Determine what happens to the potential energy of a book as it drops from a desk to the floor.
indicate how convection currents depend on density differences.
answer both, and you for your and time.

I can answer the first one, but not the second one. Sorry. Potential energy due to gravity has to do with the height of the object relative to the floor. As the book is dropped, the energy is continually transferred from potential energy into kinetic energy up until just before the book hits the floor, at which point all of the potential energy has been converted to kinetic energy, so the potential energy is 0. Hopefully this helps!

25.38 m/s

Explanation:

We have a straight line where the car travels a total distance , which is divided into two segments :

(1)

Where

On the other hand, we know speed is defined as:

(2)

Where is the time, which can be isolated from (2):

(3)

Now, for the first segment the car has a speed , using equation (3):

(4)

(5)

(6) This is the time it takes to travel the first segment

For the second segment the car has a speed ,  hence:

(7)

(8)

(9) This is the time it takes to travel the secons segment

Having these values we can calculate the car's average speed :

(10)

(11)

Finally: