got some exams tomorrow, and still trying to get a good grasp of all the mechanics, physics, chemistry side of things.

im getting there, but one thing puzzles me, so wouldnt mind someone explaining it in laymans terms?

velocity is measured in m/s. acceleration is the ‘rate of change of velocity’, and measured in m/s per second (m/s squared)

gravity is the acceleration which acts on everything, and is measured at 9.81 m/s squared (dunno how to do the ickle 2 for squared)

so…..does that not imply that once falling, something is accelerating at 9.81m/s every further second? so its not falling at a constant velocity of 9.81m/s, its getting faster each second? by an astronomical amount once in the air for a good few seconds?

sorry 😀

Sounds like you’ve got it. Were it not for wind resistance and objects reaching terminal velocity pretty quickly things would fall very fast indeed. Look at the space jump man.

until it reaches terminal velocity.

ahhh, i havent come across terminal velocity yet 🙂

EDIT: just googled it, makes sort of sense now thanks

so…..does that not imply that once falling, something is accelerating at 9.81m/s every further second? so its not falling at a constant velocity of 9.81m/s, its getting faster each second? by an astronomical amount once in the air for a good few seconds?

Well I’m not sure that I would use the term “astronomical” but essentially, yes. Plus as others have said air resistance has quickly becomes limiting as the object reaches its terminal velocity.

Objects in a vaccum / space would continue in the way you describe.

On earth the friction with the air acts as a brake and ultimately the 2 opposing forces call it a draw @ terminal velocity.

(having said that I did my A level in 1988!

Yes, an object starts at zero.
Once let go of, or dropped it will accelerate (ignoring drag) at a rate of 9.81m/s^2.
So, at one second it will be falling at a speed of 9.81m/s but it is still accelerating at 9.81m/s^2 so it gets faster & faster.

In reality, drag slows the whole sheebang down, so something that is light with a large area will not fall very fast as the force pulling it towards the ground is quickly balanced by the drag force acting in the opposite direction.
I think a person’s terminal velocity is 120 mph or so. He jumps out of a plane with an initial (downwards) velocity of 0m/s. He then accelerates at 9.81m/s^2 until he gets to 120 mph or so. At this point, drag is slowing him down just as much as gravity is trying to speed him up, so he stays at the same speed.

dunno how to do the ickle 2 for squared

Alt-253 on the numeric keypad will give you a ² or you could write it as m/s^2.

(Or you could copy and paste that one.)

Presumably drag isn’t the only thing creating terminal velocity?

Or are we saying that in a perfect vacuum an object could accelerate beyond the speed of light…?

Here: http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa_pre_2011/forces/forcemassact.shtml

And here:
http://www.bbc.co.uk/learningzone/clips/why-does-a-human-have-a-different-terminal-velocity-to-a-mouse/8751.html

The way to think about it is in terms of forces. (F=ma, as you no doubt know).

Gravitational force acts to pull things to earth (mg, the force due to gravity).
There’s generally air resistance acting to oppose this (let’s call this R).

So the resultant force on your object is mg-R (downwards).
And acceleration is given by a = (mg-R)/m.

Obviously when mg and R are equal, a = 0, and velocity is constant – terminal velocity.

Or are we saying that in a perfect vacuum an object could accelerate beyond the speed of light…?

Mass and energy get a bit mixed up as you get near the speed of light.

Edit- from wiki:
As speeds approach the speed of light, relativistic effects become increasingly large and acceleration becomes less.

I think a person’s terminal velocity is 120 mph or so. He jumps out of a plane with an initial (downwards) velocity of 0m/s. He then accelerates at 9.81m/s^2 until he gets to 120 mph or so.

Depends – if you adopt the classic skydiver’s spreadeagled position then yes. I have a number of friends who (cough) parachute (cough) for a living…they tend to jump very high (typically over 35K feet) and to lose height rapidly they will sometimes go “head first”. They can reach speeds well in excess of 200mph doing this.

Felix Baumgartner hit over 800mph on his way down but obviously this was where there was next to no air resistance.

Or are we saying that in a perfect vacuum an object could accelerate beyond the speed of light…?

The mass of objects increases as they go faster. Meaning it’s harder to accelerate them.

As it approaches the speed of light, its mass approaches infinity and so requires infinite force to accelerate beyond the speed of light.

They can reach speeds well in excess of 200mph doing this.

and when they alter body position theyll actually start ‘braking’ as it were, or ‘decelerating’?

and when the alter body position theyll actually start ‘braking’ as it were?

Yep. The force due to air resistance will go up such that it’s greater than the force due to gravity, and so they’ll have negative acceleration downwards and slow down (until the force due to air resistance equals that of gravity again).

sadexpunk – Member

They can reach speeds well in excess of 200mph doing this.

and when they alter body position theyll actually start ‘braking’ as it were, or ‘decelerating’?

Yes, assuming the altered body position makes them less aerodynamic. The drag force will increase, so they will decelerate until the drag force & the force due to gravity become equal again.

Same as a skydiver pulling his parachute. The parachute increases the drag by a lot so he will decelerate a great deal, meaning he won’t become a pulpy mess upon hitting the floor.

Has anyone mentioned planes on conveyor belts yet?

GrahamS – Member

Presumably drag isn’t the only thing creating terminal velocity?

