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What is the Physics involved when a cat dropped upside down lands on four legs?

If a cat is held upside down and dropped, it is able to execute a twist and to land upright, even if it has no initial angular momentum.

To Quote from Conceptual Physics by Paul G Hewitt:

“Zero-angular momentum twists and turns are performed by turning one part of the body against the other. While falling, the cat rearranges its limbs and tail several times to change its rotational inertia (moment of inertia) repeatedly until it lands feet downward. During this maneuver the total angular momentum remains zero. When it is over, the cat is not turning. This maneuver rotates the body through an angle, but it does not create continuing rotation. To do so would violate angular momentum conservation”

Starry Night : Why do stars twinkle? Can you answer?

We have been hearing the rhyme “twinkle twinkle little star” since we were kids.

Why stars twinkle? Can anyone give a fitting explanation?

See below a world famous painting “Starry Nights” by Van Gogh.

Alternating Current circuits

John Abraham Asks:

“a circuit has a battery of emf E and internal resistance r.two inductors of inductance L1 and L2 are connected in series to the battery but parallel to each other.what is the current through each inductor in steady state?”

Answer: A pure inductor does not offer any reactance to pure dc. Therefore the only resistance in circuit is the internal resistance. But since the two inductors are in parallel, current through each of them is half the total current in circuit, = E/2r

Another problem from kinematics

Zeen asks:

A boy jumps from rest, straight down from the top of a cliff. He falls halfway down to the water below in 0.800 s. How much time passes during his entire trip from the top down to the water? Ignore air resistance.

Answer :

let the total height be h

So, for first case

S = h/2

a = g=10 m/s^2

u=0 m/s

t = 0.8 sec

using the relation

h/2 = 0.5 x 10×0.8 ^2

h=6.4 m

In second case (considering the full motion)




u=o m/s

using the relation 
6.4 = 0.5 x 10 x t^2


t = 2 x 6.4/10 = 1.28 s

Are there any ways to pass through a laser sensor without being detected?

The sensor is triggered when the LASER beam is obstructed.

If you sense the presence of Laser beam and avoid it, you can do that. If any of the many laser beams are obstructed, it will trigger the alarm.

You can also imagine replacing the LASER beams at exact positions with your own set of beams instantly when they are obstructed with your own laser of same kind frequency and intensity (which is not easily possible)


Vector resolution and addition

Zeenath Asks:

Sir please send me the answers with the reasons and steps

  1. the drawing on the left shows two vectors A and B, and the drawing on the right shows their components.

    The scalar components of these vectors are:

    When the vectors A and B are added, the resultant vector is R, so that RAB. What are the x– and y-components of R?
  2. the displacement vectors A and B, when added together, give the resultant vector R, so that RAB. Use the data in the drawing to find the magnitude R of the resultant vector and the angle q that it makes with the +x axis.




In the first question as you can see that the X components (Ax and Bx being equal and opposite will cancel each other and therefore the X com ponent of the resultant is zero.

The Y components will add up and therefore the Y component of resultant = 3.4+3.4 = 6.8 m and this itself is the magnitude of the resultant as the X components have canceled out.

Hnit for Answer to the second question

Force on a Stationary object

Joni Aks

What would be the amount of force  required by a 3390 lb. moving object to move a stationa 4125 lb. object 18 inches?   Help, I’m old now and having a senior moment when it comes to formulas….    :}

Answer to be posted soon

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March 2011

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