A projectile is fired upward with an initial speed v0 on an air filled world. A short time later, it comes back down and has a final speed v < v0 (air resistance is not negligible). What was the sign of the work done by friction during the flight?

(A) W fric > 0
(B) Wfric=0
(C) Wfric < 0

(A) W fric > 0

Explanation:

According to the law of energy conservation, mechanical energy stays always constant. So if the ball come back to the same position (same potential energy) but with reduced speed (kinetic energy), then the kinetic energy has been affected by friction work created by air resistance. This work is non 0 and larger than 0. So the total of this friction work and kinetic energy after the fall = kinetic energy before the fall.

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.

uranium fissions/breaks up giving out energy and neutrons