# assignment 3 last question

Discussion in 'Physics' started by kaname08, Dec 1, 2010.

1. ### kaname08 Thread Starter Member

Sep 22, 2010
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0
i really need your help , there is three question in 8 items that i cant really solve hope you could help,,

1) a golf course sprinkle system discharges a water from a horizontal pipe a a rate of 7200 cm cube/s. At one point of the pipe, where the radius is 4.00 cm, the water's absolute pressure is 2.40 x 10 to the 5th Pa. At a second point in the pie, the water passes through a constriction where the radius is 2.00 cm. what is the water's absolute pressure as it flows through this constriction?

2) A 78-kg person an apparent mass of 54-kg (because of buoyancy) when standing in water that comes up to the hips. Estimate the mass of each leg. Assume that the body has SG= 1.00

3)A 48.0-kg piece of wood floats in water but is found to sink in alcohol (SG=0.79) in which it has an apparent mass of 0.047 kg. What is the SG of the wood?

thank you hope you could help,,

2. ### jpanhalt Expert

Jan 18, 2008
6,313
1,189
#2 If mass outside of water (ignoring the buoyancy of air) is 78 kg and with both legs in water is 54 Kg, then the legs are 24Kg given the specific gravity of 1.0. Now, if he is a left-footed place kicker in the NFL and the left leg is 5% bigger than the right, the right leg is 11.707 Kg and the left is 12.293 Kg.
#3 The wood displaces (48 - 0.047) Kg of alcohol with a s.g. of 0.79, which equals 60.7 L. QED.

John

3. ### jpanhalt Expert

Jan 18, 2008
6,313
1,189
Oops, I misread question #2. If it is a man, then each leg must 8 Kg. If it is a woman, then she is too fat.

John

4. ### kaname08 Thread Starter Member

Sep 22, 2010
37
0
what L QED ,and ddo i have to put if he/ she is left kicker??,,so it could be 24kg divided by two,right??

5. ### jpanhalt Expert

Jan 18, 2008
6,313
1,189
QED has several meanings, including its original Latin, quod erat demonstrandum. In the context I used, it means that given the problem worked as far as I did it, the rest should be doable by you. In other words, QED = quite easily done in its English, bastardized meaning.

As for the weight of legs (question #2), it is a silly question, requiring lots of unstated assumptions*, so I gave silly answers. Nevertheless, you should see the principle. If you don't, please clarify where you are confused.

John

*One of the assumptions one must make is to ignore the contribution to mass and volume of the pelvis and anatomical parts therein.

6. ### BillO Distinguished Member

Nov 24, 2008
994
139
#3 Do you know how to calculate density? D=Mass/Volume

John gave you the volume of the wood. You know the mass. Take the next step.

#2 12kg per leg would be enough of an answer.

#1 Where:

p1=pressure @ 4cm
V1=water velocity @ 4cm
p2=pressure @ 2cm
V2=water velocity @ 2cm

Then : p2= p1-(V2*V2 - V1*V1)/2

P1 is given
Caculate V1 from the given flow rate (f) and using the radius (4cm) to get cross-sectional area.
Calulate V2 the same way, but using the 2cm radius.

The formula for velocity (V) is;

V = f / pi*r*r

7. ### kaname08 Thread Starter Member

Sep 22, 2010
37
0
i know now what to do next... but i just wanna to clear how it happened,,

for #2 how can you say that it should only be subtracted??
#3 why it should be subtracted

thank you

8. ### BillO Distinguished Member

Nov 24, 2008
994
139

Because, since we are asked to assume the body's density is 1, any part of the body that is in the water will be weightless and will not contribute to the apparent weight.

This is a special case of Bernoullis principle (water is incompressible and has a density of 1). Please look up the equations and their derivation. The physics reason, if you want to call it that, is that, at the smaller radius, the velocity (V2) must be higher and therefore the pressure (P2) must be lower.