CAPF AC 2020 Q6: Science & Technology – Physics
- (a) Force never occurs singly in nature
- (b) When the earth pulls a stone downwards due to gravity, the stone exerts a force on the earth
- (c) There is a cause-effect relation implied in the third law
- (d) There is no cause-effect relation implied in the third law
Answer: (c) There is a cause-effect relation implied in the third law
Explanation
Newton's third law of motion
How to think: two options say the opposite of each other.
Both cannot be true, so the wrong statement is one of them.
Action and reaction arise together, so neither causes the other.
To every action there is always an equal and opposite reaction.
The two forces act on two different bodies and arise at the same instant.
So neither force is the cause of the other.
NCERT states that there is no cause-effect relation implied in the third law.
| Statement | Verdict | Fact |
|---|---|---|
| (a) Force never occurs singly in nature | True | Forces always occur in pairs |
| (b) When the earth pulls a stone downwards, the stone exerts a force on the earth | True | The stone pulls the earth with an equal force |
| (c) There is a cause-effect relation implied in the third law | NOT correct | Action and reaction are simultaneous |
| (d) There is no cause-effect relation implied in the third law | True | Either force can be called the action |
Points on the third law:
| Point | Detail |
|---|---|
| Size | Action and reaction are equal |
| Direction | Opposite |
| Bodies | Act on different bodies, so they do not cancel |
| Timing | Simultaneous |
Answer: (c) There is a cause-effect relation implied in the third law.
Why each option is right or wrong
- (a) Incorrect. This statement is true: forces always occur in pairs.
- (b) Incorrect. This statement is true: the stone pulls the earth with an equal force.
- (c) CORRECT. This is the statement that is not correct; action and reaction are simultaneous.
- (d) Incorrect. This statement is true and is taken directly from NCERT.
Key point
Newton's third law: forces occur in equal and opposite pairs, on different bodies, at the same instant; no cause-effect relation between them.
Remember
Third law: action and reaction are simultaneous; no cause and effect.
Common mistakes
- Thinking the action comes first and the reaction follows it.
- Believing action and reaction cancel out; they act on different bodies.
- Missing the word NOT in the question and marking a true statement.
Exam tip
Educated guess: (c) and (d) are opposites, so one of them must be the answer. NCERT says no cause-effect.
Sources
- NCERT Class XI Physics — Laws of Motion (Newton's third law)
- NCERT Class IX Science — Force and Laws of Motion