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# ISAAC Physics impossible question

The waves hit a wall at an angle of 30∘. The distance 𝑑 equals 12m. A new wave peak hits the wall every 2.0s. At what speed do the contact points between the wall and the wave peaks travel along the face of the wall? Give your answer to 2 significant figures.

I tried 1.5 and it says:"The speed of the contact point is not the same as a component of the wave's velocity. Make sure that you're thinking about the distance travelled by the point of contact between wave peaks hitting the wall.

If this is what you're trying to do, check your trigonometry."

(edited 3 years ago)
What is distance d?
Original post by nzy
What is distance d?

I've attached the file below.

Thank you
Could you show the method you used to get 1.5?
Original post by nzy
Could you show the method you used to get 1.5?

I'll be honest with you, I just used the fact that d covered 4 wavefront so 12m/4 = 3m (as wavelength)
Frequency = 1/period so freq = 1/2
Wavespeed = freq x wavelength
Wavespeed = 1.5 m/s
Finding the wave speed is a good start. However, trickily, since one wavelength is the distance between two adjacent peaks, you actually need to count the spaces between each peak/line in the diagram rather than the lines. So rather than there being 4 wavelengths over 12m, there are only 3 wavelengths encompassed by 4 peaks.

Once you've calculated the correct wave speed, you then have to start thinking about components since the question asks for the speed of "the contact points between the wall and the wave peaks" along the wall, which you have to realise is just a long-winded way of asking for the component of the wave speed parallel to the wall. So your final step is to resolve the wave speed into its vertical and horizontal components (and be careful of the angle at which it goes into the wall - drawing a vector diagram may help), where the 'vertical' component in this case is the wave speed along the wall.
Original post by nzy
Finding the wave speed is a good start. However, trickily, since one wavelength is the distance between two adjacent peaks, you actually need to count the spaces between each peak/line in the diagram rather than the lines. So rather than there being 4 wavelengths over 12m, there are only 3 wavelengths encompassed by 4 peaks.

Once you've calculated the correct wave speed, you then have to start thinking about components since the question asks for the speed of "the contact points between the wall and the wave peaks" along the wall, which you have to realise is just a long-winded way of asking for the component of the wave speed parallel to the wall. So your final step is to resolve the wave speed into its vertical and horizontal components (and be careful of the angle at which it goes into the wall - drawing a vector diagram may help), where the 'vertical' component in this case is the wave speed along the wall.

Thank you for your detailed response, will definitely give that a shot !
Original post by nzy
Finding the wave speed is a good start. However, trickily, since one wavelength is the distance between two adjacent peaks, you actually need to count the spaces between each peak/line in the diagram rather than the lines. So rather than there being 4 wavelengths over 12m, there are only 3 wavelengths encompassed by 4 peaks.

Once you've calculated the correct wave speed, you then have to start thinking about components since the question asks for the speed of "the contact points between the wall and the wave peaks" along the wall, which you have to realise is just a long-winded way of asking for the component of the wave speed parallel to the wall. So your final step is to resolve the wave speed into its vertical and horizontal components (and be careful of the angle at which it goes into the wall - drawing a vector diagram may help), where the 'vertical' component in this case is the wave speed along the wall.

I got 1.3m/s, but it states that is wrong?

Again, it gives the error message "The speed of the contact point is not the same as a component of the wave's velocity. Make sure that you're thinking about the distance travelled by the point of contact between wave peaks hitting the wall.

If this is what you're trying to do, check your trigonometry."
🤔
(edited 3 years ago)
Original post by AnonPhysics5364
I got 1.3m/s, but it states that is wrong?

Again, it gives the error message "The speed of the contact point is not the same as a component of the wave's velocity. Make sure that you're thinking about the distance travelled by the point of contact between wave peaks hitting the wall.

If this is what you're trying to do, check your trigonometry."
🤔

I inputted 2m/s to test and I got "This is just the speed of the waves - think about how the point of contact moves over the course of one whole wave."
I would do as it says and check your trig working again, making sure all the angles are correct (the angle you want is not 30 degrees)
Original post by nzy
I would do as it says and check your trig working again, making sure all the angles are correct (the angle you want is not 30 degrees)

Thank you so much it was 4.0 m/s !

I was working on this like forever!

Thank you
Hi,Could you explain your workings? Im still stuck on this one!Thanks
Can't you use 30 degrees by doing 2/sin(30)?
has anyone found the solution? i got stuck as well
(edited 10 months ago)
Original post by azimshurzo
has anyone found the solution? i got stuck as well