Doppler/Mechanics Questions

[900824M]

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HI

I was just wondering if you could help me out with these mechanics and doppler questions.

I'm really having a hard time figuring out how to do these questions

Mechanics

*A driver's skull(5kg) decelerates from 60km/h to 0 km/h in 0.75s during a road traffic accident. Calculate the impact force experienced by the skull.

*A mass of 5kg is dropped from a height of 20m and imbeds itself in the skull of a passerby. Calculate
(i) the velocity with which the object striked the skull
(ii)the force of impact if it comes to a complete stop within .25s (g=9.8m/s2)

I just get stuck on conversions and the velocity/displacement equations !


Doppler Questions

*A Doppler ultrasound trasmitter operating at 5MHz is used to measure blood flow in a carotid artery of cross-sectional area of 2cm2.If the reciever detects signals which are doppler shifted by 2.5kHz. What is the blood velocity along the direaction parallel to the transmitter? If at some other location the blood flow is measured as 0.5m/s . What would be the cross sectional area of the artery at this point?

*A Doppler ultrasound flow meter operating at a frequency of 6MHz measures the blood flow in an artery to be 20m/s. What doppler shift (in Hz) is detected by the system?


I dont understand what they mean by cross sectional area and direction of blood flow. Does it make a difference?


Thats it -- i know i should have some sort of answer for you guys but i just suck at physics 😳



THANKS

 

[Handinhand]

*A driver's skull(5kg) decelerates from 60km/h to 0 km/h in 0.75s during a road traffic accident. Calculate the impact force experienced by the skull.

This is asking for the impulse. You can do it multiple ways. You can find the force of the skull by finding the deacceleration then multiplying by the mass. Then with that, you use I = F * delta t. Your change in time is the duration of impact.

Impulse is also I = m * delta V, or I = mVo - mVi. I think you can figure that out from here.

*A mass of 5kg is dropped from a height of 20m and imbeds itself in the skull of a passerby. Calculate
(i) the velocity with which the object striked the skull
(ii)the force of impact if it comes to a complete stop within .25s (g=9.8m/s2)

i) For this I would just use energy conservation. Find the potential energy of the mass from 20m. Then, assume that when the skull is struck, all energy has been converted to KE. So use the PE value you calculated before, set that equal to .5mv^2, and solve for V.

ii) again, this is an impulse problem. Solve it exactly like the problem from above. I'll let you figure that out.

Doppler Questions

*A Doppler ultrasound trasmitter operating at 5MHz is used to measure blood flow in a carotid artery of cross-sectional area of 2cm2.If the reciever detects signals which are doppler shifted by 2.5kHz. What is the blood velocity along the direaction parallel to the transmitter? If at some other location the blood flow is measured as 0.5m/s . What would be the cross sectional area of the artery at this point?

*A Doppler ultrasound flow meter operating at a frequency of 6MHz measures the blood flow in an artery to be 20m/s. What doppler shift (in Hz) is detected by the system?

I think they want you to rely on your past knowledge of fluid motion. I'll let you look back in your book and find an equation (you might have to derive it, I know my book doesn't give it flat out) that relates cross sectional area and velocity to eachother.

The thing with Doppler problems with blood flow, there is actually two shifts. One where the probe sends sound to the the moving blood, and then when the moving blood reflects the sound back. So you have a moving observer, and then a moving source, both being the blood. So you have to calculate the shift first as if the blood was hearing it, use that found frequency to then shift another time as if the blood was the source and the probe was observing.

As for direction, we always just assumed the blood was moving towards the probe. Speed of sound, especially in a dense fluid (like blood), moves so fast that the duration it takes for the probe to send sound out an impulse and then receive is so minute that I doubt position is going to change. If you were asking about the impact direction makes, that's different though. If something is moving towards a sound source, the shift is going to be an increase of frequency, where as if you're moving away from the source, the shift is going to lower.

Hope this helps.

THANKS

Also, this forum isn't supposed to be for homework help. I empathized with you though, those physics help foums always take forever to answer your question.

 

[900824M]

Thanks a lot - this isnt homework these are past paper questions 🙂
i have an exam soon 🙁

 

[BerkReviewTeach]

HI

I was just wondering if you could help me out with these mechanics and doppler questions. REMOVED INSERTED IMAGE I'm really having a hard time figuring out how to do these questions

Mechanics

*A driver's skull(5kg) decelerates from 60km/h to 0 km/h in 0.75s during a road traffic accident. Calculate the impact force experienced by the skull.

*A mass of 5kg is dropped from a height of 20m and imbeds itself in the skull of a passerby. Calculate
(i) the velocity with which the object striked the skull
(ii)the force of impact if it comes to a complete stop within .25s (g=9.8m/s2)

I just get stuck on conversions and the velocity/displacement equations !


Doppler Questions

*A Doppler ultrasound trasmitter operating at 5MHz is used to measure blood flow in a carotid artery of cross-sectional area of 2cm2.If the reciever detects signals which are doppler shifted by 2.5kHz. What is the blood velocity along the direaction parallel to the transmitter? If at some other location the blood flow is measured as 0.5m/s . What would be the cross sectional area of the artery at this point?

*A Doppler ultrasound flow meter operating at a frequency of 6MHz measures the blood flow in an artery to be 20m/s. What doppler shift (in Hz) is detected by the system?


I dont understand what they mean by cross sectional area and direction of blood flow. Does it make a difference?


Thats it -- i know i should have some sort of answer for you guys but i just suck at physics 😳



THANKS

Diggidy gave you a great summary of how to attack those questions with some great insights. (Excellent job Diggidy!)

If you can make those questions into four-option multiple-choice questions, then you might find more replies. Until then, because this looks more like either a homework task or the promotion of another site, you probably won't get very many replies.

Why'd you put that website clipin tag in your message anyway?