Exam mechanics question (1 Viewer)

[cssftw]

I ran out of time. I got the 3/2 vt bit and was about to finish the question when they called pens down. So i finished off with a quick 'by inspection, this is equal to whatever'

yeah same lol, but I woulda thought you'd only need to write down 3/2 vt and then say its half of 3vt... to get full marks? srsly i thought they were pretty lax with the marking for 4U

 

[Implying]

they are, generally, as long as thye can tell you know what you're doing, which is why i'm pretty sure i only lost 1 mark

 

[caffeinated4]

I let initial v for jac = 1/3 VT
and final = 2/3 vt
initial for gil = 3vt
final = 3/2 vt

and then i sub these in into the equation and the expressions i got were equal so i said the time to do blahh blah blah is equal.

lol i dont think thats right

I think it's right because that's exactly what I did. >_>

 

[DavidWen]

Terminal velocity is by no means the maximum velocity you'll get=="
You may very well have a velovity greater than the terminal velocity, and mind you you can travel exactly at the terminal velocity! It's just the stable velocity you'll acquire after some time.

Another instance of coaching institutes teaching people wrong stuff. Forget the lim t-> infinity bollocks, it's wrong.

 

[khorne]

Terminal velocity is by no means the maximum velocity you'll get=="
You may very well have a velovity greater than the terminal velocity, approach it, oscillate about it, etc... and mind you you can travel exactly at the terminal velocity! It's just the stable velocity you'll acquire after some time.

Another instance of coaching institutes teaching people wrong stuff. Forget the lim t-> infinity bollocks, it's wrong.

terminal velocity is when a-> 0

 

[DavidWen]

terminal velocity is when a-> 0

Yes it is, never said it woznt. Just responding to the idea that terminal velocity can never be reached or exceeded.

 

[b3kh1t]

Terminal velocity is by no means the maximum velocity you'll get=="
You may very well have a velovity greater than the terminal velocity, approach it, oscillate about it, etc... and mind you you can travel exactly at the terminal velocity! It's just the stable velocity you'll acquire after some time.

Another instance of coaching institutes teaching people wrong stuff. Forget the lim t-> infinity bollocks, it's wrong.

Like Khorne said it is when the acceleration is equal to zero, therefore you cannot gain any more speed to go faster, and obviously you can travel at that speed but it's the maximum speed reachable

 

[DavidWen]

Like Khorne said it is when the acceleration is equal to zero, therefore you cannot gain any more speed to go faster, and obviously you can travel at that speed but it's the maximum speed reachable

No it is not. is there a particular reason why acceleration can't be negative in the downward direction, i.e. directed upwards? Exceed the terminal velocity and slow down. It's just the velocity at which acceleration is 0, not an unreachable velocity.
I understand the argument that one cannot exceed the terminal velocity when falling at a speed smaller than that. But if you start off with a greater velocity than the terminal velocity, there is nothing stopping the resistance from exceeding the gravity and slowing you down. THis is the case with parachute infantries jumping down from aeroplanes. They do not open the chutes till they are clear from the aeroplane, at which time they are travelling at an initial velocity faster than the terminal velocity, and by the time they touch the ground they have slowed down to the terminal velocity.

 

[b3kh1t]

No it is not. is there a particular reason why acceleration can't be negative in the downward direction, i.e. directed upwards? Exceed the terminal velocity and slow down. It's just the velocity at which acceleration is 0, not an unreachable velocity.
If it woz to be unreachable, think of this: a parachute infantry jumps out of the aeroplane and accelerates all the way (that is, the acceleration is never 0, presumably), wots the chance do you reckon he'll be alive?

Ok first I going to clarify what terminal velocity is, you have 2 forces acting on you when in mid air, air resistance and gravity, now you are always acclerating downwards due to gravity, however when the air resistance is equal to the gravity you acceleration becomes zero and you have reached (i said reached, read what I said properly it is the maximum speed reachable) and if the acceleration becomes negative when travelling downwards then you will start slowing down, deccelerating or begin to move upwards which is impossible.

 

[DavidWen]

Ok first I going to clarify what terminal velocity is, you have 2 forces acting on you when in mid air, air resistance and gravity, now you are always acclerating downwards due to gravity, however when the air resistance is equal to the gravity you acceleration becomes zero and you have reached (i said reached, read what I said properly it is the maximum speed reachable) and if the acceleration becomes negative when travelling downwards then you will start slowing down, deccelerating or begin to move upwards which is impossible.

look, i understand it. i got this question out in the HSC. I'm saying, if you start out with a velocity greater than the terminal velocity, you will slow down until you reach the terminal velocity.
Consider the sentence,
"then you will start slowing down, deccelerating or begin to move upwards which is impossible",
indeed sir it's very impossible. if the acceleration is upwards, you slow down, until youve reached the terminal velocity, assuming that you started out travelling faster than it.

Now, wot we learnt in high school is simplified. the terminal velocity for a blanket is not equal to the terminal velocity of a pin, the shape of an object affects its air resistance. The same way that the terminal velocity for a parachute is smaller than the terminal velocity of a human body. A parachutist, travelling it midair, has a velocity greater than the terminal velocity of the parachute, before the chute is opened. When the chute opens, he deccelerates until he reaches the terminal velocity. Now the acceleration is 0, and he doesnt slow down further or stop and travel upwards---it's just a fantastic vision.

 

[b3kh1t]

look, i understand it. i got this question out in the HSC. I'm saying, if you start out with a velocity greater than the terminal velocity, you will slow down until you reach the terminal velocity.
Consider the sentence,
"then you will start slowing down, deccelerating or begin to move upwards which is impossible",
indeed sir it's very impossible. if the acceleration is upwards, you slow down, until youve reached the terminal velocity, assuming that you started out travelling faster than it.

Now, wot we learnt in high school is simplified. the terminal velocity for a blanket is not equal to the terminal velocity of a pin, the shape of an object affects its air resistance. The same way that the terminal velocity for a parachute is smaller than the terminal velocity of a human body. A parachutist, travelling it midair, has a velocity greater than the terminal velocity of the parachute, before the chute is opened. When the chute opens, he deccelerates until he reaches the terminal velocity. Now the acceleration is 0, and he doesnt slow down further or stop and travel upwards---it's just a fantastic vision.

yeh exactly but first of all to begin with a speed higher than the terminal velocity you must be shot out or something but she jumped for the plane, aka initial velocity equals zero and yes obviously I get that that's what people above you explained, I didn't read the question fully and I understand the terminal velocity was that with the parachute opened and she opened it when she reached 3 times that speed, and mind you I did answer this question fully I just didn't read all that information, so I was not taught wrong stuff and my teacher is the perfect and one of the most intelligent gentleman I have ever met.

 

[DavidWen]

Oh, I must apologise for not reading through the thread properly then. It's my fault. Never meant to get your teacher involved either, just that I was taught by a coaching institute called "intuition", if you happen to have heard of it, that the terminal velocity was lim t-> infinity V. Quite outrageously wrong, and I thought it was a popular misconception. Turns out that no one is been taught these dodgy stuff but me.

 

[b3kh1t]

Oh, I must apologise for not reading through the thread properly then. It's my fault. Never meant to get your teacher involved either, just that I was taught by a coaching institute called "intuition", if you happen to have heard of it, that the terminal velocity was lim t-> infinity V. Quite outrageously wrong, and I thought it was a popular misconception. Turns out that no one is been taught these dodgy stuff but me.

hahahaha aiight cool, anyway best of luck with things

 

[DavidWen]

hahahaha aiight cool, anyway best of luck with things

Yeah, could use some. you too then.