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Suppose a rocket is launched upwards , and an object (say on the top of the rocket, inside the rocket) is at rest. If i'm correct , as soon as the rocket starts moving upwards , the object would instantly go downwards to the bottom of the rocket.

My question is , why exactly does the object attract downwards? My initial thought is because of fake forces , d'alembert force in particular , since the direction of that force is always opposite the direction of acceleration.

What do you think ? Can you explain this phenomenon? What physical principles does is rely on?

Thank you.

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  • $\begingroup$ Yes, That's what I meant... Thx for the correction $\endgroup$ – orel Zemach Sep 23 at 16:04
  • $\begingroup$ The cause of the phenomenon is inertia. The object has inertia and will stay at rest from the point of view of an inertial frame of reference, which the rocket is not since it is accelerating. $\endgroup$ – Swike Sep 23 at 16:19
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My question is , why exactly does the object attract downwards?

It doesn't. The floor of the rocket moves upward.

When the rocket accelerates upward, the object does not move relative to point in the same inertial frame as the object. The reason is Newton's first law "an object will remain at rest or in uniform motion in a straight line unless acted upon by an external force"

The external upward force acts on the rocket, not the object at rest on the ceiling. That is, at least until the floor of the rocket contacts the object at which point the object will accelerate upward along with the rocket.

Hope this helps.

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That object is not moving. Only the rocket around it is moving. This is like asking, why the cup moves towards the water, when you pour water into it; that doesn't make much sense, because it isn't the cup that is moving - it is the water.

When all the surroundings are moving, but you aren't it is easy for your brain to get tricked. It creates an illusion of you moving, because the brain assumes that the surroundings are stationary. This is the origin of the so-called but non-existing "centrifugal force" - when turning in your car, you feel squeezed towards the window. But it is actually not you moving into that window, it is rather the window moving into you (the car is turning sideways, while your body expected to continue forwards).

Imagine a roller-skater standing on wheels in a bus. The bus suddenly accelerates. From your eyes when you are sitting inside the bus, it looks like the roller-skater is moving backwards. But he isn't. He is just standing still. Rather, it is the bus-floor that is moving forwards away from him.

Likewise, the answer to your example is fairly simple: the object isn't moving, the surroundings are. Nothing makes the object "attract downwards". It is just an illusion. It isn't moving at all.

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