Go Back   Physics Help Forum > College/University Physics Help > Advanced Mechanics

Advanced Mechanics Advanced Mechanics Physics Help Forum

Reply
 
LinkBack Thread Tools Display Modes
Old Apr 12th 2009, 04:18 PM   #1
Junior Member
 
Join Date: Apr 2009
Posts: 4
Two Impulse + Momentum related questions

First Question

A 2.5 kg ball and a 5.0 kg ball have an elastic collision. The 2.5 kg ball was at rest and the other ball had a speed of 3.5 m/s. What is the kinetic energy of the 2.5 kg ball after the collision?

I did


5 x 3.5 = 17.5

so 2.5(v) = 17.5

v = 7 m/s

kinetic energy which is 1/2 m v^2

.5 x 2.5 x 7^2

= 61.25 J

I got 61.25 J, however the answers have to be..

a) 1.7 J b) 8.1 J c) 3.4 J d) 14 J e) 27 J

What is the answer? and how do you come to it?

2nd Question

A 5.0 kg boy runs at a speed of 10.0m/s and jumps onto a cart. The cart is initially at rest. If the speed, with the boy on the cart, is 2.50 m/s, what is the mass of the cart?

mv + m2v2 = m'v'

(5)(10) + m2(0) = (M2 + 5) (2.5)

50 = 2.5M + 12.5

M = 15 kg

is what I did, but the answers have to be either....

a) 150J b) 210J c) 175J d) 260 J e) 300J

what is the answer and what am I doing wrong? can someone help??

thanks
mikeg is offline   Reply With Quote
Old Apr 12th 2009, 08:53 PM   #2
Physics Team
 
Join Date: Feb 2009
Location: India
Posts: 365
Edit: post deleted as I realised it was incorrect. Correct solution posted separately

Last edited by Parvez; Apr 14th 2009 at 12:43 PM.
Parvez is offline   Reply With Quote
Old Apr 13th 2009, 10:22 AM   #3
Junior Member
 
Join Date: Apr 2009
Posts: 4
I'd love to think so too man, but those are the only options and I don't know what i'm doing wrong
mikeg is offline   Reply With Quote
Old Apr 13th 2009, 10:55 AM   #4
Physics Team
 
Join Date: Feb 2009
Location: India
Posts: 365
I again ask how the mass can be in Joul. Either the question is wrong or the answer.
Parvez is offline   Reply With Quote
Old Apr 13th 2009, 01:57 PM   #5
Junior Member
 
Join Date: Apr 2009
Posts: 4
yea sorry its a mistake by me its supposed to be 150 kg, 210 kg .... etc. so that was my fault for that
mikeg is offline   Reply With Quote
Old Apr 14th 2009, 12:41 PM   #6
Physics Team
 
Join Date: Feb 2009
Location: India
Posts: 365
First Question

M = mass of the larger block
m = mass of smaller block
u = velocity of larger block before collision
V = velocity of larger block after collision
v = velocity of smaller block after collision.
As the collison is elastic, kinetic energy and the momentum will remain conserved. Hence-
(1/2) Mu^2 = (1/2)MV^2 + (1/2)mv^2 (conservation of energy)

Mu^2 - MV^2 = mv^2
M(u+V)(u-V) = mv^2 ............(1)


Next
Mu = MV + mv (conservation of momentum)
M(u-V) = mv .................(2)

Divide eq (1) by eq (2)

(u+V) = v

V=v-u put this value of V in eq (2) and you get
v= 2Mu/(M+m) = 2*5*3.5/(5+2.5)

Kinetic nergy of this (smaller) ball = (1/2)mv^2 = 27.2 J
Parvez is offline   Reply With Quote
Old Apr 14th 2009, 12:46 PM   #7
Junior Member
 
Join Date: Apr 2009
Posts: 4
thank you so much for solving the first question.

any ideas on the secound? my initial thought would be a) only because i keep getting 15 and it seems relevant but hey maybe you got better ideas
mikeg is offline   Reply With Quote
Old Apr 14th 2009, 01:00 PM   #8
Physics Team
 
Join Date: Feb 2009
Location: India
Posts: 365
I tried my best. Seems no way to get the answer. Please check the answers once again. you might have read kg as 0j ( 150J --> 15kg)
Parvez is offline   Reply With Quote
Reply

  Physics Help Forum > College/University Physics Help > Advanced Mechanics

Tags
impulse, momentum, questions, related



Search tags for this page
Click on a term to search for related topics.
Thread Tools
Display Modes


Similar Physics Forum Discussions
Thread Thread Starter Forum Replies Last Post
Some questions related to Sound Shivrasdhar Waves and Sound 1 Oct 2nd 2010 08:45 AM
Questions Related to Coulomb's Law and Charges Distributions KellyTram Electricity and Magnetism 1 Sep 5th 2010 10:54 PM
3 Questions Related to Coulomb's Law KellyTram Electricity and Magnetism 1 Sep 1st 2010 09:49 PM
Vector Related Questions Kookehs Kinematics and Dynamics 1 May 30th 2010 07:29 PM
Momentum and impulse sean123 Kinematics and Dynamics 0 Mar 16th 2010 06:41 AM


Facebook Twitter Google+ RSS Feed