Water, rocks, large washing-up bowl, strong plastic bag without holes
Start by giving the children an opportunity to marvel at the sheer size of a blue whale, maybe by showing a short video or a picture where they can see the whale’s size compared to a human. Then, make a “whale” by putting rocks into a plastic bag and invite the children to try to pick it up. It’s so heavy! Place the bag in the bowl and carefully add water. Make sure none of the water goes in the bag. Encourage the children to try to lift the bag of stones.
What’s the STEM?
The bag feels lighter and is easier to lift when it is in water. Think about how beached whales are in trouble and need help but when they are in water, they can move around effortlessly. Ask the children how they feel in the water. Can they float? You feel less heavy in water because the water supports you.
This activity explores forces. The water pushes the bag of rocks up from underneath, just like the children’s bodies and the blue whale. The force of the water that pushes the bag of rocks up is called buoyancy and acts against gravity, which is the force pulling the rocks down. This makes the bag of rocks feel less heavy in water.
Did you know that most of the time we use the words mass and weight incorrectly? When we say, “My weight is…” or, “We need 225g of flour for the cake,” we are actually talking about mass, as weight refers to the force exerted on the object by gravity. While we would weigh less on the Moon, our mass would be the same as on Earth.
Observations, conclusions and extensions
The shape of the whale makes a difference too. Perhaps you could investigate by pulling objects through the water and looking at the drag their different shapes cause in the water. Model animals are wonderful for this. You could also make your own out of modelling clay, plasticine or even Blu Tack and test them. Our tinfoil boat challenge would also make a nice extension as the children create boats and load them with marbles, investigating how the shape of the hull affects how much cargo the boat can take.
Use your explorations to introduce this STEM vocabulary to the children:
buoyancy, force, gravity, upward thrust, heavy, light, weight, mass.