Moments and impulse

[Lee_Truss]

Hey there,

I ve got a maths exam in under a week now and I just getting various methods into my head, although I know the moments and impulse and the formulae involved I dont know how the hell they solved this.

A shell of mass 50kg is fired with speed 560ms. Given that the shell is in the barrel of the gun for 1/20 second, calculate the average force in kN, exerted on the shell by the explosive charge.

Ive been trying to work it out for a couple of hours now and there are no examples in the text book so could someone please show me the way to solve it.

Thanks

[HarryW]

Well you have the mass of the shell which we'll call m, the velocity of the shell which we'll call v, the average initial force on the shell (from the explosion in the gun) which we'll call F, and the length of time that force was exerted for, t.

Okay, so we have four variables, three of which are known. We also know two equations relating to momentum and impulse. I have forgotten what the usual letter for momentum is so I'll call it x.

• x = mv (momentum = mass * velocity)
• x = Ft (impulse = force * time)

Since the impulse is the change in momentum, and we know the shell begins at rest, we can say that momentum = impulse.

So, from these equations, we can equate them like this:

mv = Ft

and solve for F, the average force exerted on the shell:

F = mv/t

F = 50 * 560 / 0.05

F = 560000

However the question asks for kilonewtons, so the answer is 560 kN.

[Lee_Truss]

I got the 560000 but failed to pick up the units, god damn it, thanks for the help man.

[CornedBee]

Do I remember right?
monumentum = mass * velocity
energy of monumentum = mass * velocity²
right?

[DavidHooper]

Yes that's right.

Lee_Truss, what exam is it? Exam board and module? I got my Mechanics 2 and Stats 3 in two weeks!

[Lee_Truss]

Hey everyone,

Wells its Mechanics 1 under the edexcel exam board, I dont know if the A2 and AS mechanics papers are different but im in A2. Last year we did pure 1 and 2 with stats 1 and this year its pure 3 mech 1 and stats2. If I carry on as I did last year I should get a high d low c which should be enough (Along with the others) to get me into UNI along with the over courses (AVCE ICT and A2 business).

[DavidHooper]

Cool. I did OCR but, for what it's worth, M1 is pretty easy, P3 is relatively hard and S2 is easy when you know how.

[[Digital-X-Treme]]

Originally posted by Lee_Truss
...Last year we did pure 1 and 2 with stats 1...

Thats a hard start to Maths AS... Did many people drop out before starting A2?

I'm doing AS Maths now (Edexcel) and were doing Stats 1, Pure 1, and Mechanics 1 in the first year, and Pure 2, Pure 3 and Stats/Mechanics 2 in second year. *Continues revising for Stats 1 exam next week...

[HarryW]

Lee, sounds like your first year was the same as mine, I did P1, P2 and S1 in the first year. In the second year we did P3 and M1, then most people did S1 as their 6th module but if people wanted to they could do M2 instead. I did M2 since I took A-level Physics too.

Hardest module by far is P3. I made the rather large mistake of revising almost entirely integration stuff, and totally forgot about vector geometry. Quick tip: don't forget about vector geometry I think I got an E for that exam, but thankfully I did okay in the others and ended up with a B.

[Lee_Truss]

Hey there,

Well we had four groups of 25-20 and now we have two with around 8 in each, and I know of at least one didnt pass his AS (But is doing A2). The main problem came with pure 1 where we did it in the first term and did the exam in January, there was two much to learn in that module with comparison to GCSE maths and with christmas no one was prepared.

We have done a chunk of pure 3 already, including vector geometry as it helps with mechanics. We have had quite a good term, doing several topics of pure 3 and the mechanics book.

Good look to anyone who is doing any math exam these weeks.