Protons and neutrons are made from combinations of the two most common quarks, the u quark and the d quark. the u quark's charge is +2/3e, while the d quark carries −1/3e.
part a
how could three of these quarks combine to make a proton?

a. uuu
b. uud
c. udd
d. ddd
part b
how could three of these quarks combine to make a neutron?

a. uuu
b. uud
c. udd
d. ddd

Part A  B.  Part B C.

Explanation:

A) The elementary charge e (without sign) is equal to the charge of one electron (with negative sign) or to the charge of one proton (with positive sign), so the proton must have a total charge of +e.

if u = +2/3 e and d= -1/3 e, we need a combination which sum gives +3/3 e.

Combination A adds to 6/3e, so it is not possible. C adds to zero, and D gives a negative result.

The only remaining choice is udd:

uud ⇒ +2/3 e + 2/3 e -1/3 e = +3/3 e = +e

So, the statement B is true.

B) As the neutron has no net charge, we need to find a combination which sum adds to zero.

So, A and D are not possible, as they are combinations of the same type of quarks, so the sum is either positive or negative, but not zero.

uud gives +e (we chose it to make a proton in part A), so the only remaining choice is udd:

udd⇒ +2/3 e -1/3 e - 1/3 e = 0

So the statement C is true.

close lines strong magnet less close weaker

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.