Nine lives are flashing before their eyes

Unfortunately, cats fall out of windows in cities sometimes. In a famous article , The New York Times notes that the likelihood that a cat survives a fall goes down as the fall distance increases (expected) but then goes back up at very large distances (perhaps unexpected). If the statistics are correct, then there should be some physical reason this occurs. Some have suggested that terminal velocity and cat biology come into play. The article above indicates that cats have a terminal velocity of 60 miles per hour (mph). If we model the drag force F d F_d on a cat as

F d = 1 2 k A v 2 F_d=\frac {1} {2} k A v^2

where A A is the cross-sectional area of the cat, v v is its velocity and k = 1 kg/m 3 k = 1 \text{ kg/m}^3 , what is the cross-sectional area in m 2 \mbox{m}^2 of a 5 kg 5 \text{ kg} cat with a terminal velocity of 60 mph 60 \text{ mph} ?

Details and assumptions

  • The acceleration of gravity is 9.8 m/s 2 -9.8~\mbox{m/s}^2

  • 1 mile = 1.6 km 1 \text{ mile} = 1.6 \text{ km}


The answer is 0.136.

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9 solutions

Santanu Banerjee
Aug 8, 2013

First Convert miles per hour to meters per second

60 mph = 26.8224 mps

Now use the above formula...

Since the cat is falling with uniform velocity,

The gravitational force should be equal to the drag force

Thus 2 m g = A ( v ^ 2 )

=> 2 ( 5 ) ( 9.8 ) = A ( 26.8224 ^2 )

From this we get A = 0.136 square meters

I answered 0.13 and it gave me wrong ;-( !!!!

Utkarsh Bhardwaj - 5 years, 7 months ago

Lol I guessed

Horla asd - 6 years, 7 months ago

I would have preferred it is the question specified the level of precision wanted in the final answer. I first tried 0.14 as this is to two significant figures, the maximum of all the components of the question. Just an idea :) (strictly speaking I should have just said 0.1 due to the 5Kg I believe).

David Baker - 6 years, 7 months ago

Well, actually...

We are told to assume 1 mile = 1.6km, therefore 60mph = 96km/h or 26.67m/s

Substituting in, we get: Upwards force (drag) = 0.5 x 1kg/m^3 x Am^2 x (26.67m/s)^2 = 0.5A x 711.1kg.m/s^2 = 355.56Akg.m/s^2 At terminal velocity, this equals gravity at mg or 5kg x 9.8m/s^2 = 49kg.m/s^2

So 355.56A = 49 A = 49/355.56 A = 0.1378215

Andy Boal - 3 years, 10 months ago
Eric Goodall
Nov 3, 2015

I would suggest fixing the equation so that units make sense. In the end you do not come to an area in m^2, you get an answer of A=0.136kg/m... if you ignore units after getting all terms into meters and seconds it sort of works, but seriously the units should make sense in the end.

No, look at the units: Force is always in Newtons, and one Newton is equal to one kg.m.s-2 - mass is in kg and acceleration is in m.s-2

Take the drag formula given. F = 0.5kAv^2 And substitute the units (remember we are told the drag coefficient k is in kg/m^3): kg.m^-3.m^2.m^2.s^-2 The metre indices cancel down to give: kg.m.s^-2 which is in fact correct.

Andy Boal - 3 years, 10 months ago

Drug coefficient in kg/m3 is obviously missing here. I assumed it was 1 when solving this.

Dmitry Donskoy - 4 years, 6 months ago
Saad Haider
Aug 6, 2013

At terminal velocity, there is no vertical acceleration and that means that F d = W F_d = W . Since W = m g W = mg , we can safely say F d = 5 9.8 = 49 N F_d = 5\cdot 9.8 = 49N . We also have the value v v = 60 m i l / h = 60 mil/h since our gravitational acceleration is in m s 2 ms^{-2} , and the final answer has to be in m 2 m^{2} , hence we have to convert v v from m i l / h mil/h to m s 1 ms^{-1} . Letting 1 1 m i l e = 1.6 mile = 1.6 k m = 1600 km = 1600 m m 60 60 m i l / h mil/h = 60 × 1.6 × 1000 60 × 60 m s 1 = 26.7 = \frac {60\times 1.6\times 1000}{60\times 60} ms^{-1} = 26.7 m s 1 ms^{-1} . From the given formula we know that F d = 1 2 A v 2 F_d = \frac {1}{2}Av^{2} , which can be rewritten as A = 2 F d v 2 A = \frac {2F_d}{v^{2}} . Substituting in F d = 49 F_d = 49 N N and v = 26.7 v = 26.7 m s 1 ms^{-1} , we get A = 2 × 49 26. 7 2 = 0.137 A = \frac {2\times 49}{26.7^{2}} = 0.137 m 2 m^{2}

