In this section, we will explore some consequences of Newton's first law, which can be stated as, "an object moving with a constant velocity will continue to move with the same speed and in same direction unless an interaction with another object(s) produces a net external force".
Check out the OpenStax trailer about forces!
OpenStax AP Physics Chapter 4: Newton's Laws Physics Concept Trailer
Read the BoxSand Introduction and watch the pre-lecture videos before doing the pre-lecture homework or attending class. If you have time, or would like more preparation, please read the OpenStax textbook and/or try the fundamental examples provided below.
Newton's first law introduces the force concept in relation to an object's speed and direction. If an object is in motion or at rest, the only way for the object's speed or direction to change is for an external force to act upon it. For example, a stationary ball in outer space will only begin to move when a force is applied to the ball. Since the ball begins to move we can say that its velocity has changed. Additionally, a ball in motion has no reason to change its velocity, speed or direction, unless an external force acts upon it.
Newton's First Law of Motion
Prior to this section, we did not ask any questions about what causes the motion of objects to begin or to change. In this section, we will use Newton's second law develop a cause and effect relationship between forces and motion. Newton's second law can be stated as follows:
Newton's Second Law
**NOTE: Just like kinematics, the rest of PH201 works under the point particle approximation. Thus, we will not need to worry about the center of mass yet. But in PH202, will will start to consider the physical shape of objects, requiring us to determine center of masses.
Newton's Second Law
Gravitational Force
Hooke's Law
Now, take a look at the pre-lecture reading and videos below.
Here is an introduction to forces from the OpenStax textbook (Chapter 4)
OpenStax chapter 4.1 develops the force concept
Read about Newton's First Law of Motion in chapter 4.2
The most important law in forces is Newton's 2nd Law of Motion in chapter 4.3
Newton's 3rd law is the last (chapter 4.4), which talks about symmetry in forces.
Check out the normal force and tension in chapter 4.5