Physics problems

[mistermack]

But, back in my day, we had to memorise all the formulae! I frequently tell my class they don't know how lucky they are!

Yep. But it does make sense. Back in your day, you didn't have all of these things instantly available to you, with the click of a mouse. You would have to delve through a huge room full of papyrus to find the formulae, so it made sense to memorise it.

[Hermit]

You and me too, brother. I distinctly remember a few questions in preliminary tests being about no more than probing (this, by the way, is as close as I'll ever get to reading one of tatt's stories) if you know some formula or another. And then we had to apply them in other tests. No slide rules allowed either. Long divisions had to be done with pen on paper. We were allowed to consult log tables when doing calculus, and we were allowed to use rulers to prevent us skipping to the wrong fucking line, though.

[Xamonas Chegwé]

But, back in my day, we had to memorise all the formulae! I frequently tell my class they don't know how lucky they are!

Yep. But it does make sense. Back in your day, you didn't have all of these things instantly available to you, with the click of a mouse. You would have to delve through a huge room full of papyrus to find the formulae, so it made sense to memorise it.

It still makes sense. Just as learning times tables makes sense. There is no substitute for having the information in your active memory. I hate seeing a student pull out a calculator to work out 3 x 6!!1!

In the Mechanics modules of A-level Maths, there is still a requirement to know certain formulae - nowhere near as many as I would like! But Newton's equations of motion are among those which need to be known by rote.

[Brian Peacock]

I guess the lesson here is that it's important to know stuff, and to know where to find out stuff that you don't know.

[mistermack]

I wonder if Newton will still be remembered in a thousand years time?

People will have surgically implanted sshds connected up to their thoughts, so that basic knowledge like formulae will just flash up in their minds, as soon as they think of it. Or in front of their eyes in a visual display.

Which could be distracting, as you sweep out the artificially intelligent computers' room, before giving them a polish.

[Hermit]

I wonder if Newton will still be remembered in a thousand years time?

Well, Herostratus is remembered merely for being an arsonist, and that was despite the fact that the death penalty was imposed on anyone who mentioned his name in speech or writing. Herostratus died almost 2400 years ago. So, yes, I think there is a fair chance that Newton will be remembered 1000 years from now.

[rainbow]

1a would be 0.17992802878848 m/s - so 0.18 rounded. Note that I added the weight of the bullet to the sandbag/trolley to get that result - but that doesn't affect the rounded figure - I would have made sure that it did!

1b

v2 = u2 +2as

so 0 = 202500 + 2 x a x 0.08

a = -202500 / 0.16 = -1,265,625 m/s2

1c

v = u + at

so 0 = 450 - 1,265,625t

t = 450 / 1,265,625 = 0.000355555557s - which rounds to 0s by your instructions!

1d

F = ma

so F = .02 x 1,265,625 = 25,312.5N

1e

.025 x v = 50.045 x 0.22 (Note that I have assumed that the first bullet is still in the bag - and that the frictionless trolley has been stopped in between firings!)

v = 440.396 m/s - rounds to 440.40.

Can't be arsed to do the rest.

Wrong.

You didn't take into account quantum effects.

[mistermack]

I wonder if Newton will still be remembered in a thousand years time?

Well, Herostratus is remembered merely for being an arsonist, and that was despite the fact that the death penalty was imposed on anyone who mentioned his name in speech or writing. Herostratus died almost 2400 years ago. So, yes, I think there is a fair chance that Newton will be remembered 1000 years from now.

Yes, I suppose so.
Although, there has always been a fashion for studying the ancient greeks.

When the Chinese take over, they might order that all non-Chinese history be erased.

Like we did to that, uuuuh, nope, it's bin erased.

[JimC]

My answers, hopefully correct...

