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6: Applying Newton’s Laws

Learning Objectives

• Understand when an object’s motion can be modeled as one dimensional (linear).
• Be able to develop models for objects undergoing linear motion.
• Be able to develop models for objects undergoing circular motion.
• Be able to develop models for objects undergoing arbitrary three dimensional motion.
• Understand the forces involved in circular motion, and understand that “centripetal” and “centrifugal” forces are not really forces.

In this chapter, we take a closer look at how to use Newton’s Laws to build models to describe motion. Whereas the previous chapter was focused on identifying the forces that are acting on an object, this chapter focuses on using those forces to describe the motion of the object.

Newton’s Laws are meant to describe “point particles”, that is, objects that can be thought of as a point and thus have no orientation. A block sliding down a hill, a person on a merry-go-round, a bird flying through the air can all be modeled as point particles, as long as we do not need to model their orientation. In all of these cases, we can model the forces on the object using a free-body diagram as the location of where the forces are applied on the object do not matter. In later chapters, we will introduce the tools required to apply Newton’s Second Law to objects that can rotate, where we will see that the location of where a force is exerted matters.

prelude

If a person swings on a swing where the ropes are damaged, where are the ropes most likely to break?

1. at the bottom of the trajectory, when the speed is the greatest.
2. at the top of the trajectory, when the speed is zero.
3. at the point in the trajectory where the speed is one half of its maximal value.

• 6.1: Statics
In static situations, the acceleration of the object is zero. By Newton’s Second Law, this means that the vector sum of the forces (and torques, as we will see in a later chapter) exerted on an object must be zero. In dynamic situations, the acceleration of the object is non-zero.
• 6.2: Linear motion
We can describe the motion of an object whose velocity vector does not continuously change direction as “linear” motion. For example, an object that moves along a straight line in a particular direction, then abruptly changes direction and continues to move in a straight line can be modeled as undergoing linear motion over two different segments. An object moving around a circle, with its velocity vector continuously changing direction, would not be considered to be undergoing linear motion.
• 6.3: Uniform circular motion
• 6.4: Non-uniform circular motion
• 6.5: Summary
• 6.6: Thinking about the material
• 6.7: Problems and Solutions

Thumbnail: Stock cars racing in the Grand National Divisional race at Iowa Speedway in May, 2015. Cars often reach speeds of 200 mph (320 km/h)