Thomas Jones
Aug 5, 2013

When an object is falling at terminal velocity it experiences no acceleration so there are no resultant forces acting on the object. This means that the weight of the cat is equal to the force of drag on the cat at terminal velocity:

5 g = 1 2 A v 2 [ 1 ] 5g = \frac{1}{2}Av^{2} [1]

In order to work out the cross-sectional area in m 2 m^{2} , the 60mph has to be converted into m s 1 ms^{-1} . This is done by first dividing the 60mph by 3600 to find it in miles per second and then multiplying by 1609.344 (conversion of miles to m) to find it in m s 1 ms^{-1} :

60 m p h = ( 1 60 1609.344 ) m s 1 = 26.8224 m s 1 60mph = (\frac{1}{60}*1609.344)ms^{-1} = 26.8224ms^{-1}

Rearranging equation [1] for A, we get:

A = 10 g v 2 = 98 719.44 A = \frac{10g}{v^{2}} = \frac{98}{719.44}

By evaluating this you find that:

A = 0.1362 m 2 A = 0.1362m^{2}

Noel Quirol
Aug 10, 2013

A=(2F)/v^2 =2(5)(9.8m/s^2)/(26.817m/s)^2 = (98m/s^2)/(719.15m^2/s^2) = 0.136

Yash Kumar Gupta
Aug 10, 2013

F_d= m a = A (v)^2/2 => 5 9.8= A (711.11111111111111111111111111111)^2 since v=60 miles per second so 1miles = 1.6 km = 1600m so 60 miles per second = (1600*60)/3600 => A= 0.136 m^2 :D

Rogers Epstein
Aug 8, 2013

At terminal velocity, the cat's drag force will equal the gravitational force. This allows the cat to maintain the same falling velocity, as no net force acts on it. Since the gravitational force is the acceleration due to gravity * the cat's mass, the gravitational force is 9.8 * 5 N = 49 N. This must equal the drag force, which is (A* v^2)/2. We are given that v = 60 mph, which me must convert to m/(s^2). (60 mi/ 1 hr) = (60 * 1600 m )/ (3600 s) which is about 26.822 m/s. Putting into our formula:

49 = (A * 26.822^2)/2, and by simple algebra, A=.139 m^2

However, I must question the units in the problem, as the given formula for drag implies force is in units of m^4/s^2. I'm surprised my solution worked due to this.

Rogers Epstein - 7 years, 10 months ago
Andrias Meisyal
Aug 6, 2013

It's simple to get the answer. Follow up the formula above: F d = 1 2 A v 2 Fd = \frac{1}{2}Av^{2} . F d Fd in here, is the weight force of the cat, you can find it using the weight force as the mass of cat multiply the gravity of the place being. Finally, don't forget to change the conversion of velocity from miles per hours to meter per second . (Hint: 1 mile = 1.609,344 meters) .

Tim Vermeulen
Aug 5, 2013

At the terminal velocity, the overall acceleration is 0 0 , i.e. the gravitational force is of the same magnitude as the drag force, but in the other direction, so

F g = F d m g = 1 2 A v 2 A = 2 m g v 2 2 5 9.8 ( 60 0.447 ) 2 0.136 m 2 . F_g = F_d \implies mg = \frac{1}{2}Av^2 \implies A = \frac{2mg}{v^2} \approx \frac{2 \cdot 5 \cdot 9.8}{(60 \cdot 0.447)^2} \approx \boxed{0.136}\text{ m}^2.

Note : 1 mph 0.447 m/s . 1 \text{ mph} \approx 0.447 \text{ m/s}.

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