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I'm not a physicist and I'm a little bit puzzled with the image below: enter image description here

I know it's a joke but if we assume we don't have air resistance what exactly will happen if someone jump in the upward direction from a roof of on a moving car? Unless a force is applied to him will he land in the same spot he jumped from on the car? And why (in technical terms)?

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  • $\begingroup$ @ Oбжорoв But then we'd all be deprived of the nice pictures and of admiring the explanatory skills of the answerers! $\endgroup$ Feb 13, 2022 at 15:23
  • $\begingroup$ Funny. As the vehicle seems to be (graphically) going at the same speed, it's probably the effect of air resistance indeed. What strikes me lol is the undauntedness of the pool observer :) $\endgroup$
    – Arc
    Feb 14, 2022 at 13:58
  • $\begingroup$ Air resistance is important here, at least if the car is moving fast. $\endgroup$
    – JAlex
    Jul 14, 2022 at 21:25

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Newton's first law of motion answers your question, the principle of inertia. If the car is at constant velocity everything inside the car will follow with the same velocity. If no other forces are applied to the person, once it jumps, it will still have the car's velocity due to inertia and will land at the exact same spot over the car. It doesn't matter what kind of situation it is, if you can assure there is no net forces acting, the result of the jump will be the same.

If the car is accelerating, however, it changes. If the car is gaining speed, then once the person jumps, it will lag behind the car because the car is still gaining speed while the person isn't. If the car is losing speed, once the person jumps it will get a lead over the car.

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If we assume that the car is not accelerating (and is therefore an inertial frame of reference) then the person would fall into the pool.

Why? Because the same happens when they're on the ground. There is no difference between the laws of physics in two inertial frames (moving with constant velocities relative to each other). The earth itself is (approximately) an inertial frame of reference.

In a non-inertial frame, however, the situation would be different. Depending on whether the van is accelerating or decelerating, the person would fall backwards or forwards respectively.

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  • $\begingroup$ Thank you for your answer. Earth is an inertial frame, but what about the mass? If your are on a boat that is moving constantly with a speed of 120km/h you are in inertial frame. If you jump in the upward direction will you land in the boat or fall in the water? $\endgroup$
    – graham89
    Feb 13, 2022 at 13:27
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    $\begingroup$ Assuming that the boat is an approximately inertial frame (it really is moving at a steady 120 km/h and there is no accelerating or decelerating going on), then yes you should land in the boat and not in the water. Think about it this way, what would happen if you jumped inside a train (assuming the train isn't turning)? The answer would be the same. $\endgroup$ Feb 13, 2022 at 14:13
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I know your question asks what happens without air resistance, but the picture is more realistic than that. As soon as the man jumps, air resistance will apply a braking force, and he will move backwards relative to the pool.

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