# Very challenging physics question

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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

rep to best answer definitely!!!!

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

rep to best answer definitely!!!!

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#2

(Original post by

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

rep to best answer definitely!!!!

**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

rep to best answer definitely!!!!

what exam board?

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(Original post by

what exam board?

**raman_17**)what exam board?

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#4

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(Original post by

Is the answer 10?

**langlitz**)Is the answer 10?

please could you explain what that means

there are unfortunately no answers for this section in the book

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#9

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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

**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

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#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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#11

**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

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#12

(Original post by

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?

**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?

==C====||A====B

......| 1km |

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(Original post by

Yeah I got 3 km/h

**langlitz**)Yeah I got 3 km/h

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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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?

More questions follow but you need to have answered this one...

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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#15

(Original post by

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? :'

**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? :'

In this scenario we are solving it from above which is effectively the same as observing from the shore. I hope that makes sense

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#16

(Original post by

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?

More questions follow but you need to have answered this one...

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 :'(((((

**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?

More questions follow but you need to have answered this one...

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 :'(((((

**no phase shift**when light is reflected at the surface of a lower refractive index material"

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(Original post by

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

**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"
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#18

(Original post by

So which one would it be because on both reflections it is from air to glass?

**Lola1244**)So which one would it be because on both reflections it is from air to glass?

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(Original post by

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

**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

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#20

(Original post by

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?

**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?

See here:

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

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