UPSC Prelims 2013 Q95: Science & Technology – Physics
The known forces of nature can be divided into four classes, viz., gravity, electromagnetism,
weak nuclear force and strong nuclear force. With reference to them,
which one of the following statements is not correct?
- (a) Gravity is the strongest of the four
- (b) Electromagnetism acts only on particles with an electric charge
- (c) Weak nuclear force causes radioactivity
- (d) Strong nuclear force holds protons and neutrons inside the nucleus of an atom
Answer: (a) Gravity is the strongest of the four
Explanation
The four fundamental forces of nature
| Force | Relative strength | Acts on | Range |
|---|---|---|---|
| Strong nuclear | Strongest | Quarks; holds protons and neutrons in the nucleus | Very short |
| Electromagnetic | Second | Charged particles | Infinite |
| Weak nuclear | Third | All quarks and leptons; causes radioactive (beta) decay | Very short |
| Gravity | Weakest | Everything with mass | Infinite |
The options checked:
| Option | Verdict |
|---|---|
| (a) Gravity is the strongest of the four | Not correct; it is the weakest |
| (b) Electromagnetism acts only on particles with an electric charge | Correct |
| (c) Weak nuclear force causes radioactivity | Correct |
| (d) Strong nuclear force holds protons and neutrons inside the nucleus | Correct |
Gravity seems strong in daily life only because it always attracts and adds up over huge masses.
Answer: (a) Gravity is the strongest of the four.
Why each option is right or wrong
- (a) CORRECT. This statement is not correct: gravity is the weakest force.
- (b) Incorrect. This statement is true: electromagnetism acts on charged particles.
- (c) Incorrect. This statement is true: the weak nuclear force causes radioactivity.
- (d) Incorrect. This statement is true: the strong nuclear force binds the nucleus.
Key point
Forces by strength: strong nuclear, electromagnetic, weak nuclear, gravity. Gravity is weakest but has infinite range and always attracts.
Remember
Gravity is the weakest force.
Common mistakes
- Thinking gravity is strongest because it holds planets in orbit.
- Missing the word 'not' in the question.
- Confusing the weak force (decay) with the strong force (binding).
Exam tip
Strength order: Strong, Electromagnetic, Weak, Gravity. Gravity is last.
Sources
- NCERT Class XI — Physics Part I, Ch 1 (Physical World: fundamental forces in nature)
- CERN — The Standard Model