Force And Motion – Session 4

Mass is the amount of stuff or matter that an object contains. It is a measure of how much stuff there is in an object. When you pick up a heavy rock, you can feel by it’s size that it has more mass compared to a small pebble. Similarly, a big watermelon has more mass than a tiny grape.

The standard unit for measuring mass is the grams. To measure the mass of something, we use a special tool called a scale or a balance. We put the object with known mass on one side of the scale. The object whose mass is unknown is put on the other side of the scale. We increase or decrease the number of known masses on one side until both sides are balanced. Balancing of both sides indicate that the mass of the unknown object is equal to the mass of the known object.

The most precise way of measuring the mass is by using a digital scale. For example, we want to measure the mass of the tomatoes. We shall put the tomatoes on the digital scale. The reading on the digital meter tells the mass of the tomatoes. The mass can also be measured in kilograms. There are one thousand grams in one kilogram.

Have you ever wondered why objects fall-down onto the ground? Well, this is due to gravity. Gravity is like an invisible force that pulls things toward each other. It is what keeps us on the ground.

Very long time ago, a smart man named Sir Isaac Newton started thinking about why things move and how they work. Here’s one tale about him that explains gravity. One day, he noticed an apple fall-down from a tree. He wondered why it fell-down to the ground instead of going up or sideways. This got him thinking. He realized that there must be an invisible force that pulls things, like the apple, toward the Earth. He called this force gravity.

Newton started to study and experiment with gravity. He found out that everything with mass has this special force. This force exists between the anything that has mass. It is like a magic pull that keeps us on the ground.

That is why when you jump up, you come-back downwards. The gravity of the earth is strong enough to bring you to the ground. And that is why the moon goes around the Earth. Both moon and the earth both have gravity. Then why does the moon revolve around the earth?.

The object with greater mass has a stronger force of gravity. The moon revolves around the earth because the moon is less in mass compared to earth. The mass of earth is greater than the mass of the moon. The moon revolves around the earth because there is a stronger force of gravity of from earth as compared to the force of gravity from the moon. Moreover, the forward motion of the moon and the force of gravity of the earth causes moon to revolve in an orbit around the earth.

Do you know what causes the tides in the oceans. The tides in the ocean are due to the force of gravity of the moon. As we have studied, objects, especially with large mass, have a force of gravity that pull things toward them.The moon also has mass. But the mass of moon is less than the mass of the earth. So the force of gravity of the moon also pulls the earth. This pull causes the big tides in the oceans because the moons gravity pulls on the ocean water.

Do you know you are taller in the morning as compared to the evening. Is your height in the morning really different than your height in the evening? Yes, that is correct. Do you want to try this? Well, measure your height immediately after waking up in the morning with the help of a ruler. After that, measure your height again in the evening. You will see a difference in your height.

You might be wondering why we are taller in the morning as compared to the evening? Well the answer to this question lies in the force of gravity. When you sleep all of your body muscles are relaxed. But during the day you are also walking and doing your other work. During the day your body is in upright standing position. The gravity of the earth pulls you downwards. As a result your spine is compressed a little bit. Due to this, you become a little shorter in the evening. Once you sleep, your spine can relax because the gravity is not pulling it downward vertically. Therefore it goes again to it’s original length.

You might be wondering why we are taller in the morning as compared to the evening? Well the answer to this question lies in the force of gravity. When you sleep all of your body muscles are relaxed. But during the day you are also walking and doing your other work. During the day your body is in upright standing position. The gravity of the earth pulls you down. As a result your spine is compressed a little bit. Due to this, you become a little shorter in the evening. Once you sleep, your spine can relax because the gravity is not pulling it downward vertically. Therefore it goes again to it’s original length.

We know that the force of gravity depends on the mass of the objects. It also depends on the distance between two objects. Suppose you are going very far away from the earth into the space. You will be afloat in space. You will not fall-down onto the earth. Can you tell why you will not fall-down onto the earth in this case? This is because the distance between you and the earth is very large. Due to this reason the force of gravity of the earth will not pull you.

If you are close-by to earth, the force of gravity between you and the earth is stronger. Suppose you are standing on the earth. When you jump, you come-back to the ground. This is because the distance between you and the earth is very small. Due to this reason, the gravity of the earth pulls you stronger.

Suppose we drop a ball from a very tall office-building. Can you tell the speed at which the ball moves towards the earth? Does the ball move towards earth at constant speed?.We have already discussed that the force gravity will be stronger between the objects that are closer to each other. As the ball gets closer to the ground, the earth pulls the ball more strongly towards itself. As a result the speed of the ball increases.

The speed of the ball dropped from the office-building will increase with the passage of time. We can also say that the ball will accelerate. This acceleration of the ball is due to the force of gravity. Acceleration of any object due to the force of gravity is nine point eight meter per second square. This means that the speed of a free falling object will increase by nine point eight meter per second after every second.