Or are we saying that in a perfect vacuum an object could accelerate beyond the speed of light…?

er, nearly, sort of, yes, but no.

if you could accelerate an object at a constant rate, eventually it would reach light-speed.

however, as an object approaches light speed, the mass of the object increases, approaching infinity the closer to light speed the object get.

you need more force to accelerate more mass, an object with nearly infinite mass would need nearly infinite force to accelerate only a tiny bit.

dropping something into a blackhole is about as close as you could get to this, and then it all gets a bit hawkins/einsteinian anyway and my brain begins to melt.

(but the object falling towards a black hole still doesn’t reach light speed)

but, yes, it’s only drag that leads to a ‘terminal velocity’ – as far as objects falling through our atmosphere are concerned.

oooo-kay, youve passed 😀

next baffler is maybe the way its written in my course book. we’re onto water power now. the book states the equation for water power is 100 X L X P divided by 60. L is litre per min, pressure is bar. it doesnt explain why, so rather than just trying to learn it off pat, i want to understand it so ill remember it.

i googled water power, and read that it only ends up around 60% of its starting or ‘brake power’. so i assumed that thats why we’re dividing by 60. but then i thought, well, the 100 doesnt want to be on the top line, surely itll be L X P, then 100/60 for 60%.
so i tried working it out both ways, and they dont match.

say the flow is 100 l/m and the pressure is 5 bar, what do you make the water power?

EDIT: by the book, its 80 odd, ‘my’ thought process says 300.

It looks to me like the 60 is a conversion from l/min to l/s and the 100 is a combination of the conversion from bar to N/m2 (multiply by 100000) and l/s to m3/s (divide by 1000). Dimensionally this will give the units of Power.

It looks to me like the 60 is a conversion from l/min to l/s and the 100 is a combination of the conversion from bar to N/m2 (multiply by 100000) and l/s to m3/s (divide by 1000). Dimensionally this will give the units of Power.

gulp :-/

so should i just assume the answers 80 odd and not think too much into it? 😀

All I’ve done is a quick dimenional analysis, it’s a quick and useful way of making sure that your equations are correct and that you’ve used all the correct units and conversions to make sure you get the answer right. It may be a little beyond what you’ve been taught though (that sounds a lot more condecending that it is supposed to, sorry). Incidently if I’m right then that “100” will have units associate with it.

Not sure where you get the 80 odd from though as if I plug those numbers you gave into the equation you gave then I get

100*100*5/60 which is 833.33 by my calculator. This is the theoretical power that it would take to raise the pressure of that flow of water by 5 bar.

yep, sorry, i made it 833 too, just i looked at it, then tried it the other way and just remembered it wrong 🙂

thanks

they tend to jump very high (typically over 35K feet) and to lose height rapidly they will sometimes go “head first”.

HALO

gravity is the acceleration which acts on everything, and is measured at 9.81 m/s squared (dunno how to do the ickle 2 for squared)

so…..does that not imply that once falling, something is accelerating at 9.81m/s every further second? so its not falling at a constant velocity of 9.81m/s, its getting faster each second? by an astronomical amount once in the air for a good few seconds?

Just to be clear and confirm your correctness

Ignoring air reistance

The velocity is not constant it changes by 9.81 m/s every second

The acceleration is constant. It always speeds up by 9.81 m/s every second

Actually the acceleration isn’t constant, as it reduces as you get further away. Felix Baumgartner jumped from about 39km, so was about 6410km from the centre of the earth, compared to 6371 (average) at the ground. Gravity reduces by the inverse square of distance, so would be down to about 98.8% of the 9.81m/s² or 9.69m/s².

water power:

right then,

energy = force x distance moved (this is a fundamental thing)

imagine a bicycle pump, you apply a force to the handle and create a pressure.

pressure = force/area of pump head.

you push the handle a distance.

you’ve moved a force through a distance, so there’s your energy.

energy = force x distance (right?)

but, you can also say it like this;

energy = (pressure x area) x distance

energy = pressure x area x distance

and, area x distance is a volume

so now you can say that:

energy = pressure x volume

so,

power = pressure x volumetric flowrate

as long as it’s all in metres, Newtons, and seconds, you’ll be fine!

with every second that passes the objects speed increases by 9.8 m/s

with every second that passes the objects speed increases by 9.8 m/s

Until it hits the ground 😉 when you accelatation could be something in the order of -982 m/s/s

There are plenty of things that make you accerlerate faster than gravity

http://en.wikipedia.org/wiki/Orders_of_magnitude_%28acceleration%29

For you physics and maths bores that haven’t seen this yet check out What-If at http://what-if.xkcd.com/

Be sure to click through Previous to find the hypothetical answers to questions such as

“How much would the sea level fall if every ship were removed all at once from the Earth’s waters?”

“What if I took a swim in a typical spent nuclear fuel pool? Would I need to dive to actually experience a fatal amount of radiation? How long could I stay safely at the surface?”

“In Armageddon, a NASA guy comments that a plan to shoot a laser at the asteroid is like “shooting a b.b. gun at a freight train.” What would it take to stop an out-of-control freight train using only b.b. guns?”

For you physics and maths bores that haven’t seen this yet check out What-If at http://what-if.xkcd.com/

Best blog on the web.