Newton's Second Law states "The acceleration of an object is directly proportional to the resultant force acting on it and inversely proportional to its mass". We may write this in the form of an equation:
or 
We can use this equation to define the
the unit of force, written
equals the force required to produce an acceleration of
on a mass of
Balanced forces
There are many forces acting on objects on Earth, even stationary ones. When an object on the Earth is stationary, two forces are acting upon it. One is the force of gravity. This force pushes an object down towards the centre of the Earth. At the same time, the surface the object is sitting on pushes upwards with an equal force. This keeps the object from falling through the surface. In this case, the two forces acting upon the object are balanced because they act in opposite directions with the same magnitude.
Unbalanced forces
When the forces are unbalanced – they have different magnitudes - the object accelerates. An example of this is when a car accelerates from a stop. The driving force from the wheels of the car provides a forward force. This force then cancels out the forces that push in the opposite direction, such as air resistance and friction. If the driving force is more than the forces of air resistance and friction, the car will accelerate forward.
The Difference Between Newton's First and Second Law
An important distinction that Newton's Second Law makes in conjunction with his First Law (objects at rest tend to stay at rest, while objects in motion tend to stay in motion) is that force is not required for motion to continue. If an object is already moving, it does not require any force to continue moving. In fact, if no forces are imposed on a moving object, it will continue moving in the same direction and at the same speed until a force acts upon it.