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## Homework Statement

Q1. A little girl is holding a helium-filled balloon on a string while

riding in a closed elevator going down a very tall building at

constant speed. The elevator shaft is maintained under vacuum.

Suddenly the elevator cable snaps, sending the elevator

into free fall. In her surprise the girl lets go of the string. She

is even more surprised to see

(a) the balloon rising.

(b) the balloon floating downward.

(c) the balloon remaining stationary.

(d) the balloon bouncing slowly between the floor and the

ceiling.

## Homework Equations

Newton's 2rd and 3rd laws

## The Attempt at a Solution

The answer is C which confuses me.

That means the balloon itself is accelerating in a stationary frame, then there must be some force acting on the balloon. Would it be the air in the elevator that is exerting the constant force on the balloon?

## Homework Statement

Question 10

At the shallow end of a swimming pool, the water is 70.0

cm deep. The diameter of the cone of light emerging from

the water into the air above, emitted by a light source 10.0

cm in diameter at the bottom of the pool and measured by an

observer on the edge of the pool 2.50 m away, is’

(a) 1.60 m. (b) 1.70 m.

(c) 1.75 m. (d) 1.80 m.

## Homework Equations

n1sin(a1)=n2sin(a2)

## The Attempt at a Solution

Sorry, but i don't get waht it means by "light cone" in the question

Thanks in advance! :tongue: