1
$\begingroup$

I am getting confused as to why a ball still feels gravity when inside a moving car. The point of a reference frame is to reinterpret all the forces acting on a particle in one frame only. Hence all the equations of forces get modified to still be able to accurately explain and predict motion. But pay attention to the following scenario:

Suppose there is a ball in a moving cart and suddenly the ball moves backwards. Using my understanding of reference frames, I can safely deduce that the frame, or the cart here, is accelerating forward. Now suppose we drop the ball from some height and it moves south west. We know, from its motion towards west, that the cart is accelerating towards east. But shouldn't we, using the same analogy, deduce that the car is also accelerating upwards (because the ball falls down)? The car is actually feeling no acceleration at all in the vertical direction because the gravitational force is countered by the normal force. But the ball still falls whereas its reference frame feels no acceleration in vertical direction.

One way I tried to think of it is relating the fact that the ball also has mass. So it must still feel the gravitational force of earth. To me, this feels kind of a failure because it undermines the usefulness of reference frame concept because we still have to worry about the Earth. Why can't I think of gravity In some other terms?

It means that the gravitational force transcends frames of reference. So shouldn't we also worry about other forces accumulating like the force due to earths revolution (the centripetal force) also?

Why does the ball still feel gravity? I am really tripping over this interpretation of reference frame. Help me.

$\endgroup$
  • $\begingroup$ Why can't I think of gravity In some other terms? You can! You can use general relativity. But it's simpler to use Newtonian physics, unless you need extremely accurate results, or are dealing with extremely strong gravity. FWIW, when standing on the Earth, you don't feel gravity, you feel the normal force of the ground (or floor) that's supporting you against gravity, and which is preventing you from freefalling to the centre of the Earth. $\endgroup$ – PM 2Ring Apr 18 at 10:54
  • $\begingroup$ But that's just our sensory nerves, right. Its the same reason why we don't feel the 1 atm pressure on our heads. We are accustomed to it. I am more confused about the ball's movement instead. $\endgroup$ – HitchHiker224 Apr 18 at 11:01
  • $\begingroup$ No, it's not just our sensory nerves. Do a handstand, you still feel the normal force, on nerves that don't normally get that stimulus. In GR, the natural reference frame near the Earth's surface is in freefall. A frame that's at rest relative to the Earth's surface is accelerating upwards relative to the natural frame. Gullstrand–Painlevé coordinates make that explicit. $\endgroup$ – PM 2Ring Apr 18 at 11:41
  • $\begingroup$ So let me summarize. You refer to a Natural frame of reference near Earth because a body dropped would stay in position. Kind of like in ISS. The body is in rest in this frame and this is the simplest frame we can choose. Hence the most "natural"? A frame at rest w.r.t surface would be moving upwards with respect to that frame and the ball would hence feel acceleration g downwards. Am I correct? I am a high school student and I have not read GR yet. I mean to do so though. $\endgroup$ – HitchHiker224 Apr 18 at 14:39
  • $\begingroup$ I reread the question again and now I realize how clumsy my concepts were. I can think about it clearly now. Thanks $\endgroup$ – HitchHiker224 Apr 18 at 14:45

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.