4. A boy raises a 20.0-kg rock 1.50 m above the ground. a. What force does the boy use to raise the rock? Hint: What is the rock's weight?
b. How much work does he do?

we need to know the time interval during which the force acts. impulse of force = momentum change

= initial pressure

= final pressure = 2

= initial volume

= final volume = /4

= initial temperature = 20 c = 20 + 273 = 293 k

= final temperature = ?

using the equation

the answer is the first electromagnet (a). this is because it has more coils around the ferromagnetic material. increasing the number of coils increases the surface area in which the metal interacts with the current in the coil. this enables more atoms in the metal to become aligned hence becoming more magnetized.