Newton’s First Law of Motion
Every body will remain at rest or in a uniform state of motion unless acted upon by a force.
This does not sound quite clear to a novice in the field. We can formulate this law using different words.
If no force acted on a body it:
a) remains at rest
or
b) continues motion along a straight line with constant speed.
I am sure it is still not clear to everyone. Newton’ First Law of Motion is very difficult to understand. We will try to give you a more detailed explanation.
There is ONE cause (reason) – no force acting – but this ONE cause can result in TWO mutually exclusive effects: state of rest or state of uniform motion.
How this can be? First we must widen the term “no force acting”. It is better to state: “the resultant (net) force acting on a body is ZERO”. This means the same thing, but for this statement we can easily find an example from our “neighborhood”. To find a body with “no force acting” upon it we would have to think about an object outside our Universe – completely unrealistic reasoning.
To clear the situation we “redefine” the 1st Law to the form:
If the resultant force acting on a body is zero, the body remains in
a) a state of rest
or
b) uniform motion.
The question remains, when and why do we have situation a) and when b).
a) If the object was at rest when the acting forces compensated, it will remain in this state.
b) If an object was moving when the acting forces compensated, it will continue to move with the velocity which it had at the moment when the resultant force dropped to zero.
In other words Newton’s First Law expresses the idea of inertia. In physics inertia is a well defined term, but it is also has another broader meaning. See for example the definition of inertia in the Wikipedia.
As far as case a) seems quite logical to anyone, case b) is much more difficult to understand.
One of the best experiments that help to understand Newton’s First Law of Motion can be performed with the Atwood Machine when masses on both sides of pulley are equal – see the Figure below.
In such a case the net force exerted on masses m is zero. Masses are not moving – state of rest – left part of figure.
Now, let’s pull down delicately, for a moment, the mass on the right side of a pulley. They start moving and will continue to move until the mass on the left strikes the pulley or the other one hits the floor. During this motion the resultant force is ZERO.
The force is not required to keep an object moving.
This statement sounds strange to many people, but it is true. The force is required only to change the state of motion – its speed or direction.
So why does a car need an engine to move with constant speed along a straight road?
Because the force is needed to overcome the forces of friction and of air resistance. Later on we will solve problems which make this statement quite obvious.