It is more difficult to stop an object moving at high speed than it to stop the same object moving at low speed. If the runaway baby carriage happened to be moving at a speed of one million miles per hour, the force required to stop it would be the same as the one needed to stop the truck moving at a much smaller speed. Observations such as the above lead us to the conclusion that what matters is the product of mass times velocity. Well, it turns out that mass does change with time if the object is moving close to the speed of light. Conservation of Momentum Momentum is an especially important physical quantity because, under certain conditions, it is conserved.
Conservation of momentum is a direct consequence of Newton's third law.
To see this, consider the example of firing a gun. As the bullet takes off, the gun kicks back. The gun exerts a force on the bullet, due to the explosives. The value of such a conserved quantity can be calculated by noting that, prior to firing the gun, both the bullet and the gun are at rest and therefore their total momentum value is zero. We can get this without any knowledge of the complex forces during the explosion and without solving Newton's equations.