Newton's first law of Motion

Newton's first law is also called the law of inertia. It states that if the vector sum of all forces (that is, the net force) acting on an object is zero, then the acceleration of the object is zero and its velocity is constant.

Consequently:

• An object that is at rest will stay at rest until an unbalanced force acts upon it.
• An object that is in motion will not change its velocity until an unbalanced force acts upon it.

In the first point, the phrase unbalanced force refers to a set of forces which do not have a zero sum (net force zero) or whose torques about the center of mass of the object do not have a zero sum. Indeed, without the torque requirement, a net force of zero will not accelerate the center of mass of an extended object, but may cause the object to rotate.

The second point seems to violate everyday experience. For example, a hockey puck sliding along ice does not move forever; rather, it slows and eventually comes to a stop. According to Newton's first law, the puck comes to a stop because of a net external force applied in the direction opposite to its motion. This net external force is due to a frictional force between the puck and the ice, as well as a frictional force between the puck and the air. If the ice were frictionless and the puck were traveling in a vacuum, the net external force on the puck would be zero and it would travel with constant velocity so long as its path were unobstructed.

Implicit in the discussion of Newton's first law is the concept of an inertial reference frame, which for the purposes of Newtonian mechanics is defined to be a reference frame in which Newton's first law holds true.

Momentum of a body

It is observed experimentally that the force required to stop a moving object depends on two factors: (i) mass of the body and (ii) its velocity

A body in motion has momentum. The momentum of a body is defined as the product of its mass and velocity. If m is the mass of the body and , its velocity, the linear momentum of the body is given by

Momentum has both magnitude and direction and it is, therefore, a vector quantity. The momentum is measured in terms of kg m s ^-1 and its dimensional formula is MLT^-1.

When a force acts on a body, its velocity changes, consequently, its momentum also changes. The slowly moving bodies have smaller momentum than fast moving bodies of same mass.