A stone is dropped vertically downwards. What is the velocity of the rock after 4 seconds ? What is the displacement of the rock after 4 seconds ?

Vf = 39.2 m/s

h = 78.4 m

Explanation:

Using first equation of motion for vertical motion:

Vf = Vi + gt

where,

Vf = Final Velocity = ?

Vi = Initial Velocity = 0 m/s

g = 9.8 m/s²

t = time taken = 4 s

Therefore,

Vf = 0 m/s + (9.8 m/s²)(4 s)

Vf = 39.2 m/s

Now, we use second equation of motion

h = Vi t + (1/2)gt²

where,

h = height covered = ?

Therefore,

h = (0 m/s)(4 s) + (1/2)(9.8 m/s²)(4 s)²

h = 78.4 m

answer: the correct answer is 1.

explanation: there are various process in which a radioactive nuclei decays:

1) alpha decay: in this process, a heavier nuclei decays into lighter nuclei by releasing alpha particle. the mass number is reduced by 4 units.

2) electron capture: in this decay process, a parent nuclei absorbs an electron and gets converted into a neutron. simply, a proton and an electron combines together to form a neutron. mass number does not change in this process.

3) gamma ray emission: it is a decay process in which an unstable nuclei gives excess energy by a spontaneous electromagnetic process. this decay releases . this process does not change the mass number.

4) positron emission: it is a type of decay process, in which a proton gets converted to neutron and an electron neutrino. this is also known as -decay. in this the mass number remains same.

from the above information, we see that only alpha-decay is producing a change in the mass number of the nucleus.