A particle is found to be at rest when observed from a reference frame S₁, and the same particle is found to move with a constant velocity when observed from another frame S₂. Which of the following statements is true?
- AS₁ is an inertial frame, but S₂ is a non-inertial frame.
- BS₂ is an inertial frame, but S₁ is a non-inertial frame.
- CBoth S₁ and S₂ are non-inertial frames.
- DBoth S₁ and S₂ are inertial frames.
Show answer and explanation
Correct answer: (d) Both S₁ and S₂ are inertial frames.
Explanation
An inertial frame is one in which Newton's first law holds: a body with no net force on it stays at rest or moves with a constant velocity.
The particle (with no net force on it) is at rest in S₁, so S₁ is inertial. In S₂ the same particle moves with a constant velocity, so S₂ moves with a constant velocity relative to S₁, without acceleration.
Any frame moving with a constant velocity relative to an inertial frame is also inertial. So both S₁ and S₂ are inertial frames. Only an accelerating or rotating frame is non-inertial.