eli5, when a moving object bounces off of another, does it momentarily stop moving?
In: 415
Yes.
If at one point of time the velocity of the object is positive, and at another point in time the velocity is negative (i.e. moving in the opposite direction) than mathematically there must be a moment in between those two points where the velocity is zero.
This is the [intermediate value theorem](https://en.wikipedia.org/wiki/Intermediate_value_theorem).
Each part of it stops moving momentarily, but not all at the same time. By the time the back of the object stops and changes direction, the front of the object is already going back the other way.
You can probably find videos of squishy objects colliding and bouncing off each other. You can certainly imagine it. It’s almost like the front of the ball turns around and goes back to tell the back of the ball that it hit something and it needs to go back. Objects that seem rigid to us aren’t actually perfectly rigid, they do this too, but with less physical compression involved
Not completely unless perfectly moving along the same axis. Like a piston in an engine. If deflecting at an angle, then only one or two axes will have a zero velocity momentarily, but it will still have movement in the other axes.
Only in the situation in which two objects hit each other at 180 degrees from each other — think a perfect head on collision. In this case the two objects will first come to a complete stop before they bounce off each other and return along the exact same line they were traveling. Another way to think about this is two tennis balls with a string running through them – push each ball toward each other along the string (assuming zero friction) and they’ll stop.
If it’s off by even a fraction of a degree, you might not get a total “stop” of motion because of the rebound will now occur at an angle, and the object might not lose all its forward momentum.
Yes, [here’s a video](https://youtube.com/shorts/mU2TQf6tf2Y?si=NQv-MNMZyFBhRMFE) of a golf ball hitting a wall in super slow mo, it kind of shows what you’re wondering about.
No. If an object is moving very fast in a straight line, and you measure it’s speed at an exact moment, it’s speed is zero (because it hasn’t moved any distance in that exact moment). The perception that it has stopped comes from the fact that we’ve eliminated time while measuring.
This is because at an exact moment, the time taken to travel a distance is zero, and it cant go anywhere in zero time.
This happens even if the object changes direction, like when it bounces. If you measure at an exact moment, no distance is traveled so the speed is zero.
Mathematically, if you measure in the tiniest amount of time possible, very close to the moment it bounces, it’s distance travelled will be small, but having traveled in both directions it will have traveled distance and will have speed.
We could talk about momentum and vectors and that this was a big problem in math centuries ago, and let to the invention of calculus.
But this is ELI5
**Edit** downvotes ?
Wow down with science !!!
I’ve always wondered about a fly hitting a car windscreen, it hits it and pretty much instantaneously is going in the other direction. How can it at some point be going zero speed when the car never stopped moving?
Some answers are not really talking about reality, where macroscopic things are usually _continuous_, meaning they don’t instantly change or jump. What actually happens when things collide is that they deform a bit. The energy of impact gets temporarily turned into deformation of the atom structure. This is like a rubber ball, but harder materials deform just as well, but much less, corresponding to even slight changes already storing lots of energy (and if that energy gets too high, things crack or get deformed permanently).
At some point, the deformation reaches its maximum and begins un-deforming, turning its energy back into movement. At that moment and assuming a frontal perfect collision, the object would be stationary. As another answer correctly mentions, the different ends of the object would actually be moving relative to each other, as part of (un-)deforming. If we account for that, one needs to ask what “stationary” means, where I would go with the center of mass. By similar reasons as above, it would also not move at some point in time.
Yes. If I throw an object to my right, and it eventually bounces back to me (ie the object moves to the left since that’s the only way it gets back to me), there’s a very very very small fraction of a second where it doesn’t move.
That’s because if it’s going to the right, then moves left, at some point it needs to stop moving to the right, which is when it stops moving. As soon as that moment is done, it starts it’s movement to the left.
If you are travelling on a train from cityA to cityC with the train going through city B without stopping, would you say that you be to cityB? There is an instant where by the object’s velocity 0 followed by another where it’s zero + delta. However the object has not stopped as it accelerated from -delta through 0 to delta.
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