Do all heavy things sink?

In our lesson this week, we will be considering why objects float or sink. We will be learning about a new force, upthrust.

Watch this clip to learn about upthrust: www.bbc.co.uk/education/clips/zrrg9j6

These are the key ideas to understand so far:

Key Ideas:

• All forces are either pushes or a pulls.
• Forces usually occur in pairs.
• Objects have mass (how much ‘stuff’ there is). This is measured in kilograms (kg). Mass isn’t a force.
• Gravity acts upon mass and gives objects weight.
• Weight is a force. It is measured in Newtons.
• Gravity pulls all objects (including gases) towards the centre of the earth.
• A ship floats in water because the water pushing it up (upthrust) is equal to the force of gravity (weight) pulling it down.
• Friction occurs when objects rub together.
• Friction also occurs when objects move through air or water. This is called air resistance or water resistance.

Revision link: www.bbc.co.uk/bitesize/ks2/science/physical_processes/forces/read/6/

What is happening when a boat floats?

Floating ships

When something is placed in the water, it displaces the water (moves it out of its place; like when you climb into the bath). This displaced water is pushed upwards and to the side of the object. However, the water doesn't ‘want to be there’ because gravity is trying to pull it back down underneath the object. This creates an upward reaction force called UPTHRUST.

If weight and upthrust are balanced forces (are the same size as each other) an object will float.

To float an object must displace (push out) its own weight in water so that that water can push back against the boat with the same force as the downward pull of gravity.

When we float in the swimming pool, it is because the upthrust from the water is equal to our weight pulling it down. But what happens when we swim in salty water? Click on this link to find out. www.bbc.co.uk/education/clips/z44c87h

In class we will be weighing objects in the air and water to establish that they weigh less in water. We will then explore different shaped pieces of play-dough to discover whether the shape of an object effects the amount of water it displaces and therefore its ability to float. Once this has been established, we will briefly discuss how adding salt to water can alter the upthrust on an object.

Homework: Complete the homework quiz sent to you as a task on firefly and submit it.