Ultrasound waves and the kidney

[Kolika28]

Homework Statement

In a kidney study, ultrasound was used with frequency 3.5 MHz. In normal body tissue, the speed of the ultrasound is 1.50 km / s, while the speed in the kidney tissue is 1.55 km / s. Reflected signals came as follows: after 20 microseconds from the skin, after 75 microseconds from the front of the kidney, and after 152 microseconds from the back of the kidney.
Find out how long the kidney is across.

Homework Equations

S=v*t

The Attempt at a Solution

(152-75-20)/2=28,5
S=v*t=(1,55*1000)*(28,5*10^(-6)=0,044175

But the answer is 6 cm

 

[gneill]

The Attempt at a Solution

(152-75-20)/2=28,5
S=v*t=(1,55*1000)*(28,5*10^(-6)=0,044175

Please elaborate your reasoning; can you explain in words what your calculation is trying to accomplish?

Have you made a sketch of the sound paths?

 

[PeroK]

Homework Statement

In a kidney study, ultrasound was used with frequency 3.5 MHz. In normal body tissue, the speed of the ultrasound is 1.50 km / s, while the speed in the kidney tissue is 1.55 km / s. Reflected signals came as follows: after 20 microseconds from the skin, after 75 microseconds from the front of the kidney, and after 152 microseconds from the back of the kidney.
Find out how long the kidney is across.

Homework Equations

S=v*t

The Attempt at a Solution

(152-75-20)/2=28,5
S=v*t=(1,55*1000)*(28,5*10^(-6)=0,044175

But the answer is 6 cm

Explain to yourself the path of the ultrasound, broken down into stages with the time for each.

 

[Kolika28]

Ok, I can try to explain. 152 microseconds is the time it takes to the back of the kindney and back, therefore I divide with 2, because I only need one way. I don't need to find the length from the skin to the back of the kidney, so therefore I subtract 75 and 20. Then I multiply seconds I'm left with with the speed through the kidney tissue. I don't see what I have done wrong. Could someone tell me?

 

[Kolika28]

My bad, I should have read the question better. It should be (152-75)/2. Thank's for your help!