In this physics video lecture for iit jee, we will be discussing the topic Newton’s laws of motion. The topics discussed in this video are,

• Force
• types of forces
• field forces
• contact forces
• string and spring forces
• spring force
• free body diagram

So what is a force? Force is basically responsible for any kind of motion. For any motion to happen, force is necessary. Force is only required to start the motion and not maintain it. Force is a vector quantity. In order to add or subtract two forces, we must pay attention to,

1. The magnitude of force,
2. the direction of force,
3. and the point of action

In this physics video lecture for iitjee we have discussed the various points of action in the video diagrammatically. Next, we have discussed types of forces.

The various types of forces are,

• Field forces: The best example for it is gravitational force. For this force to act no contact between the object. There are various types of field force like electrostatic force, magnetic force, etc. To understand this force, let’s answer this simple question, why is there a gravitational pull? This is because of the law of lowest potential energy. The potential energy depends on the position of the object. The field is present in the direction of the lowest potential energy. This field can be a gravitational field, magnetic field, etc. the object tries to move in the direction where it has the lowest potential energy.  Lower the potential energy of an object is, more stable it is.

• Contact forces: it is due to the contact between two objects. If two objects are in contact, and a force is applied, then the first object applies a force on the second object. Contact forces  can be of two types,
1. Normal force: if two spheres are in contact, then the force will act perpendicular to the surface of contact.
2. Frictional force: If there is an object on the surface, and if a force is applied tangentially, then this force will be a frictional force.

There are cases where both normal force and frictional force are present together. For such cases, we will find the resultant of these two forces. In the lecture of physics for iitjee we will see more on types of forces.

• String and spring forces: The force in the string is known as tensile force. It is called so because the force is created due to tension. If we pull a string from one end, then tension will be created. Here we are just considering about massless and frictionless string. If the string is massless, then the force on any point of the string will be same. If suppose the string is attached to a wall, then it will apply tension on the wall. If the string is a massive string then the string will have different forces on different points, and as a result, the tension will also be different. If we have a string with friction then it will cause a difference in the tension. This can be understood well by taking the example of a string and pulley system.

How does tensile force act? The string always pulls the object it is attached towards itself. Thus the tensile force always pulls. We have discussed the various cases possibly by taking the example of string and pulley system.

• Spring force:  When we pull a spring attached to a wall, it applies a force F. this force is also known as restoring force.

F =- kx

If we pull the spring in the positive x-direction, then the force will act on the negative x direction. If we compress the spring, then the force will act in the x direction. The spring can have both tensile and compressive forces. On tensile force, the spring elongates and on compressive forces, it will shorten. Even here the spring is massless or there is no acceleration.  If we have a string attached to an object. Then if a force applied on one end will be equal and opposite in the other end.

The last topic of iit jee physics lecture discussed is free body diagram. This is very important to understand. So what is a free body diagram? When we draw all the forces present in a body or system, such a diagram is called a free body diagram.  If we have an object of mass ‘m’, then the forces acting on it will be

• Gravitational force (mg) passing through the center of gravity in the downward direction.

N the upward direction, we have a normal force (if the object is in rest).