# First Law of Motion

*not*a law of physics:

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 thatâ€™s physics.

**First Law of Motion**

What does it mean for the forces to be balanced? Before we answer that question, letâ€™s look at a few cases. In the following figures (Figures 3-1aâ€“f) we denote the motion of an object by "motion marks", so that 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.

**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.

**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.

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

**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.

**Case f.**The forces in all three directions are balanced. The objectâ€™s velocity vector is constant.

*F*

_{net}by

**F**_{net} = *F*_{1} + *F*_{2} +â€¦

*F*

_{1},

*F*

_{2}, â€¦, are all the forces acting on an object. The vector

*F*

_{net}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 *F*_{grav} is the force of gravity, and *F*_{kick} 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.

**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.

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

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