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Laws of Motion Newton gave three laws to describe the motion of a body when some forces are acting on it. They give the relationship between the force and the motion. Following are the three laws of motion: Newton’s First Law Definition: A body would remain in the state of rest or of uniform motion until an external force is applied on it. This law states that if the vector sum of all the forces acting on a body is equal to zero then, the body will remain at rest if it is in rest or will continue to be in motion with a constant velocity. Newton’s Second Law Definition: If a force F is applied on a body of mass m, then the body will undergo acceleration a, such that, the magnitude of the acceleration is directly proportional to the force F and inversely proportional to the mass m of the body. The direction of motion would be same as the direction of the force. Mathematically, it can be represented as follows: F = m.a Derivation: We can also say that, the force F is a time derivative of the linear momentum p. F = dp/dt ---------------- eq 1 Now, p = m.v Putting the value of p in eq 1 we get, F = d(m.v)/dt = m. dv/dt Now we know dv/dt = a, and hence F = m.a Newton’s Third Law This law is also called “action-reaction” law. It states that every action has an equal and opposite reaction. Definition: If a body exerts a force F (i.e the action) on another body then, the other body exerts a force –F (i.e the reaction) on the first body. The magnitude of the force would remain same but the direction of the force would be opposite.