# The Naked Scientists Forum

### Author Topic: Direction of pressure of a ship floating on the sea.  (Read 9580 times)

#### labview1958

• Sr. Member
• Posts: 104
##### Direction of pressure of a ship floating on the sea.
« on: 20/08/2008 12:13:08 »
Is the direction of the pressure on aship foating in the sea upwards, sideways or both?

#### daveshorts

• Moderator
• Neilep Level Member
• Posts: 2583
• Physics, Experiments
##### Direction of pressure of a ship floating on the sea.
« Reply #1 on: 20/08/2008 12:27:04 »
The pressure on the ship will act at right angles to any surface. So the simple answer is both.

The pressure will increase as you go down as there is more water above you to provide the pressure.

The difference in pressure between the top and the bottom of the boat is what makes it float, the pressures in from the side always cancel each other out.

The effect is possibly more obvious with a submarine

The pressure acts all around it, trying to crush it, but slightly more at the bottom than the top, so it floats.

#### lyner

• Guest
##### Direction of pressure of a ship floating on the sea.
« Reply #2 on: 20/08/2008 19:33:11 »
Step into a deep puddle in wellies - you'll feel the pressure down the sides of your legs.

• Jr. Member
• Posts: 17
##### Direction of pressure of a ship floating on the sea.
« Reply #3 on: 21/08/2008 14:25:59 »
i think the pressure will act in the part that is immersed in the water in all direction

#### lyner

• Guest
##### Direction of pressure of a ship floating on the sea.
« Reply #4 on: 21/08/2008 15:09:57 »
Yes; the reason that a submerged object experiences upthrust is that the pressure on the bottom facing bits times the area is greater than the pressure on the upward facing bits times the pressure, because the pressure is a bit higher at the bottom of the object.
If an object is only just floating (all but the top .001%, say) it will tend to end to end up horizontal rather than vertical because the difference in pressure is much greater when it's upright.

#### labview1958

• Sr. Member
• Posts: 104
##### Direction of pressure of a ship floating on the sea.
« Reply #5 on: 24/08/2008 15:35:42 »
"The pressure on the ship will act at right angles to any surface. So the simple answer is both."

What proof that it is right angles?

#### lyner

• Guest
##### Direction of pressure of a ship floating on the sea.
« Reply #6 on: 24/08/2008 18:09:26 »
Which other direction would you choose? Normal is the only direction with no particular 'bias'.

#### LeeE

• Neilep Level Member
• Posts: 3382
##### Direction of pressure of a ship floating on the sea.
« Reply #7 on: 24/08/2008 21:53:43 »
If the pressure wasn't at right angles to the surfaces there'd be an imbalance of the forces acting upon any floating body and it would move of it's own accord e.g. if you put a ball in a tank of still water, it would start spinning.  Once you figured out how the get the water to only apply pressure in the direction you wanted you could do away with engines and all our energy needs would be sorted.

#### labview1958

• Sr. Member
• Posts: 104
##### Direction of pressure of a ship floating on the sea.
« Reply #8 on: 25/08/2008 14:35:48 »
I believe the pressure is in ALL directions. Thus it will balance out.

#### lyner

• Guest
##### Direction of pressure of a ship floating on the sea.
« Reply #9 on: 25/08/2008 14:54:15 »
that's another way of looking at it. On a small part of the surface, the water molecules are arriving from all directions. Each one gives a small impulse. The net effect is to cancel all impulses except from the normal - which doesn't provide any outward impulse.

#### LeeE

• Neilep Level Member
• Posts: 3382
##### Direction of pressure of a ship floating on the sea.
« Reply #10 on: 25/08/2008 16:34:32 »
I believe the pressure is in ALL directions. Thus it will balance out.

It can't apply pressure from all directions - it can't apply pressure from the direction of the surface that it's applying pressure to.

#### lyner

• Guest
##### Direction of pressure of a ship floating on the sea.
« Reply #11 on: 25/08/2008 18:40:27 »
I believe the pressure is in ALL directions. Thus it will balance out.
If the ship is not moving up or down the weight force is balanced by the vector sum of all the forces on the hull due to pressure . The vertical components add up to the weight. The horizontal components add to zero.

#### The Naked Scientists Forum

##### Direction of pressure of a ship floating on the sea.
« Reply #11 on: 25/08/2008 18:40:27 »