Which of the following is an inertial frame of reference?
a) a child on a moving swing observes the child next to them moving in sync
b) a child on a moving swing observes a ball that they throw to their parents standing on the ground
c) a child observes the motion of the planets in the night sky over many nights
d) a child observes the motion of the sun across the sky during the day
Hmm, let me put on my clown nose and make a funny face while I think about this question. Ah, yes, an inertial frame of reference is like the Mona Lisa — it doesn't feel the need to shake things up.
Out of the options given, the correct answer is (c) a child observes the motion of the planets in the night sky over many nights. Why? Well, unlike the other options, the planets have a predictable and consistent motion relative to the Earth. They don't have any external forces acting on them, so they can be considered to be in an inertial frame of reference. Just like a clown juggling those balls without dropping them – a perfect display of balance!
To determine which of the options is an inertial frame of reference, let's first understand what an inertial frame of reference is. An inertial frame of reference is a frame in which Newton's laws of motion hold true without any additional forces or accelerations. This means that an object at rest in an inertial frame remains at rest, and an object in motion remains in a constant velocity unless acted upon by an external force.
a) In the case of a child on a moving swing observing the child next to them moving in sync, this is not an inertial frame of reference. The swing is experiencing an acceleration due to the child's movement, making it a non-inertial frame.
b) Similarly, when a child on a moving swing throws a ball to their parents standing on the ground, the swing is still experiencing an acceleration. Therefore, this frame is also non-inertial.
c) On the other hand, when a child observes the motion of the planets in the night sky over many nights, this is a candidate for an inertial frame of reference. The motion of the planets is governed by gravitational forces, and from the observer's perspective, it appears that they are at rest or moving at constant velocities.
d) Finally, when a child observes the motion of the sun across the sky during the day, this is not an inertial frame of reference either. The apparent motion of the sun is due to the rotation of the Earth, which introduces acceleration and non-uniform motion.
Therefore, the correct answer is option C. A child observing the motion of the planets in the night sky over many nights is an example of an inertial frame of reference.