Acement block accidentally falls from rest from the ledge of a 15.0m high building. when the block is 14.0m above the ground, a man who is 1.71m tall looks up and notices that the block is directly above him. how much time does he have to move out of the way?

Given that,

For free falling body, acceleration is equal to acceleration due to gravity.

a = g = 9.81 m/s²

Distance to be covered, s = 15 - 1.71

s = 13.29 m

At top initial velocity, Vi = 0

Since,

s = Vi *t + 1/2 at²

13.29 = 0 + 0.5 * 9.81 * t²

t = 1.646 s

The man have 1.65 seconds to move out of the way.

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.

mistakes are ok