Grain is pouring into a silo to be stored for later use. Due to the friction between pieces of grain as they rub against each other during the pouring process, one piece of grain picks up a charge of 6.0 E -10 C and another piece of grain picks up a charge of 2.3 E -15 C. What is the electric force between them if the pieces of grain are 2 cm apart?

F = 3.105 10⁻²⁰ N

Explanation:

For this exercise let's use Coulomb's law

          F = k q₁ q₂ / r²

in this case each of the grains has a charge

         q1 = 6.0 10⁻¹⁰ C

         q2 = 2.3 10⁻¹⁵ C

and they are separated by a distance r = 2 cm = 0.02 m

let's substitute

          F = 9 10⁹ 6.0 10⁻¹⁰ 2.3 10⁻¹⁵ / 0.02²

         F = 3.105 10⁻²⁰ N

air exerts forces on falling objects near earth’s surface.

explanation:

the term 'free falls' refer to object that are falling towards the earth, in a situation in which gravity is the only force acting on the object.

for an object in free fall, the acceleration is constant (g, the gravitational acceleration), and so the velocity of the object constantly increases.

for objects near earth's surface, there is always another force acting on the object: the resistance due to the air. this force is opposed to the direction of fall of the object, and it is proportional to the velocity of the object, so as the object falls down, this force increases up to a point when it balances the force of gravity, and the object continues its motion at constant velocity (called terminal velocity). since in this case gravity is not the only force acting on the object, we are not in a situation of free fall.