Social Sciences: Class 4 — Chapter 7: How Things Work

Some objects spin easily, while others do not. We can observe and test different objects to understand how they spin.

• Objects that spin: Coins, bangles, and wooden spinners (tops) can spin.
• Objects that do not spin properly: Erasers, pencils, and flat pieces of stone.

The position of the center of rotation is critical. A spinner with a hole exactly at the centre remains upright and spins well, whereas a spinner with a hole away from the centre wobbles and fails to spin properly.

Historical & Cultural Examples:

• Charkha: A traditional spinning wheel used to twist cotton fibre into thread, famously used by Mahatma Gandhi to make handmade clothes.
• Lattu: A popular traditional Indian spinning top, historically made of clay and commonly made of wood today.

Whether an object floats or sinks in water depends on multiple factors, not just its weight.

• Common Observations: Lighter objects like leaves float, while heavier objects like iron nails sink.
• Exceptions: An empty steel bowl (which is relatively heavy) floats, while a tiny iron pin (which is light) sinks.

The Role of Shape:

By changing the shape of an object, we can change whether it floats or sinks. This is demonstrated using aluminium foil:

• Spread out foil: Floats
• Pressed tightly into a ball: Sinks (trapping as little air as possible increases its density)
• Cup-like shape: Floats

Different materials can be used to construct toy boats, each with unique strengths and challenges:

• Ice-cream stick boat: Strong but requires adhesive.
• Coconut shell boat: Naturally hollow and floats well, but can be heavy.
• Walnut shell boat: Small, lightweight, and easy to float.

Classification Exercise: Classify the following objects based on whether they float or sink in water:

• Things that float: Wax, Thermocol, Candle, Cork, Leaf, Ice-cube, Pumpkin.
• Things that sink: Marble, Coin, Eraser, Spoon, Potato, Tomato, Lemon.