Turn on thread page Beta
 You are Here: Home >< Physics

# Buoyancy watch

1. I have two identical in mass and size mouliable materials. I mould one into a boat shape, so that it floats. And the other into a ball, so that it sinks.

What EXACTLY IS it about the surface areas that causes this to happen. Why would a flatter surface area be able to float? What forces are actually in play?

What I'm supposing happens is that the bouyancy force for the ball is less than its weight? But if this is the case how because I thought the buoyancy force depended on water displacement. . And when the objects are first placed on water... isn't this the same... until they sink/float at which point the force has already been overcome...?
2. It's the same as trying to push an inflated a balloon underwater. The density of the object is less than that of the volume it's trying to replace.

For a boat, you make it a hollow 'boat' shape and it's basically a massive empty space with very low density, displacing water which is a much higher density.

Lighter things always stay on top.

The force acting on the boat is always the same, whether sinking or floating.The only difference is at what point the equal and opposite force causes equilibrium to occur.

The main point is that it's not really about surface area, it's about volume and density. By moulding something into a hollow shape, yes it has a higher surface area but the main factor is its decreased its density.
3. (Original post by stolenuniverse)
I have two identical in mass and size mouliable materials. I mould one into a boat shape, so that it floats. And the other into a ball, so that it sinks.

What EXACTLY IS it about the surface areas that causes this to happen. Why would a flatter surface area be able to float? What forces are actually in play?
The ball displaces exactly its volume of water, but sinks. We therefore know that the material is denser than water.

By making a hull from it, we include a volume of air - much like making a half-balloon, so the average density is greatly reduced. The hull can displace more than the weight of the material, so it floats.

It's to do with the average density, not surface area. If you took the solid ball of material and made it into a balloon, it would float once the volume of air caused the average density (the weight of the combined object divided by its volume) to be less than that of water.

Turn on thread page Beta

### Related university courses

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: February 6, 2017
Today on TSR

### Exam Jam 2018

Join thousands of students this half term

### Solo - A Star Wars Story

Poll

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE