You are Here: Home

# M3 impulsive strings and conservation of momentum Tweet

Maths and statistics discussion, revision, exam and homework help.

Announcements Posted on
Enter our travel-writing competition for the chance to win a Nikon 1 J3 camera 20-05-2013
IMPORTANT: You must wait until midnight (morning exams)/4.30AM (afternoon exams) to discuss Edexcel exams and until 1pm/6pm the following day for STEP and IB exams. Please read before posting, including for rules for practical and oral exams. 28-04-2013
1. M3 impulsive strings and conservation of momentum
Hi guys i'm having a bit of a problem with an impulsive tension question and i'm wondering whether you'd be able to help me out.

http://mathsathawthorn.pbworks.com/f/M3.pdf It's question 6 on that Paper.

I started by solving for the speed of block B just before the string became taught, I found its value to be 5.6 which agrees with the mark scheme.

Then, to calculate the velocity after the impulse I used conservation of momentum.

mass of Block A=7kg (stationary before impulse)
Mass of block B=3kg (velocity of 5.6 before impulse)

Using conservation of momentum: 7(0)+3(5.6)=7(-v)+3(v)
Where V is the common velocity post impulse and also taking downwards as +ve

thus v=-4.2 thus the speed is 4.2

however this is incorrect, the mark scheme has the value as 1.68 which is the result you get if you take momentum after impulse to be 7(v)+3(v)

Can anyone explain why ?

2. M3 impulsive strings and conservation of momentum
It's been a while since you posted and nobody's replied yet...maybe you should check out MarkedbyTeachers.com, TSR's sister site. It has the largest library of essays in the UK.

They've got over 181,000+ coursework, essays, homeworks etc.. all written by GCSE, A Level, University and IB students across all topics. You get access either by publishing some of your own work, or paying £4.99 for a month's access. Both ways give you unlimited access to all of the essays.

All their documents are submitted to Turnitin anti-plagiarism software, so it can't be misused, and the site's used by hundreds of thousands of UK teachers and students.

What's more, you can take a look around the site and preview the work absolutely free. Click here to find out more...
3. Re: M3 impulsive strings and conservation of momentum
I think because it's on a string over the pulley, you can't take the directions to be opposite like that.
Consider only the direction relative to the string. Take "A to B" as positive and "B to A" as negative
If the string were horizontal, and B was accelerated horizontally at g, this problem is the same and you can see why the signs are the same.