembering how to do it. I take it your doing your A-levels? Second year?

Ah the good ol physics memories lol

Hope you do well with your studies
Back to top
Profile PM Email
Ilya
21
Years of Service
User Offline
Joined: 10th Aug 2003
Location:
Posted: 24th Oct 2004 05:33
Link
Quote: "A) 2.45 m/s

Gain in gravitational potental energy = Loss in kinetic energy
Therefore mgh = 0.5mv^2
v = sqrt(2gh)
v = sqrt(2 x 9.8 x 0.3)
v = 2.45

B) 101 m/s (Think your mistake was rounding error here)

m1.u1 + m2.u2 = m3.v
0.05 x u1 + 2 x 0 = 2.05 x 2.45
u1 = 5.03 / 0.05
u1 = 101

C) 81.8 N

a = ((v^2) - (u^2)) / (2s) Work out decceleration of bulletin wood
a = (0 - 101) / (2 x 0.03)
a = -1677

t = (v - u) / a Time it takes for bullet to deccelerate
t = (0 - 101) / 1677
t = 0.06

F = m(v - u) / t Average force produced by wood
F = 2(0 - 2.45) / 0.06
F = 81.8

D) 253 J (before) 6.03 J (After)

E = 0.5mv^2 + 0.5mv^2 E = mgh
E = 0.5 x 0.05 x 101^2 E = 2 x 9.8 x 0.3
E = 253 E = 6.03"


What?!??!?!
What's ^?
Do you get to use a calculator?

The default editor is fine.

Quote: "I've seen the word programming and I'm not sure what it means. Anybody please explain?"
Back to top
Profile PM Email Website
Neofish
20
Years of Service
User Offline
Joined: 7th Apr 2004
Location: A swimming pool of coke
Posted: 24th Oct 2004 05:50
Link
^ is to the power of

[center]:: Life is a box-plot :: Want Gmail? Email me ::
[center]
Back to top
Profile PM Email Website
Ilya
21
Years of Service
User Offline
Joined: 10th Aug 2003
Location:
Posted: 24th Oct 2004 06:08 Edited at: 24th Oct 2004 06:09
Link
Oh.

What happened to little numbers?

The default editor is fine.

Quote: "I've seen the word programming and I'm not sure what it means. Anybody please explain?"
Back to top
Profile PM Email Website
Neofish
20
Years of Service
User Offline
Joined: 7th Apr 2004
Location: A swimming pool of coke
Posted: 24th Oct 2004 06:51
Link
its harder to do, i dont know how to do it with HTML and it may be blocked (dunno, unless it uses the & and ; sys)

[center]:: Life is a box-plot :: Want Gmail? Email me ::
[center]
Back to top
Profile PM Email Website
bitJericho
21
Years of Service
User Offline
Joined: 9th Oct 2002
Location: United States
Posted: 24th Oct 2004 07:23
Link
..besides, it's the proper way to do powers when you're programming


No plot? No problem! Come write with the worst of us!
Back to top
Profile PM Email Website
Neofish
20
Years of Service
User Offline
Joined: 7th Apr 2004
Location: A swimming pool of coke
Posted: 24th Oct 2004 07:30
Link
was going to say tht but that would mean typing it

[center]:: Life is a box-plot :: Want Gmail? Email me ::
[center]
Back to top
Profile PM Email Website
Phaelax
DBPro Master
21
Years of Service
User Offline
Joined: 16th Apr 2003
Location: Metropia
Posted: 24th Oct 2004 16:51
Link
Quote: "man physics sucks, why cant they just take it as it is, there is a bullet in the wood, "


Given the type of bullet(by looking at it) and figuring out the speed of the bullet, police could estimate where the shooter was at. It may not seem like it, but this is a very good and practical question that does have its usefullness.

also, the wood's density and size would help. plus you gotta consider string tension. Not sure if the length of the string would make a difference thought.

"eureka" - Archimedes
Back to top
Profile PM Email Website
Robin
21
Years of Service
User Offline
Joined: 22nd Feb 2003
Location: United Kingdom
Posted: 25th Oct 2004 00:28
Link
Yeah I'm in my second year of A-levels It's not bad - my teacher pretty decent too.
btw. I think you forgot to square the 101 in part c)
Robin

http://www.thegameszone.tk | robin@thegameszone.tk
Back to top
Profile PM Email Website
pizzaman
20
Years of Service
User Offline
Joined: 18th Feb 2004
Location: Gateshead, UK
Posted: 25th Oct 2004 06:44
Link
Your right I forgot to square 101 , altough it was quite late when I did those questions. Did my A-level in physics last year, got a B in the end so it wasn't too bad, however I wanted an A, o well thats how it goes I guess.
Back to top
Profile PM Email
Phaelax
DBPro Master
21
Years of Service
User Offline
Joined: 16th Apr 2003
Location: Metropia
Posted: 25th Oct 2004 18:25
Link
I just remembered about this website. I visit it occationally.
http://physicsforums.com/

"eureka" - Archimedes
Back to top
Profile PM Email Website
Back to top

Login to post a reply

Server time is: 2024-09-22 22:21:22
Your offset time is: 2024-09-22 22:21:22