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Introduction to Newton's Laws of Motion

Newton's First Law of Motion

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Every body continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it.
- Newton's First Law of Motion, translated from the Principia's Latin
This is sometimes called the Law of Inertia, or just inertia. Essentially, it makes the following two points:
  • An object that is not moving will not move until a force acts upon it.

  • An object that is in motion will not change velocity (including stopping) until a force acts upon it.
The first point seems relatively obvious to most people, but the second may take some thinking through, because everyone knows that things don't keep moving forever. If I slide a hockey puck along a table, it doesn't move forever, it slows and eventually comes to a stop. But according to Newton's laws, this is because a force is acting on the hockey puck and, sure enough, there is frictional force between the table and the puck, and that frictional force is in the direction opposite the movement. It's this force which causes the object to slow to a stop. In the absence (or virtual absence) of such a force, as on an air hockey table or ice rink, the puck's motion isn't hindered.

Here is another way of stating Newton's First Law:

A body that is acted on by no net force moves at a constant velocity (which may be zero) and zero acceleration.
So with no net force, the object just keeps doing what it is doing. It is important to note the words net force. This means the total forces upon the object must add up to zero. An object sitting on my floor has a gravitational force pulling it downward, but there is also a normal force pushing upward from the floor, so the net force is zero - therefore it doesn’t move.

To return to the hockey puck example, consider two people hitting the hockey puck on exactly opposite sides at exactly the same time and with exactly identical force. In this rare case, the puck would not move.

Since both velocity and force are vector quantities, the directions are important to this process. If a force (such as gravity) acts downward on an object, and there's no upward force, the object will gain a vertical acceleration downward. The horizontal velocity will not change, however.

If I throw a ball off my balcony at a horizontal speed of 3 m/s, it will hit the ground with a horizontal speed of 3 m/s (ignoring the force of air resistance), even though gravity exerted a force (and therefore acceleration) in the vertical direction. If it weren't for gravity, though, the ball would have kept going in a straight line . . .

. . . at least until it hit my neighbor's house.

Newton's Three Laws of Motion

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