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All moving objects have momentum. Momentum can be defined as the tendency of an object to keep moving in the same direction and it is a vector quantity. It can be calculated using: Momentum = mass × velocity, and its unit is kgm/s.
Total momentum is conserved in collisions and explosions unless an external force is exerted on the system. This is called the Law of Conservation of Momentum. The law states that: For an isolated system, provided that there are no external forces acting and no energy is provided, momentum will remain constant.
While moving along the same line, moving objects may collide with each other. When they collide, they exert a force on each other. By Newton's Third Law, these two forces are equal in size and opposite in direction. By Newton's Second Law, since a force is acting on these objects, their momentum changes. A collision can either elastic or inelastic: an elastic collision is one in which kinetic energy is conserved, whereas in an inelastic collision, there is no conservation of kinetic energy. The kinetic energy before and after a collision can be calculated using KE = mv² and compared to decide whether a collision is elastic or inelastic. There is conservation of momentum regardless of whether kinetic energy is conserved or not. An explosion is when a rapid change occurs to a stationary system causing objects to move apart.
By Newton's Second Law, a force is required to stop a moving object. This force is given by F = m × a. Substituting the acceleration equation into this gives F × t = mv − mu. Given that v is the final velocity and u is the initial velocity, mv − mu = change in momentum. Change in momentum is also known as impulse and force is also known as the rate of change of momentum. Newton's Second Law can be rewritten as: A force acting on a body is proportional to the rate of change of momentum it produces in its direction.
Impulse is defined as the change of momentum of an object and is given in Ns. It can be calculated using: Impulse = Force × time, and it is equal to the area under a force-time graph.
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