The Physics PHYA2 thread! 5th June 2013

Reply 2040

Possibly but I think that's specifically for that mystery liquid. Ball A didn't have an deceleration with oil, so I don't think B should either.

That's because A started in oil b started in air

Reply 2041

Davelittle

13

Original post by ClarkyC...

Was it not decelerating?

Isn't decelerating when its velocity decreases?

Reply 2042

NabRoh

11

Original post by Bookaky

For the graph about the ball and the oil I think it is debatable. Surely the ball would continue to accelerate in the oil for only a short period of time and then level off for terminal velocity?

This is correct if we're talking about the motion of ball B.

- Straight (linear) (+ve) acceleration up to hitting oil
- Vertical drop in velocity as ball hits oil
- Slight curve upwards as ball accelerates in the oil
- Levels out as new terminal velocity reached
- Vertical drop in velocity as ball hits bottom

(edited 10 years ago)

Reply 2043

ClarkyC...

1

Original post by Davelittle

Isn't decelerating when its velocity decreases?

It's velocity is decreasing

Reply 2044

Davelittle

13

Original post by ClarkyC...

It's velocity is decreasing

Are you talking about the 6 marker or the graph? :P

On the 6 marker it's velocity didn't decrease at all

Reply 2045

1029384756

Original post by Davelittle

Isn't decelerating when its velocity decreases?

yep, how many marks you think I'll get if I basically did the other way around and said the velocity is largest at start and it decelerates? (Yeah I completely messed up)

Reply 2046

StalkeR47

5

Original post by NabRoh

This is correct if we're talking about the motion of ball B.

- Straight (linear) (+ve) acceleration up to hitting oil
- Vertical drop in velocity as ball hits oil
- Slight curve upwards as ball accelerates in the oil
- Levels out as new terminal velocity reached
- Vertical drop in velocity as ball hits bottom

Wait..... How can it be a straight line? If the ball is dropped from the rest, it should accelerate until it hits the oil surface. So, the graph is curve showing that velocity is increasing more with less time.

Reply 2047

ClarkyC...

1

Original post by Davelittle

Are you talking about the 6 marker or the graph? :P

On the 6 marker it's velocity didn't decrease at all

Sorry no the velocity didnt decrease on the 6 marker but I'm not sure whether its decelerating or not. I'm going off another past paper as to why I think it's deceleration

Reply 2048

masryboy94

8

Original post by StalkeR47

Wait..... How can it be a straight line? If the ball is dropped from the rest, it should accelerate until it hits the oil surface. So, the graph is curve showing that velocity is increasing more with less time.

that is what i thought too, but maybe they are considering acceleration to be due to gravity hence you get a straight line?

Reply 2049

FLLF

For the very last question, I put that:

There would be a spectrum of colours rather than just red
The fringe spacing will be smaller because the maxima are wider and the minima are smaller (I think this is wrong)
The central fringe would be white rather than red

How many marks would this be?

Reply 2050

ClarkyC...

1

Right the diameter was 2.2x10-3 of the sphere and the density was 8100kgm-3. I can't get to 0.44N what am I doing wrong

Reply 2051

JD1234

Original post by Boop.

Thats what I put I think I remember it from an older past paper, I said that the ball didnt decelerate, but that it still accelerated in the fluid, the acceleration was decreasing with time, not sure if im right though..

Small balls of this type (hehe).. reach terminal velocity very fast, so when B was released it would have accelerated with constant acceleration and almost reached terminal velocity.. when it hit the oil the drag would increase by ALOT... so it's speed decreases as it moves through the oil, we know this acceleration is not constant/is non-uniform as the curve for A shows this.

A accelerated with non-uniform acceleration, so when B hits the oil it will decelerate with non-uniform deceleration. So the graph for B, I think, should be a straight line up to the point where it hits the oil then a curve slopping downwards and leveling off at terminal velocity.

Reply 2052

Davelittle

13

Original post by StalkeR47

Wait..... How can it be a straight line? If the ball is dropped from the rest, it should accelerate until it hits the oil surface. So, the graph is curve showing that velocity is increasing more with less time.

a is constant so it would be a straight line (think about it, if gradient=a and a=g=9.81m/s^2 then the gradient has to be constant, what else is accelerating it?)

Reply 2053

Davelittle

13

Original post by ClarkyC...

Right the diameter was 2.2x10-3 of the sphere and the density was 8100kgm-3. I can't get to 0.44N what am I doing wrong

Find r-> find V-> find m-> W=mg

Reply 2054

FLLF

Original post by ClarkyC...

Right the diameter was 2.2x10-3 of the sphere and the density was 8100kgm-3. I can't get to 0.44N what am I doing wrong

The diameter was 2.2x10-2

So W = (4/3) * pi * (2.2x10-2/2)^3 * 8100 * 9.81

Reply 2055

MeerkatM

how many squares did people get under the graph?

Reply 2056

Davelittle

13

Original post by ClarkyC...

Sorry no the velocity didnt decrease on the 6 marker but I'm not sure whether its decelerating or not. I'm going off another past paper as to why I think it's deceleration

Its acceleration decreases, I don't think that is the same as deceleration?

If someone decelerates from 10m/s to 0m/s its velocity goes down, not up.

Reply 2057

Davelittle

13

Original post by MeerkatM

how many squares did people get under the graph?

42 i think?

Reply 2058

1029384756

Original post by FLLF

For the very last question, I put that:

There would be a spectrum of colours rather than just red
The fringe spacing will be smaller because the maxima are wider and the minima are smaller (I think this is wrong)
The central fringe would be white rather than red

How many marks would this be?

Full marks the reasons not right for the second one but nothing is "inncorect" the reasons becuase the average wavelength of white light is smaller than red light