# Why does water not follow the law of motion?

Gold Member
So I was in the bus going to school. I sat in the back not directly underneath the emergency exit but more like behind it. Anyways, since it’s been raining consistently, the emergency exits sometimes leak. So I’m sitting down and my jacket sleeve gets wet with a drop from the emergency exit. I remember the law of motion states that and object in motion stays in motion and and object at rest stays at rest until acted upon by an unbalanced force. (I asked a similar question in the past about throwing a ball in a plane)

Wouldn’t that mean that the water, which is “connected” to the bus would go with the motion of the bus? But that doesn’t solve my question to why it flew back towards me. If it’s supposed to go with the motion of the bus, then it would’ve just fallen straight down right? If it doesn’t follow the bus’s motion, then would it be because water is a polar molecule? Or maybe it’s because a bus isn’t as fast as an airplane so it doesn’t have the same result?

kent davidge

Homework Helper
So I was in the bus going to school. I sat in the back not directly underneath the emergency exit but more like behind it. Anyways, since it’s been raining consistently, the emergency exits sometimes leak. So I’m sitting down and my jacket sleeve gets wet with a drop from the emergency exit. I remember the law of motion states that and object in motion stays in motion and and object at rest stays at rest until acted upon by an unbalanced force. (I asked a similar question in the past about throwing a ball in a plane)

Wouldn’t that mean that the water, which is “connected” to the bus would go with the motion of the bus? But that doesn’t solve my question to why it flew back towards me. If it’s supposed to go with the motion of the bus, then it would’ve just fallen straight down right? If it doesn’t follow the bus’s motion, then would it be because water is a polar molecule? Or maybe it’s because a bus isn’t as fast as an airplane so it doesn’t have the same result?
It is difficult to speculate about an event for which we have very limited details. But might the bus have been accelerating forward while the water droplet was falling?

russ_watters, CWatters and doglover9754
Mentor
So I was in the bus going to school. I sat in the back not directly underneath the emergency exit but more like behind it. Anyways, since it’s been raining consistently, the emergency exits sometimes leak. So I’m sitting down and my jacket sleeve gets wet with a drop from the emergency exit. I remember the law of motion states that and object in motion stays in motion and and object at rest stays at rest until acted upon by an unbalanced force. (I asked a similar question in the past about throwing a ball in a plane)

Wouldn’t that mean that the water, which is “connected” to the bus would go with the motion of the bus? But that doesn’t solve my question to why it flew back towards me. If it’s supposed to go with the motion of the bus, then it would’ve just fallen straight down right? If it doesn’t follow the bus’s motion, then would it be because water is a polar molecule? Or maybe it’s because a bus isn’t as fast as an airplane so it doesn’t have the same result?
@jbriggs444 beat me to it. Was the bus accelerating at the time? If the bus wasn't moving, the drop would fall straight down, and that should also be the case if the bus was moving at a constant speed. If the bus happened to be decelerating, the drop would appear to fall in front of you.

The polarity of water has nothing to do with its motion.

doglover9754
Staff Emeritus
2022 Award
Another possible cause is air currents. If a window was open/cracked, or if the bus has air vents, it could have generated a current of air at the top of the bus, pushing the droplet backwards.

Chestermiller and doglover9754
Homework Helper
Gold Member
2022 Award
We are all speculating here, but it's also possible that the water leaked through quickly so that it hadn't acquired all the bus's momentum.

Certainty, if it came in through a definite gap, it would be going backwards relative to the bus.

doglover9754
Gold Member
It is difficult to speculate about an event for which we have very limited details. But might the bus have been accelerating forward while the water droplet was falling?
Actually, yes. I’m not sure if it was at a solid consistent speed, but it shouldn’t have too much effect right?

Gold Member
Another possible cause is air currents. If a window was open/cracked, or if the bus has air vents, it could have generated a current of air at the top of the bus, pushing the droplet backwards.
I never thought of that. Huh. The busses that are in Hawaii don’t have windows that can open. But I assume there was at least a little because of the fact that water was able to deep through.

Homework Helper
Actually, yes. I’m not sure if it was at a solid consistent speed, but it shouldn’t have too much effect right?
It depends on how much effect you expect. One tenth of a gee of forward acceleration would result in a 6 degree rearward deflection. You could add additional deflection if the bus were on an up-slope.

doglover9754
Gold Member
We are all speculating here, but it's also possible that the water leaked through quickly so that it hadn't acquired all the bus's momentum.

Certainty, if it came in through a definite gap, it would be going backwards relative to the bus.
As a safety hazard, busses are required to have their emergency exits closed unless there is an actual emergency. The hatches usually have a seal to prevent leaks but I don’t think that the bus that I was in had a good seal.

Gold Member
It depends on how much effect you expect. One tenth of a gee of forward acceleration would result in a 6 degree rearward deflection.
The bus was moving forward. I couldn’t tell the difference in speed so I’m not too sure if the speed was consistent enough.

Staff Emeritus
Gold Member
Another question would be how smooth was the bus ride? A drop of water clinging to a surface could have come loose due to a small jolt or vibration of the bus. The drop would have taken on some of the motion of that jolt relative to the bus. In effect, it could have been "tossed" backwards by whatever caused the drop to come free.

Comeback City, doglover9754 and sophiecentaur
Homework Helper
Gold Member
2022 Award
As a safety hazard, busses are required to have their emergency exits closed unless there is an actual emergency. The hatches usually have a seal to prevent leaks but I don’t think that the bus that I was in had a good seal.

If you watch raindrops on the side window of a moving bus, they move backwards. The air outside may be squeezing them thru the seal at an angle.

doglover9754
Homework Helper
If you watch raindrops on the side window of a moving bus, they move backwards. The air outside may be squeezing them thru the seal at an angle.
Not sure I buy that one. Raindrops on a side window tend to have their slow downward creep (surface tension/viscosity versus gravity) biased rearward by wind. The resulting rearward velocity is rather small and likely to be completely negligible after having squeezed through a leak in a seal.

Homework Helper
Gold Member
2022 Award
Not sure I buy that one. Raindrops on a side window tend to have their slow downward creep (surface tension/viscosity versus gravity) biased rearward by wind. The resulting rearward velocity is rather small and likely to be completely negligible after having squeezed through a leak in a seal.

Perhaps, but if there were a draught thru the seal, then the cold air would also be disobeying Newtons laws in the same way.

doglover9754
But that doesn’t solve my question to why it flew back towards me.
Did it actually fly directly towards you, or did it maybe flow along the ceiling, and dropped on you?

DaveC426913
Gold Member
Another question would be how smooth was the bus ride? A drop of water clinging to a surface could have come loose due to a small jolt or vibration of the bus. The drop would have taken on some of the motion of that jolt relative to the bus. In effect, it could have been "tossed" backwards by whatever caused the drop to come free.
I don’t remember a bump.

Gold Member
Did it actually fly directly towards you, or did it maybe flow along the ceiling, and dropped on you?
It flew towards me.

Gold Member
It flew towards me.
The salient part of that question was: do you know from where it fell?
Can you be certain it fell directly from the crack, or could it have dribbled along the underside of the ceiling a few inches?

Gold Member
The salient part of that question was: do you know from where it fell?
Can you be certain it fell directly from the crack, or could it have dribbled along the underside of the ceiling a few inches?
I am very sure it fell from the crack. Not directly from the crack, but like right outside of it. Like you know how a crack in an emergency exit is like a crevice? So it literally rolled down the crack and eventually flew towards me. I was watching it as there is not much entertainment on a public bus.