How much kinetic energy do you think a ball will have if it were dropped from a 2-m shelf

That depends on how much mass the ball has, and what planet it's on.

The kinetic energy it has when it hits the floor is the same as the potential energy it has when it's still on the shelf.  That's . . .

Potential energy = (mass of the ball) (gravity) (height off the floor)

Gravity on Earth is 9.8 m/s² .  

So if the shelf is on Earth, then the ball's kinetic energy when it hits the floor is . . .

Kinetic energy = (mass of the ball, in kg) (9.8 m/s²) (2 m)

Kinetic energy = 19.6 · (mass of the ball, in kg)  Joules

Up in the first answer, SWIMMY says it would have about 5 Joules.  That's the kinetic energy it would have IF the mass of the ball is 0.255 kilogram.  

Is that a reasonable mass for a ball ?  Well, an NFL football is 0.40-0.43 kilogram.  An NBA basketball is 0.62 kilogram.  Two major-league hardballs are 0.284-0.298 kilogram.  And 5 golf- balls are  0.229 kilogram.  So I guess 0.255 kilogram is not too unreasonable for a ball, and SWIMMY's estimate of 5 Joules of kinetic energy isn't bad ... after a fall from 2 meters.

real, smaller and upside-down.   so the answer should be b.

when a device is used in a circuit in which the voltage is 81 v, the current flowing through the device is 3 a. what is the resistance of the device?

a. 27 ω

b. 40.5 a

c. 243 ω

d. 243 a

the correct answer is a. 27