# Summary

- First Law of Motion: An object continues to be in a state of rest or of uniform motion along a straight line unless acted upon by an unbalanced force.
- The natural tendency of objects to resist a change in their state of rest or of uniform motion is called inertia.
- The mass of an object is a measure of its inertia. Its SI unit is kilogram (kg).
- Force of friction always opposes motion of objects.
- Second Law of Motion: The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of the force.
- The SI unit of force is kg m s
^{â€“2}. This is also known as Newton and represented by the symbol N. A force of one Newton produces an acceleration of 1 m s^{â€“2}on an object of mass 1 kg. - The momentum of an object is the product of its mass and velocity and has the same direction as that of the velocity. Its SI unit is kg m s
^{-1}. - Third Law of Motion: To every action, there is an equal and opposite reaction and they act on two different bodies.
- In an isolated system, the total momentum remains conserved.