## Does gravity travel instantaneously?

Does gravity travels instantaneously? Or it travels equal to the speed of light.

No clear answer now. If we believe in the Theory of relativity, Gravity (Gravitational Waves travel with the same speed as that of light in vacuum. If gravitons are like photons, then it makes sense too.

However I invite you to think more and post your thoughts as comments here. Let the discussion begin.

I found the following links to know more

• http://metaresearch.org/cosmology/speed_of_gravity.asp
• http://en.wikipedia.org/wiki/Speed_of_gravity
• http://www.desy.de/user/projects/Physics/Relativity/GR/grav_speed.html

## Motion in a vertical circle and conservation of energy

A stone  tied to a string of length l is whirled around a vertical circle with the other end of the string at the centre. At a certain instant of time the stone is at the lowest position and has a speed u. What is the magnitude of change in its velocity as it reaches a position where the string is horizontal?

Let’s assume that the potential energy at the lowest position be zero. So, when the string is horizontal, the stone has risen by a vertical height l, the length of the string which is also the radius of the vertical circle.

If v is the magnitude of velocity at the horizontal position, then according to the law of conservation of energy,

KE+PE at the lowest position = KE+PE at the horizontal position

$\frac{1}{2}mu^{2}=\frac{1}{2}mv^{2}+mgl$

From the equation above, v-u can be calculated.

$v=\sqrt{u^{2}-2gl}$

The following links will help you for deeper understanding and you can browse through some solved problems from the topic too.

## Addition of vectors – can you add 3 unit vectors to get a unit vector ?

“Can you add 3 unit vectors to get a unit vector ? discuss in terms of velocity, force etc”

-Sindhuja posted

## When several transparent materials are kept above each other, the resultant behaves like an opaque object.Why?What’s the Optics behind it?

When light falls on a object, three things happen -

• A part of the light is absorbed
• Another part is reflected ad
• The remaining part is transmitted.

A transparent medium transmits most of the light incident on it. However, it absorbs a small part as well as reflect a small part.

Therefore, when a number of transparent sheets are stacked, the amount of light absorbed will be considerably large and the arrangement tends to be opaque.