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Nuclear force-definition, types of nuclear force, nuclear stability, practice problems, FAQs

Each component in the universe is bound to each other by means of forces. Electric charges, for instance, are held together by Coulombic forces between them. Humans are bound to the earth by means of gravitational force between them and earth. Similarly, protons and neutrons are held tightly together inside a nucleus by means of nuclear force. The magnitude of nuclear force is huge; that is why a large amount of energy is needed to separate protons and neutrons. This huge energy needed to overcome nuclear forces finds applications when energy needs are large; one good example would be the nuclear reactor. We will study more about nuclear forces in this article.

Table of contents

  • What is nuclear force?
  • Properties of nuclear force
  • Types of nuclear force
  • Nuclear stability
  • Practice examples
  • FAQs

What is nuclear force?

The force that exists between the nucleons,.i.e the positively charged protons and the neutrons (which have no charge) is called nuclear force. Due to its like charge, protons repel each other. So there exist strong forces inside the nucleus to counter the repulsive forces between the protons which is called nuclear force.

Properties of nuclear force

  • Nuclear force does not depend upon the nature of the charge. Let indicate force between proton and proton, indicate the force between a proton and a neutron, and indicate the force between two neutrons, then .
  • It is a short range force. Over distances greater than the force vanishes. 
  • Nuclear force is attractive in nature.
  • It is a non central force, meaning it does not depend upon the distance between the particles but rather upon their orientation. 
  • Nuclear force is stronger if the spin is parallel; and weak if it is anti parallel.
  • It is attractive in nature for distances upto but for distances less than fermi( fermi the nuclear force becomes repulsive

Graph showing variation of nuclear force with distance

Types of nuclear force

1)Strong nuclear force

  • Strong nuclear force, or strong interaction as it is called, is the force that acts between particles such as quarks (A quark is any of a group of subatomic particles believed to be among the fundamental constituents of matter. Similar to how protons and neutrons make up atomic nuclei, these particles themselves are thought to consist of quarks). It is the strongest out of the four fundamental forces of interaction. 
  •  At distances greater than , it is about times greater than the weak nuclear force. Strong nuclear force holds the nuclei of atoms together.
  • Its magnitude is far greater than electromagnetic force and gravitational force. 

Fig. shows nucleons held together by strong nuclear force

 2)Weak nuclear force

  • It exists between subatomic particles that are involved in radioactive decay. 
  • It exists during beta decay, where one nucleus having too many protons or neutrons, emits beta particles and forms another nucleus.
  • Weak nuclear force does not show parity-symmetry. 
  • Weak nuclear force is neither attractive nor repulsive.

Nuclear stability

  • The belt or region on which stable nuclei lie is known as the stability belt. The nuclei on the stability belt are more stable than other nuclei. In the above figure, the straight line approximately shows nuclei which have equal number of protons and neutrons Only lighter nuclei on this line are stable. Heavy nuclei are more stable if

Note: Nuclear force is effective in short range, hence it is sufficient to overcome Coulombic repulsion that exists between nucleons. 

Practice problems

1) Identify the correct statement (s) amongst the following:

(i) At greater distances, the nuclear forces are negligible.

(ii) Nuclear forces are non central forces.

(iii) Nuclear forces are weakest of all forces.

(iv) Nuclear forces depend upon the charge of the particles. 

(a) (i),(ii)

(b) (ii),(iii) 

(c) (iii),(iv)

(d) (i), (iv)

Solution) a 

Nuclear forces become lesser in magnitude with increase in distance. Nuclear forces are non central forces unlike gravitational force. 

2) Why are nuclear forces called non-central forces?

Solution) Nuclear forces are called non-central forces because their direction does not depend upon the line joining the center of the subatomic particles. 

3) The nuclear force 

(a) Obeys the inverse square law.

(b) Varies inversely as the third power of the distance between the particles.

(c) Is a short range force. 

(d) Is equal to the strength of the electromagnetic force.

Solution) C

 It is a short range force. 


Q. It is very difficult to separate protons and neutrons in a nucleus. Why so? 
Ans) Strong nuclear force which binds nucleons is very huge, hence it is difficult to separate nucleons.

Q.What force keeps particles in the universe intact?
Ans) Strong nuclear force, the strongest force known to man, is responsible for keeping particles in the universe together. 

Q. Is the force between a proton and a neutron the same as that of a neutron and neutron?
Ans) Nuclear force is independent of charge of the particles. This means that the force between a proton and a neutron is the same as that of a neutron and a neutron.

Q. What is the unit of nuclear force? 
Ans) The unit of a physical quantity does not depend on its strength. We know that unit of force is Newton. So the unit of nuclear force is also Newton (

Q. Weak nuclear force is greater than gravitational force. Why?
Ans) Gravitational force is the weakest force known to man. Weak nuclear force is stronger than electromagnetic and gravitational but weaker than strong nuclear force.

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