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First Law of Motion

The following is not a law of physics:
Most people think the above statement is a law of nature. Some very intelligent thinkers thought it was a law of nature, including Aristotle (ancient Greek, no intellectual lightweight) and more recently Descartes (famous philosopher). Because it is common sense (right?) that if nothing pushes on an object, it eventually slows to a stop.

But sometimes closer scrutiny conflicts with common sense, and when that happens we have to change our thinking, to retune our intuition, so that what once seemed wrong now seems right. That can be difficult, but that’s physics.
First Law of Motion
If the forces on an object are balanced, then the object moves with constant velocity (constant speed in a straight line). Conversely, if an object has constant velocity, then the forces on it are balanced. Some people use the term inertia to describe this property of matter.
Galileo discovered this law, although it’s generally called Newton’s first law of motion.


What does it mean for the forces to be balanced? Before we answer that question, let’s look at a few cases. In the following figures (Figures 3-1a–f) we denote the motion of an object by "motion marks", so that Image means the object is moving to the right.


Case a. There are no forces. In this case think of a rock in deep space moving along. The velocity vector is constant, meaning the rock continues traveling at constant speed to the right indefinitely.


Figure 3-1a

Case b. There are two opposed forces, equal in magnitude, perpendicular to the motion. In this case think of a marble rolling along a smooth level floor (no friction). Gravity pulls down, but the floor pushes up. The velocity vector is constant.


Figure 3-1b

Case c. This scenario is a nonexample, in which the right force is larger than the left force (hence unbalanced). The object will speed up.


Figure 3-1c

Case d. This scenario is also a nonexample with the left force larger than the right force. The object will slow down.


Figure 3-1d

Case e. Two opposed forces, left and right, are equal in magnitude. The case is between cases c and d. The object has constant velocity, that is, it will keep its speed indefinitely. Think about this one for a while. This stumps many people.


Figure 3-1e

Case f. The forces in all three directions are balanced. The object’s velocity vector is constant.


Figure 3-1f

The forces are balanced if the vector sum of all the forces on the object is zero. We define Fnet by

Fnet = F1 + F2 +…

where F1, F2, …, are all the forces acting on an object. The vector Fnet is the total force on the object.

A woman kicks a soccer ball, and it rolls for a while at constant speed, then another woman stops it. Draw a diagram showing all the forces on the ball at the three times: kicking, rolling, and stopping.


Part a: The ball is kicked (see Figure 3-2). The vector Fgrav is the force of gravity, and Fkick is the force of the foot on the ball. The symbol N stands for "normal", a physics word meaning perpendicular to the ground. It is force the ground exerts on the ball.



Figure 3-2

Part b: The ball rolls along. It has only two forces acting on it (Figure 3-3). The ball does not remember (or care) what started it rolling. According to the first law, the balanced forces guarantee it will keep rolling indefinitely at constant speed.



Figure 3-3


Part c: The ball is stopped. Now there is a force of a foot on the ball as well (Figure 3-4).



Figure 3-4

Over the next several chapters there will be many problems to test your intuition on the first law.

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