The First Law is concerned with changes in velocity caused by non-zero net forces.
Isaac Newton stated three laws of motion; the first law deals with forces and changes in velocity. For just a moment, let us imagine that you can apply only one force to an object. That is, you could choose push the object to the right or you could choose to push it to the left, but not to the left and right at the same time, and also not up and to the right at the same time, and so on.
Under these conditions the first law says that if an object is not pushed or pulled upon, its velocity will naturally remain constant. This means that if an object is moving along, untouched by a force of any kind, it will continue to move along in a perfectly straight line at a constant speed.
In the animation above we see an object (blue box) that is already moving along when we encounter it. As long as we do not push or pull it, as long as it does not run into something else or rub against something else, it will continue to move in a straight line at constant speed. It will continue to do this on its own without the need for any influence from other agents.
The first law also means that if an object is standing still and is not contacted by any forces, it will continue to remain motionless.Actually, a motionless object is just a special case of an object that is maintaining constant velocity. Its velocity is constantly 0 m/s.
In the scene above we see an object (blue box) that is standing still. On its own it will continue to stand still as long as we do not influence it with a force, that is, with an unbalanced, non-zero force.
Understand that we are saying if no outside force is applied, the velocity of the object (which is its speed and direction) will remain constant.
An object traveling in a straight line at constant speed has a constant velocity.
An object standing still has a constant velocity too. Its constant velocity is zero in value.
In order to change either of these examples of constant velocity, you need to apply a non-zero force to the object. Then, when the force is applied, the velocity will change, and the object experiences an acceleration.
Several forces can act at once on the object.
Now, what about if there is more than one force on the object? You really can push an object, say, to the left and down at the same time, so, what happens then?
Under these conditions we must realize that a group of forces on an object adds up so that all the forces appear to the object as one force. This one force that is the sum of all the forces is called the net force. (You might want to read some information on net force. It is here.) The word net in this context means total. It is this net force that may change the velocity of the object. Let us look at some examples.
Imagine that two forces act at the same time on an object. One is a very strong force to the left, and the other is a weaker force to the right. These two forces add up to one net force. Since the force to the left is stronger, the net force is to the left.
Exactly Northwest
