Force And Motion – Session 5

Imagine you have a toy car. When you pick it up, you feel that it has some heaviness to it. That heaviness you feel is what we call the weight. Weight is how heavy or light something feels when you hold it in your hand. So, why do things have weight? Well, it is because of a special force called gravity. Weight depends on the amount of stuff in an object and the strength of gravity.

But wait, do not confuse weight with the mass. Mass is the amount of stuff in an object. It is like how much stuff is packed inside the box of your toys. The more toys you put inside, the greater the mass. Your mass stays the same no matter where you travel in the universe. However, the weight depends on the mass and the force of gravity.

We can measure the weight of an object by multiplying its mass with the acceleration due to the gravity of earth. We know that the acceleration of free falling object on earth due to gravity is nine point eight meter per second square. So let us say your mass is 40 kilograms. Your weight is three hundred ninety two Newtons. We denote Newtons using the symbol N.

Now, here is something exciting. If you travel to the moon, you will weigh much less than on the earth. That is because the moon has weaker force of gravity. Astronauts who visit the Moon experience this. They can jump much higher on the moon because they weigh less on the moon.

Have you ever wondered why some things feel heavy while others feel light, although they are of same size? For example, take two boxes of the same size. Fill one box with woods. Fill another box with rocks. Which box will be heavy? Definitely the box filled with rocks will feel heavy. But why?.

Let us understand the concept of density to answer this question. Density is a measure of how much mass is packed into a given volume. It tells us how much stuff is there in a specific space. The box filled with the rocks has a higher density compared to the box filled with the wood. This is because the density is calculated by dividing the mass of an object by its volume. Since the volume of both boxes is the same, the box filled with rocks, has more mass. It will have a greater density.

Let us now calculate the density of a piece of iron. We need to know the mass and the volume of a piece of iron to measure its density. Suppose the mass of the iron is fifty five grams. Its volume is seven cubic centimeters. Then the Density of this piece of iron will be 7.8 g/cm³.

Density of one kilogram of iron is greater than the density of one kilogram of wood. This is because one kilogram of iron occupies less space. While one kilogram of wood occupies more space. So high density means that the object having a mass occupies less space. Meanwhile, low density means that the object having a mass occupies more space.

Let us say you mix oil with water. Oil will not dissolve in water. Two layers will be formed. One layer will be oil. Other layer will be water. The layer of oil is above the layer of water. We can also say that the oil floats on water. Can you tell why oil floats on water?.

The oil floats on water due to the difference in density. The density of oil is less than the density of water. This is because similar mass of oil and water takes up different volume. This means that one kilogram of oil occupies more space than one kilogram of water.

Suppose we are mixing honey with water. Two separate layers of honey and water are formed. The density of honey is greater than the density of water. Can you tell if the honey forms the upper layer or the lower layer? Honey will be seen in the lower layer. This is because the density of water is lower than the density of honey.

Density also influences the floating and the sinking behavior of the objects in liquids. Have you ever wondered why some objects stay afloat on water while the others sink-down to the bottom? This is due to the relationship between the density of the object and the density of the liquid in which the object is placed. If the density of the object is greater than the density of liquid, the object will sink-down in the liquid. For example, a metal coin sinks in water. This is because the density of the metal coin is greater than the water.

If the density of the object is less than the density of liquid, the object will stay afloat on the liquid. For example, a piece of wood does not sink-down in water. It floats on water. This is because the density of the wood is less than the density of the water.

Have you ever wondered why a needle sinks in the water while a large ship floats on the water? Although the mass of the ship is greater than the mass of the needle. The needle sinks in the water because its density is greater than the density of water. The needle is small and compact. It has relatively high mass compared to its volume. That is why it sinks in water.

The ship floats on the water because its density is lower than the density of the water. Ships are designed with a hollow, large structure. This means they have a large volume in comparison to their mass. So we understand that the mass of the object does not affect its sinking or floating on water. The density of the object affects the sinking or floating behaviour of the objects.

You might have seen balloons going upward in air. Do you know why the balloons travel upward in air?Balloons are filled with the helium gas. Density of the helium gas is less than the air. Due to low density, the helium gas moves upward in the air.