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This problem set deals with drag
resistance. |
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| 1. |
Resistance generated by a solid moving
through a fluid is known as drag resistance.
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| 2. |
Drag forces on a car moving through air
can be reduced by (c.) streamlining. |
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| 3. |
A drag force of 200 N is experienced by
a hydrofoil boat moving through water at a speed of 12
m/s. A hydrofoil is a wing-like structure made to lift
the boat hull out of the water at high speeds. Find the
drag resistance of the hydrofoil.
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| Knowns |
Unknowns |
Equations/Remarks |
FD = 200
N
v = 12 m/s |
RD = ? |
RD = FD/v |
| RD = FD/v RD = (200
N)/(12 m/s)
RD = 16.7
N/(m/s)
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Substitute in the known values
and calculate the answer. |
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| 4. |
For the same hydrofoil as in problem 3,
find the drag force when the speed is doubled to 24 m/s.
Note this is still a low speed and there is little
turbulence.
Since there is little turbulence RD
is constant, so doubling the speed doubles the drag
force.
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| 5. |
What drag force is necessary to reduce
the downward acceleration of a 150 kg skydiver to 1 m/s2?
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| Knowns |
Unknowns |
Equations/Remarks |
m = 150 kg
a = 1 m/s2
g = 9.8 m/s2
The downward direction is positive |
FD = ? |
Fnet = FD + w
w = mg
Fnet = ma |
w = mg
Fnet = ma Fnet = FD + w
FD = Fnet – w = ma – mg = m(a – g)
FD = (150 kg)(1 m/s2 – 9.8
m/s2)
FD = –1320
N (the negative means that the drag force is
upwards, opposite to the direction of motion)
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Use the net force equation to
find the drag force. Substitute in the equations
for weight and net force. The sign convention
(which direction is positive and which is
negative) was chosen so that the direction of
travel is the positive direction. By this
convention the drag force has a negative value. Substitute
in the known values and calculate the answer.
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| 6. |
The drag force on an automobile is such
that a force of 50 lb is required to keep it moving at a
speed of 25 mph.
- What is the drag force (FD) when the
automobile moves at 75 mph? Assume that FD
is proportional to velocity.
- If FD varies as the square of the
velocity, what is FD when the
automobile moves at 75 mph?
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| Knowns |
Unknowns |
Equations/Remarks |
F1 = 50
lb
v1 = 25 mph
v2 = 75 mph |
a. F2 = ?
(linear relation)
b. F2 = ? (square relation) |
F2 = F1(v2/v1)
F2 = F1(v2/v1)2
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a. F2 = F1(v2/v1) = (50
lb)(75 mph/25
mph)
F2 = 150
lb b. F2 = F1(v2/v1)2
= (50 lb)(75 mph/25
mph)2
F2 = 450
lb
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Substitute in the known values
and calculate the answer for each case. |
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