Force And Motion – Session 9

Imagine you are at a birthday party. There are colorful balloons everywhere. You grab one of those balloons and blow it up. But instead of tying the balloon, you let it go. The balloon moves around the room in all sorts of different directions. But wait a minute. Why does it do that?.

Inside the inflated balloon, the air is squeezed and tightly packed. When you let it go, the air rushes out of the balloon very fast. The air rushing out of the balloon pushes the balloon forward. This push is called the thrust force. Thrust is like a big push that helps things move.

Imagine a rocket standing tall on its launchpad. It is preparing for its journey. All its systems are getting checked. It is loaded with the necessary fuel for its voyage. When the engines light up, they begin to burn the fuel of the rocket. This process releases a tremendous amount of energy in the form of heat and light.

The intense light is the fire we see. The smoke is the exhaust and other by-products of the burnt fuel. The roar is the sound of all that energy being released. This bright and loud event is not simply for exhibition. It has an important reason behind it.

A vast quantity of hot gas is also produced due to burning of fuel. This gas is under a lot of pressure because there is so much of it in a confined space. The gas needs to escape. This gas escapes through the nozzles of the rocket. As the gas shoots out downwards at high speed, it exerts an equal and opposite force on the rocket. This opposite force pushes the rocket upward. The upward movement of rocket due to this is called the thrust.

When an airplane gets ready to fly, it needs a push to move forward. This push is called the thrust force. Planes with jet engines create this thrust by drawing in air, mixing it with fuel, and igniting it. This produces a forceful blast of air out the behind of the engine. This propels the plane forward.

On the other hand, planes with propellers achieve thrust by rapidly spinning the propellers. As these propellers spin, they push air backward. This activity of pushing the air backward provides a forward motion for the plane. We can also say that backward pushing of air produces thrust in forward direction.

Contact force refers to any force that arises due to the interaction between two objects when they are in direct touch or contact. Contact forces act only when two interacting systems are physically touching each other. There are several types of contact forces. Can you give any example of contact force?.

Well, the friction is an example of contact force. We know that the friction is a force that resists the motion of an object. For example, suppose a ball is moving on the ground. After some time the ball stops moving. This is because the force of friction between the ball and the ground is acting against the motion of the ball. It causes the ball to stop. This force of friction only occurs when the ball is in-contact with the ground. We can say that the friction is a contact force.

Drag force is also an example of the contact force. For example, when you are riding a bicycle, the air tries to push you backward. This push of air against your motion is called the drag force. There is a physical-contact between air and your body. We can call this drag force as the contact force.

Let us say that you are playing football. When you kick the ball, the ball moves forward. This is due to force. This force is produced due to the physical-contact between your kicking-foot and the ball. So we can also call this force as the contact force.

Can you tell any force that does not involve any physical-contact between the two objects? Or in simple words, is there any force that is not a contact force? The force of gravity is not a contact force. As we know, gravity is due to the mass of of an object. For example, the moon revolves around the earth due to the force of gravity. There is no physical-contact between the moon and the earth. Yet the force of gravity of the earth is acting on the moon.

The resultant force is the sum of two or more forces acting on a body. The resultant force depends on the direction in which the force is applied. Imagine you want to move the table from one place to another. You and your friend apply force to push the table. The table will move more easily as compared to if you move the table alone. You both are applying force in similar direction. So your force and the force of your friend adds up. As a result the resultant force becomes greater than the individual forces.

Now imagine you both are pushing the table from opposite sides in opposite direction? Will the table move fast? No, the table will not move in this case. No doubt, you both are applying force. You both are applying force in opposite direction to each other. These two equal forces in opposite direction cancel out each other. As a result the resultant force is zero.