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Large objects have inertia and tend to keep moving-Newton's BI0 first law. Life is very different for small microorganisms that swim through water. For them, drag forces are so large that they instantly stop, without coasting, if they cease their swimming motion. To swim at constant speed, they must exert a constant propulsion force by rotating corkscrew-like flagella or beating hair-like cilia. The quadratic model of drag of Equation 6.15fails for very small particles. Instead, a small object moving in liquid experiences a linear drag force, F→drag=(bv, the direction opposite the motion), where bis a constant. For a sphere of radius R, the drag constant can be shown to be b=6πηR, where ηis the viscosity of the liquid. Water at 20∘Chas viscosity 1.0×10−3Ns/m2.

a. A paramecium is about 100μmlong. If it's modeled as a sphere, how much propulsion force must it exert to swim at a typical speed of 1.0mm/s? How about the propulsion force of a 2.0-μm-diameter E. coli bacterium swimming at 30μm/s?

b. The propulsion forces are very small, but so are the organisms. To judge whether the propulsion force is large or small relative to the organism, compute the acceleration that the propulsion force could give each organism if there were no drag. The density of both organisms is the same as that of water, 1000kg/m3.

Short Answer

Expert verified

Part(a);

  • Propulsion force for the sphere of diameter d=100μmswimming with the speed of 1mm/sis 9.4×10−10N.
  • Propulsion force of 2μmdiameter coil swimming at 30μm/sis 5.7×10−13N.

Part (b):

  • Acceleration of the orgasm for the propulsion force for the sphere of diameter d=100μmsimming with the speed of 1mm/sis 1.8m/s2.
  • Acceleration of the orgasm for the propulsion force of2μmdiameter coil swimming at30μm/sis130m/s2.

Step by step solution

01

Part (a) Step 1: Given.

Viscosity of the liquid, η=1×10−3Ns/m2.

02

Part (a) Step 2: Formula used.

The propulsion force is calculated as:

F=6πηRv.

Here,

Ris the diameter.

ηis the viscosity.

vis the velocity.

03

Part(a) Step 3: Calculation.

Propulsion force for the sphere of diameter d=100μmswimming with the speed of 1mm/sis calculated as,

F=6πηRv

Plugging the values in the above equation

F=6π×1×10−3×100×10−62×1×10−3=9.4×10−10N

Propulsion force of 2μmdiameter coil swimming at 30μm/sis calculated as:

F=6πηRv

Plugging the values in the above equation:

F=6π×1×10−3×2×10−62×30×10−6=5.7×10−13N

04

Part(b) Step 4: Given.

Density of the liquid,ÒÏ=1×103kg/m3.

05

Part(b) Step 4: Formula Used.

The acceleration of the orgasm is calculated by the formula

F=ÒÏVa

Here,

ÒÏis the density

Vis the volume

ais the acceleration

06

Part(b) Step 5: Calculation.

Acceleration of the orgasm for the propulsion force for the sphere of diameter d=100μmswimming with the speed of 1mm/sis calculated as:

F=ÒÏVa

Plugging the values in the above equation

9.4×10−10=1000×43×π×(100×10−62)3a.

a=1.8m/s2.

Acceleration of the orgasm for the propulsion force o2μmdiameter coil swimming at30μm/sis calculated

as,

the speed of 1mm/sis calculated as:

F=ÒÏVa

Plugging the values in the above equation

9.4×10−10=1000×43×π×(100×10−62)3a.

a=1.8m/s2

5.7×10−13=1000×43×π×(2×10−62)3a

a=130m/s2.

07

Conclusions.

Part (a):

  • Propulsion force for the sphere of diameter d=100μmswimming with the speed of 1mm/sis 9.4×10−10N.
  • Propulsion force of 2μmdiameter coil swimming at 30μm/sis5.7×10−13N.

Part (b):

  • - Acceleration of the orgasm for the propulsion force for the sphere of diameter d=100μmsimming with the speed of 1mm/sis 1.8m/s2.
  • Acceleration of the orgasm for the propulsion force of 2μmdiameter coil swimming at 30μm/sis130m/s2.

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Most popular questions from this chapter

An object moving in a liquid experiences a linear drag force: Fudrag=(bv,direction opposite the motion), where bis a constant called the drag coefficient. For a sphere of radius R,the drag constant can be computed asb=6phR, where his the viscosity of the liquid.

a. Use what you've learned in calculus to prove that

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