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## Newton’s 2nd Law, 2 Crates held by a Pulley

Two crate A and B are connected with a rope with negligible mass over a frictionless pulley as represented below. The crates are at rest. Crate A has an unknown mass and hangs vertically to the right of the slope and crate B with mass 22 kg, hangs on the slope to the left. The slope makes and angle of 67 degrees with the horizontal. The static coefficient of friction between crate B and the surface of the slope is 1.716. The frictional force is up the slope.”

The Answer is suppose to be 5.5Kg, but I don’t know how to get it.

## Does light experience friction?

Does light experience friction?

Asked rashmi (VIII B, KV AFS Bidar)

FRICTION  is defined as the force which opposes relative motion between two surfaces in contact.

If you mean friction in the normal sense as above (as discussed in class), then the answer is “NO“.

Light is a form of energy and not a particle. But it is true that light shows some particle behaviour also.

But, there is a slowing down when light interacts with matter. But, in a homogeneous medium, light has a constant speed. The speed of light is different in different medium.  If we go to the atomic or molecular picture of matter, then we can imagine that light is interacting with a molecule, during the time of interaction, the light can be assumed to have been absorbed by the particle and then it is re-radiated. the re-radiated light travels to the next atom and interacts/absorbed. this slows down the average speed of light, but is constant for a particular homogeneous medium.

The speed of light is maximum in vacuum where there is no matter for light to interact.

So, in between two atoms also, light travels with its branded 3L kmps speed.

Also refer:

1. http://phasing.org/2009/09/25/does-light-have-friction/

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