3.2.1.2
The strong nuclear force holds the nucleus together
Atomic Structure & Nuclear Stability — AQA A-Level Physics
Key Definition
Strong nuclear force — An attractive force between nucleons that acts at short range (0.5 to 3.0 fm) and overcomes electrostatic repulsion to hold the nucleus together.
- Protons repel each other via the electrostatic force. Gravity between nucleons is negligible.
- The strong nuclear force acts between quarks inside nucleons, binding them together.
- It is roughly the same between all nucleon pairs: proton-proton, neutron-neutron, and proton-neutron.
How the strong force varies with separation
- Below 0.5 fm: highly repulsive (prevents nucleons from overlapping).
- 0.5 to 3.0 fm: attractive. Maximum attraction at about 1.0 fm, the typical nucleon separation.
- Above 3.0 fm: effectively zero. The strong force has no effect beyond this range.
- The equilibriumAn object is in equilibrium when the resultant force on it is zero. The object is either stationary or moving at constant velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹.. position (resultant $force = 0)$ is at about 0.5 fm.
Comparison with the electrostatic force
- Between 0.5 and 3.0 fm, the strong force dominates. Its attraction far exceeds electrostatic repulsion.
- The electrostatic repulsion between protons has a much larger range but only becomes significant beyond about 2.5 fm.
- The electrostatic force depends on chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. The strong force does not — it acts equally on protons and neutrons.
- For protons, the equilibriumAn object is in equilibrium when the resultant force on it is zero. The object is either stationary or moving at constant velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹.. between electrostatic repulsion and strong attraction occurs just below 0.5 fm.
Examiner Tips and Tricks
- Learn the key numbers: range = 0.5 to 3.0 fm, typical nucleon separation = 1.0 fm, 1 fm = $1 \times 10^{-15}$ m.
- Say the force becomes 'zero' beyond 3 fm, not 'negligible'.
Common Mistake
MEDIUM
Students often: Don't say the strong force is always attractive.
Instead: It is repulsive below 0.5 fm. This short-range repulsion prevents nucleons from being squeezed into each other.
Instead: It is repulsive below 0.5 fm. This short-range repulsion prevents nucleons from being squeezed into each other.