Is this a reference frame problem ?

[Monsterboy]

Homework Statement

Rain in falling vertically. A man running on the road keeps his umbrella tilted but a man standing on the street keeps his umbrella vertical to protect himself from the rain. But both of them keep their umbrella vertical to avoid the vertical sun-rays. Explain

Relevant Equations

-

v1 - velocity of the rain with respect to the ground
v2 - velocity of the man with respect to the ground
v3 - velocity of the rain with respect to the man

So, v1 = v3 + v2 is this right ?

So, for the man moving with a velocity v2 with respect to the ground, the rain will have a horizontal component.

Why wouldn't the same logic apply to light falling vertically ? Because the speed of light is so much greater than that of the man ?

 

[PeroK]

Why wouldn't the same logic apply to light falling vertically ? Because the speed of light is so much greater than that of the man ?

Essentially yes, it's the speed. The same logic, however, does apply:

https://en.wikipedia.org/wiki/Aberration_(astronomy)

 

[kuruman]

There is a related reason why the running person holds the umbrella at an angle: the relative airspeed that creates a wind in the man's reference frame. By turning the umbrella into the "wind", he eliminates the need to provide a counter-torque which makes holding the umbrella easier. Arguably, in the man's reference frame, it is this wind that causes the raindrops to fall at an angle.

 

[hmmm27]

Arguably, in the man's reference frame, it is this wind that causes the raindrops to fall at an angle.

Or the other way'round ; heavy raindrops dragging the air along.

 

[kuruman]

Or the other way'round ; heavy raindrops dragging the air along.

How would that work in the man's reference frame if it stops raining but he is still running? There will still be a wind.

 

[hmmm27]

How would that work in the man's reference frame if it stops raining but he is still running? There will still be a wind.

How would it work if the wind at ground level matched the man's velocity as he ran, but the heavy raindrops still fell almost vertically (ground reference, so still at an angle in the man's ref fr) ? Change the parameters, change the result.

Meanwhile, with the original parameters, how is the man supposed to tell if - in his reference frame - the wind is pushing the raindrops, or the rain is pushing the wind ? or, maybe he doesn't think about it because the bottom of his trousers are still getting wet.