Force And Motion – Session 7

Imagine you have a piece of string. If you pull the ends of the string in opposite directions, you are applying the force to it. The string will fight-back and try to stay in one piece. The force inside the string that tries to pull-back and keep it from breaking is called the tension.

When you are sitting on a swing, the ropes on either side are pulled tight. That is because they are experiencing tension. The ropes are trying to hold you up. They resist the pull of the gravity. That is why you can safely swing-back and forth without falling.

When you hang wet clothes on a line to dry, the line sags a little under the weight. However, it does not usually break. That is because the line has tension keeping it together. This tension force balances out the pull of the clothes by gravity.

Imagine you have a rubber band. Hold one end of the rubber in each hand. When you pull the ends of the rubber band away from each other, you are stretching it. This stretching creates tension in the rubber band. And guess what happens when you release the ends of the rubber band? The tension stored in the rubber band makes it snap-back quickly to its original shape.

When you throw a yo-yo toy downward, it spins and then returns to your hand. But how does this happen? When you throw the yo yo toy down, it starts to unwind the string. The yo yo toy spins as it goes down. The spinning yo yo pulls the string tight, creating tension. When the yo yo reaches the end of the string, the tension in the string is at its highest. The string wants to pull itself back-together. The tension force winds the string back-onto the spool of yo yo. Finally it climbs-back up to your hand.

Now let’s talk about another interesting phenomenon like tension. Have you ever played with a balloon at a birthday party? Of course you have. But have you ever noticed something strange happening after you rub a balloon against your hair? Let us do a little experiment. Take a balloon and rub it against your hair. Then, slowly bring the balloon near some tiny bits of paper. Can you tell what will happen? The paper starts to move towards the balloon as if by magic. But is it really magic? No, it is something much more interesting. This happens due to electrostatic force.

Let us understand electrostatic force. Everything around us is made of tiny little things called atoms. Some parts of atoms have a positive charge, some have a negative charge. Imagine positive charges and negative charges like two opposite ends of a magnet. Positive charges attract the negative charges. Positive charges repel the other positive charges. The same goes for the negative charges. Negative charges repel the other negative charges.

Electrostatic force is the push or pull that happens between these charges. The attraction between positive and negative charges is called electrostatic force. The repulsion between two similar charges is also called electrostatic force.

When you rub the balloon on your hair, some of the tiny negative charges move from your hair to the balloon. These charges move from hair to the balloon due to force of friction. Now, the balloon has extra negative charges. it wants to find some positive charges to attract.

The paper has some positive charges. The negative charges in the balloon attracts the positive charges on the paper. As a result the balloon pulls the paper towards it. We can say that the electrostatic force of attraction exists between the balloon and the paper.

Imagine you are getting ready in the morning. You have finished brushing your dry hair with a plastic comb. After brushing your hair, the comb now has a special power. It has collected some of the tiny negative charges from your hair.

Now, turn on the tap so that a thin stream of water is flowing from it. Take the comb that you have used on your hair. Slowly bring this comb near the stream of water without actually touching it. You will see something amazing. The water starts to bend towards the comb. Can you tell why the water is bending towards the comb?.

What is happening here is another example of electrostatic force in action. When you brushed your hair, the comb collected those negative charges from your hair. Now the comb has these extra negative charges. Water has some positive charges too. Negative charges of the comb pull the positive charges in the water towards it. This causes the water to bend and follow the comb as you move it.

We have been talking about the electrostatic force due to attraction of the opposite charges. Electrostatic force is also due to repulsion between similar charges. Imagine you have two balloons. You rub both of them on your hair. By rubbing them on your hair, both balloons collect extra negative charges from hair.

Now, try bringing the two balloons near each other. Instead of sticking together, they will push each other away. This happens because both balloons now have extra negative charges from your hair. Remember, similar charges do not want to be near each other. They want to move apart, similar to two similar poles of a magnet. This shows that the repulsion between two similar charges is also the electrostatic force.