# 5: Further Applications of Newton's Laws- Friction, Drag, and Elasticity


It is difficult to categorize forces into various types (aside from the four basic forces discussed in previous chapter). We know that a net force affects the motion, position, and shape of an object. It is useful at this point to look at some particularly interesting and common forces that will provide further applications of Newton’s laws of motion. We have in mind the forces of friction, air or liquid drag, and deformation.

• 5.0: Prelude to Further Applications of Newton’s Laws
Describe the forces on the hip joint. What means are taken to ensure that this will be a good movable joint? From the photograph (for an adult), estimate the dimensions of the artificial device.
• 5.1: Friction
Friction is a force that is around us all the time that opposes relative motion between systems in contact but also allows us to move (which you have discovered if you have ever tried to walk on ice). While a common force, the behavior of friction is actually very complicated and is still not completely understood. We have to rely heavily on observations for whatever understandings we can gain.
• 5.2: Drag Forces
You feel the drag force when you move your hand through water. You might also feel it if you move your hand during a strong wind. The faster you move your hand, the harder it is to move. You feel a smaller drag force when you tilt your hand so only the side goes through the air—you have decreased the area of your hand that faces the direction of motion. Like friction, the drag force always opposes the motion of an object.
• 5.3: Elasticity - Stress and Strain
A change in shape due to the application of a force is a deformation. Even very small forces are known to cause some deformation. For small deformations, two important characteristics are observed. First, the object returns to its original shape when the force is removed—that is, the deformation is elastic for small deformations. Second, the size of the deformation is proportional to the force—that is, for small deformations, Hooke’s law is obeyed.
• 5.E: Further Applications of Newton's Laws (Exercises)

Thumbnail: Weight (W), the frictional force (Fr), and the normal force (Fn) impacting a cube. Weight is mass (m) multiplied by gravity (g). (CC-SA-BY-3.0; Email4mobile).

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