# Very challenging physics question

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#1
I found this in the stretch and challenge section of my textbook and I have no clue how to do it! A family cruising upstream on a river (against the river flow) pass a bridge at point A where they drop their sandwich box... they do not notice this for ten minutes until they are at point b where they turn (no time wasted) and pick up the sandwich box at point C. C is 1km away from point the bridge i.e. point A. Calculate the velocity of the river assuming the boat has the same velocity relative to the water up and downstream.

Apparently you do not need to know the velocity of the boat but you DO need to set up equations to calculate the time taken to catch up with the sandwich box

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5 years ago
#2
(Original post by Lola1244)
I found this in the stretch and challenge section of my textbook and I have no clue how to do it! A family cruising upstream on a river (against the river flow) pass a bridge at point A where they drop their sandwich box... they do not notice this for ten minutes until they are at point b where they turn (no time wasted) and pick up the sandwich box at point C. C is 1km away from point the bridge i.e. point A. Calculate the velocity of the river assuming the boat has the same velocity relative to the water up and downstream.

Apparently you do not need to know the velocity of the boat but you DO need to set up equations to calculate the time taken to catch up with the sandwich box

what exam board?
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#3
(Original post by raman_17)
what exam board?
AQA but managed to solve 😅
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5 years ago
#4
(Original post by Lola1244)
AQA but managed to solve 😅
I also do AQA.

So how was it solved then.
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5 years ago
#5
So much to pick a box of soggy sandwiches and give students a hard time.
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5 years ago
#6
(Original post by Lola1244)
AQA but managed to solve 😅
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#7
(Original post by langlitz)
See i got 3 km per hour but looking back on it i think i have misunderstood the meaning of 'same relative velocity'
please could you explain what that means
there are unfortunately no answers for this section in the book
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5 years ago
#8
I got 3 kmph as well.
Do we know what the correct answer is?
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5 years ago
#9
(Original post by Lola1244)
See i got 3 km per hour but looking back on it i think i have misunderstood the meaning of 'same relative velocity'
please could you explain what that means
there are unfortunately no answers for this section in the book
Lol I didnt read it properly. I calculated the time it took for the boat to return to the sandwich, hang on, I'll try it again
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5 years ago
#10
Can someone show me how this is done? Really interesting - or at least give me hints. Is C after B or between A and B?
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5 years ago
#11
(Original post by abhishekagarwal)
I got 3 kmph as well.
Do we know what the correct answer is?
(Original post by Lola1244)
See i got 3 km per hour but looking back on it i think i have misunderstood the meaning of 'same relative velocity'
please could you explain what that means
there are unfortunately no answers for this section in the book
Yeah I got 3 km/h
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5 years ago
#12
(Original post by h8skoooooool)
Can someone show me how this is done? Really interesting - or at least give me hints. Is C after B or between A and B?
<-- River flow
==C====||A====B
......| 1km |
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#13
(Original post by langlitz)
Yeah I got 3 km/h
Great! So when it says the boat has the same velocity relative to the river... lets say the boats velocity going down is x and the velocity of the river is y...
the velocity of the boat going from A to B is x-y
and the velocity of the boat going from B to C is x+y

i just dont understand what it means by the same velocity reltive to the river, does it just mean that it has the same velocity up and down?

I have a really hard waves question too if anyone is up for that? :')
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#14
(Original post by abhishekagarwal)
I got 3 kmph as well.
Do we know what the correct answer is?
Unfortunately we don't because the book doesn't have answers for this section! I have a question on waves though that is also confusing me ://

the question is two microscope slides of length L are placed flat with one on top of the other. One end of the top slide is raised through a small distance D, enclosing a wedge of air between the two slides. Monochromatic light shines onto the slides and an interference pattern of parallel, equally spaced fringes can be seen.
Light reflects in two places: from the bottom face of the upper slide and from the upper face of the lower slide. Which reflected ray undergoes a phase change of 180 degrees?
I mean when light enters the gap surely it will reflect of the upper face of the bottom slide undergoing a phase change of 180 and then again from the lower face of the upper slide :'(((((
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5 years ago
#15
(Original post by Lola1244)
Great! So when it says the boat has the same velocity relative to the river... lets say the boats velocity going down is x and the velocity of the river is y...
the velocity of the boat going from A to B is x-y
and the velocity of the boat going from B to C is x+y

i just dont understand what it means by the same velocity reltive to the river, does it just mean that it has the same velocity up and down?

I have a really hard waves question too if anyone is up for that? :'
Yes that's basically it. It all depends on the position of the observer. When the boat is moving upstream: an observer on the shore would see the boat moving at x-y relative to the shore, and the water moving at -y. Whereas an observer in the water would see the boat moving at x, the water as stationary and the shore moving past at y.

In this scenario we are solving it from above which is effectively the same as observing from the shore. I hope that makes sense
1
5 years ago
#16
(Original post by Lola1244)
Unfortunately we don't because the book doesn't have answers for this section! I have a question on waves though that is also confusing me ://

the question is two microscope slides of length L are placed flat with one on top of the other. One end of the top slide is raised through a small distance D, enclosing a wedge of air between the two slides. Monochromatic light shines onto the slides and an interference pattern of parallel, equally spaced fringes can be seen.
Light reflects in two places: from the bottom face of the upper slide and from the upper face of the lower slide. Which reflected ray undergoes a phase change of 180 degrees?
I mean when light enters the gap surely it will reflect of the upper face of the bottom slide undergoing a phase change of 180 and then again from the lower face of the upper slide :'(((((
From my notes: "There is a phase shift of half a wavelength, pi, when light is reflected at the surface of a higher refractive index. Note that there is no phase shift when light is reflected at the surface of a lower refractive index material"
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#17
(Original post by langlitz)
From my notes: "There is a phase shift of half a wavelength, pi, when light is reflected at the surface of a higher refractive index. Note that there is no phase shift when light is reflected at the surface of a lower refractive index material"
So which one would it be because on both reflections it is from air to glass?
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5 years ago
#18
(Original post by Lola1244)
So which one would it be because on both reflections it is from air to glass?
No it reflects from the bottom of the upper slide, that's from glass to air. And the top of the lower slide, that's from air to glass
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#19
(Original post by langlitz)
No it reflects from the bottom of the upper slide, that's from glass to air. And the top of the lower slide, that's from air to glass
How? So it refracts from the glass into the air, strikes the lower slide where it reflects and then strikes the upper side where it will reflect again?
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5 years ago
#20
(Original post by Lola1244)
How? So it refracts from the glass into the air, strikes the lower slide where it reflects and then strikes the upper side where it will reflect again?
I don't think you are imagining this correctly. The light is shining from above

See here:
http://www.schoolphysics.co.uk/age16...ges/index.html
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