Pre-lab: Motion of Objects through Fluids         Name:

 

You are going to drop marbles in a 50 cm tall beaker of water.  These marbles have a density of 1.67 g/cc with a diameter of 9/16”.  The marbles are encased in a very thin plastic cylinder so that the cylinder is vertically aligned while dropping through the water.  Below is the results of the experiment.

FW = m g;

What is the weight of one marble?

At terminal velocity, what is the drag

force compared to the weight?

 

Then calculate Fdrag = FW - FB

 

ρ = m/V           or        m = ρ (4/3 π r3)

      V = 4/3 π [½ 9/16”(.0254m / 1”) ]3

 

FBouyant = FW displaced fluid

FBouyant =        ρ         (4/3 π r3)      g

FBouyant = 1000kg/m3 (4/3 π r3) 10 m/s2

 

# of

dropped marbles

Fdrag

(Newtons)

time

(sec)

± 0.02

v_final

(m/s)

± 0.003

Which equation best describes the dependence?

            FD   ∝   1 / v2

            FD   ∝   1 / v

            FD   ∝   v

            FD   ∝   v2

 

slope =

y-int =

 

1

 

2.35

0.216

2

 

1.68

0.303

3

 

1.40

0.375

4

 

1.23

0.433

7

 

0.97

0.573

10

 

0.85

0.685

 

Similar to the lab plot drag force vs speed and log drag force vs log speed to be able to answer the above questions.    (Hint:  Best fit lines only apply to graphs with linear lines, not curved)

Note:  lab requests Resistant Force vs speed where Resistance Force = Fdrag + FBouy.  But buoyant force is constant, so how would buoyant force appear in your slope-intercept form equation?

 

 

                    Drag force vs speed                                                log(Drag force) vs log(speed)