A polonium isotope with an atomic mass of 211.988868 u undergoes alpha decay, resulting in a daughter isotope with an atomic mass of 207.976652 u. Ignoring any recoil of the daughter, find the kinetic energy of the emitted alpha particle in megaelectronvolts (MeV).

 K = 9.53 MeV

Explanation:

The kinetic energy that the alpha particle has emitted, is the energy in excess after removing the resting energy of the atoms and the helium nucleus that forms the alpha particle

Since energy and masses are related and cannot be

          m₀ c² = c² + m_He c²+ K

          K = c² (m₀ - m_{f} - m_He)

the mass of the Helium atom is 4 u

           K = (3 10⁸)² (211,988868 -207.976652 - 4,002) 1,661 10⁻²⁷

           K = 14,949 10⁻¹¹ (0.0102)

          K = 1,527 10⁻¹² J

let's reduce 1 J = 6,242 10¹² MeV

           K = 9.53 MeV

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.