1. a) 0.18 m/s b) –1,265,625 m/s2 c) 0.00036 s
d) 25,312.5 N e) 440.35 m/s

2. a) 18.09 kg.m/s b) 18.09 N.s c) 18.09 kg.m/s
d) 1206 N e) 1206 N, slowed it slightly f) 606 J

3. a) KE = 10,800,000 J, momentum = 18,000 kg.m/s b) For both masses,
momentum unchanged. KE (10 kg) = 16,200,000 J, KE (30 kg) = 5,400,000 J

4. a) 900,000 J b) 823,200J c) 76,800 J
d) The PE (gravity) e) 91.47%

5. a) 176.19 m b) 5.08 m/s2 c) 48.33 m/s
d) 81,766.5 J e) 120,866.34 J f) 39,099.84 J

6. a) 0.745 J b) 0.745 J c) 8.63 m/s
d) 0.745 J e) 2.29 N f) 3.53

[Xamonas Chegwé]

My answers, hopefully correct...

1. a) 0.18 m/s b) –1,265,625 m/s2 c) 0.00036 s
d) 25,312.5 N e) 440.35 m/s

2. a) 18.09 kg.m/s b) 18.09 N.s c) 18.09 kg.m/s
d) 1206 N e) 1206 N, slowed it slightly f) 606 J

3. a) KE = 10,800,000 J, momentum = 18,000 kg.m/s b) For both masses,
momentum unchanged. KE (10 kg) = 16,200,000 J, KE (30 kg) = 5,400,000 J

4. a) 900,000 J b) 823,200J c) 76,800 J
d) The PE (gravity) e) 91.47%

5. a) 176.19 m b) 5.08 m/s2 c) 48.33 m/s
d) 81,766.5 J e) 120,866.34 J f) 39,099.84 J

6. a) 0.745 J b) 0.745 J c) 8.63 m/s
d) 0.745 J e) 2.29 N f) 3.53

Please state which assumptions you have made in calculating these answers...

[Hermit]

Or, to put it in Jim's own words, "Working must be shown."

[JimC]

1. I assume that there is a material universe (sorry, jamest...) that follows Newtonian physics at modest speeds and gravitational strengths, and at a macro scale. In these problems, we assume a value for g of 9.8 m/s/s, and that frictional effects, other than those clearly stated, are negligible. We also assume that the systems as described are closed, with no input of energy or momentum from outside..

2. Showing working is indeed vital for grubby little schoolboys who wish to get full marks. This, of course, does not apply to me...

[mistermack]

My answers, hopefully correct...

1. a) 0.18 m/s b) –1,265,625 m/s2 c) 0.00036 s
d) 25,312.5 N e) 440.35 m/s

2. a) 18.09 kg.m/s b) 18.09 N.s c) 18.09 kg.m/s
d) 1206 N e) 1206 N, slowed it slightly f) 606 J

3. a) KE = 10,800,000 J, momentum = 18,000 kg.m/s b) For both masses,
momentum unchanged. KE (10 kg) = 16,200,000 J, KE (30 kg) = 5,400,000 J

4. a) 900,000 J b) 823,200J c) 76,800 J
d) The PE (gravity) e) 91.47%

5. a) 176.19 m b) 5.08 m/s2 c) 48.33 m/s
d) 81,766.5 J e) 120,866.34 J f) 39,099.84 J

6. a) 0.745 J b) 0.745 J c) 8.63 m/s
d) 0.745 J e) 2.29 N f) 3.53

I'm so glad I don't like gin, if that's what it does to you.

[laklak]

Answer: It never strikes the sandbag because the sandbag has a concealed carry permit and he blasts the fuck out of whoever is shooting the gun.

[Xamonas Chegwé]

1. I assume that there is a material universe (sorry, jamest...) that follows Newtonian physics at modest speeds and gravitational strengths, and at a macro scale. In these problems, we assume a value for g of 9.8 m/s/s, and that frictional effects, other than those clearly stated, are negligible. We also assume that the systems as described are closed, with no input of energy or momentum from outside..

Don't you work at a catlick school? You missed the "We assume there is a god that can override the rules of physics at will." clause...