# Can you close a car door underwater?

1. May 14, 2015

### ScoobyVelma

Hi there,

I was wondering if it is possible to close a car door underwater? I understand that, due to differential pressure, you cannot open a car door until the pressure equalises. But is it possible to close a car door underwater?

Sorry, I know it must sound like a rather silly question but it has been playing on my mind for a while and thought I would ask some people who are knoweldgeable in the world of physics.

- ScoobyVelma

2. May 14, 2015

### Drakkith

Staff Emeritus
I would think so, considering that there would be no pressure difference since the car interior is open to the water.

3. May 14, 2015

### ScoobyVelma

Thank you for that, that was what I was thinking. What has me a little stumped though would be the force of the water acting on the car door? Would force be relevant in this situation or would it be cancelled out as the water would be applying the force from all directions?

Sorry I'm new to physics, so please forgive my 2 cent reasonings haha

4. May 14, 2015

### Drakkith

Staff Emeritus
If would be cancelled out since water is pressing in on all directions, just like the air is normally. The only thing you'd have to work against is moving the water through the door, which is obviously harder than moving it against the air, just like walking through water is harder than walking out of the water.

5. May 14, 2015

### SteamKing

Staff Emeritus
If the car door is already open, and the car is underwater, what is keeping the water out of the car?

You are only going to have a difference in pressure (or force) acting on the door if there is air on one side and water on the other.

6. May 14, 2015

### BobG

Yes, it's possible to close a car door underwater.

However, there will be a pressure differential and it will be harder than in open air. Closing the door will compress the water inside the car, increasing the pressure and making it more difficult to close.

You have the same scenario in air, except:

1) It's easier to compress air than water

2) It's easier for air to escape through small openings than water

All of this is assuming you're talking about a modern car designed for optimal fuel efficiency. If you're talking about a 1960's car, you have so many openings for the water to escape through you'll barely notice a difference. And I guess we must be talking about a car that has its windows rolled up.

7. May 14, 2015

### mrspeedybob

It would be very difficult at best.

Car doors actually take a fairly large amount of force to close. We can close them easily because the force is produced by rapidly decelerating the door. Car manufacturers design their latches this way so that most of the kinetic energy of the closing door is absorbed by the latch mechanism and very little is left to cause reverberations, rattles, or other unwanted noises.

Try closing your car door so that only the first part of the latch catches. Now try closing it the rest of the way. Odds are you'll have to give it a hip-bump or something like that to get it to close, it will be very difficult to close it with smoothly applied pressure.

If you tried to close the door underwater, the door would have to push the water around. A lot of the kinetic energy you tried to put into the door would be immediately transferred to the water and lost to turbulence as the water tried to flow around the the door.