Physics please help!!! (1 Viewer)

[kashkow]

Hi guys,

Can someone explain why the answer is C? Is the HSC answers wrong or am I just missing something of the slingshot effect?



Like I thought velocity changes but speed of the satellite remains constant in this process... (This q is a multiple choice q from http://mcq.bostes.nsw.edu.au/)

Although now that I think about it, I realise that the speed doesn't change relative to the planet but picks up velocity relative to everything else... Is this why the "speed" changes?

If someone could explain that would be great!

Thanks!

 

[trecex1]

What do you mean the velocity changes but the speed remains constant?
Assuming these are all relative to the sun, a space probe's velocity increases from the transfer of kinetic energy and momentum from the planet, its direction also changes as it's 'curving' around the planet. Velocity is just speed with a direction, and speed increases while it's direction also changes so it would be C.

 

[kashkow]

Well, relative to the planet, the speed does not change. ie. It will approach the planet at x, swing around and leave the planet at x speed in a new direction (thus speed is same but velocity changes). It is only because the planet is moving relative to the sun that the probe's speed increases relative to the SUN. Unless I'm mistaken and speed relative to the planet also changes, which I don't think it does?

 

[trecex1]

Well, relative to the planet, the speed does not change. ie. It will approach the planet at x, swing around and leave the planet at x speed in a new direction (thus speed is same but velocity changes). It is only because the planet is moving relative to the sun that the probe's speed increases relative to the SUN. Unless I'm mistaken and speed relative to the planet also changes, which I don't think it does?

It doesn't say relative to the planet, if the planet is used as the frame of reference then the slingshot effect won't appear to have any effect since it doesn't increase the speed.
That's why the speed and direction both change.

 

[kashkow]

It doesn't say relative to the planet, if the planet is used as the frame of reference then the slingshot effect won't appear to have any effect since it doesn't increase the speed.
That's why the speed and direction both change.

Yeah ok, that is what I was thinking... I feel like this question is vague for not indicating the reference frame; because the answer depends on what reference frame is used (especially since B is there). Is it expected in HSC physics to use the reference point as the sun or some other point in space (besides the planet) when dealing with questions like these? Or is it just worded badly?

I thought it might've been a trick question by making most people think C (since most people think, "slingshot effect, changed velocity") but the use of "speed" rather than "velocity" made me think the answer was B (I was also thinking that slingshot effect only changes direction; forgot relative to the sun it increases actual velocity/speed as well).

Anyway, thanks for your help. I just wanted to make sure there were no holes in my understanding

 

[DatAtarLyfe]

Generally slingshot questions are asked from the frame of the sun, unless stated otherwise.
So yeh, speed and direction changes

 

[kashkow]

Generally slingshot questions are asked from the frame of the sun, unless stated otherwise.
So yeh, speed and direction changes

Oh, makes sense now. Cheers