### Newton’s First Law of Motion

An object is either at rest or moves at a constant speed in a straight line unless it is acted upon by an external force.

### Newton’s Second Law of Motion

The rate of change of Momentum of an object is proportional to the force that caused it.

In the above animation, the car is subjected to two forces: driving force, D, that acts forward and the resistive force, R, the combination of friction and drag.

When D > R, there is a resultant force, F, acting on the car and according to the F = ma, there is a corresponding acceleration – Newton’s Second Law.

When D = R, however, the resultant force, F, is zero. Therefore, according to F = ma, the acceleration is zero. As a result, the car continues to move at a constant velocity – in line with Newton’s First Law.

Not only does the above animation illustrate Newton’s First and Second Laws of Motion, but also shows that the two laws are compatible.

You can play with the animation here:

### Food for thought:

In certain parts of the world, where public busses have two open doors, I have seen people jumping off the buses as they slow down before stopping – without falling to the ground. Why do they keep running forward for a while?

When a car starts moving forward, passengers tend to move backward. Why does it happen?