Question

Yo bois, its your friendly neighbourhood UnknownDude!

I have a question for you guys, about relative motion.

When it rains, a person who is walking must hold his umbrella at an angle to prevent getting wet, while a person who is standing still holds the umbrella perpendicular to the ground.

However, on a sunny day, one must hold an umbrella perpendicularly to protect oneself from sunlight, regardless of whether one is walking or not.

Explain why, using the concept of relative motion.

11th standard, Kinematics.

Answer 1

While raining when you run/walk the relative velocity between you and the rain drops is not zero anymore...hence it creates angle with vertical...

while the case with the sunlight is the rays are travelling from a huge distance ...u need to run a huge speed to bring about a considerable change in the angle with the vertical ...hence u have to keep the umbrella always perpendicular...

Answer 2

The angle of inclination to which the umbrella is to be bend depends upon the  relative velocity between the raindrops and velocity of person holding the umbrella. 


Here in this picture rain is falling down at an speed of 4 m/s If a man is walking towards south He will feel as if rain is going backward so he has to keep umbrella facing the direction he wants to move. In this case it is South since person is moving towards south. So we can find the inclination of umbrella  by  Finding the angle in this triangle which in this case is taninverse(velocity of rain /velocity of man) will give us angle of umbrella with ground. 
In case of Sunlight The velocity of sunlight is 3,00,000,000 m/s hence finding the angle for man standing  or running will give you same answer that is 90 degrees. Hence there will be no significant angle of inclination for sunlight.