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6: The Laws of Motion

Last updated

Jun 23, 2019
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- 5.12: Multiple Velocities
- 6.1: Introduction

Boundless
Boundless

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Topic hierarchy

6.1: Introduction
There are three laws of motion that describe the relationship between forces, mass, and acceleration.
6.2: Force and Mass
Force is any influence that causes an object to change, either concerning its movement, direction, or geometrical construction.
6.3: Newton’s Laws
Newton’s first law of motion describes inertia. According to this law, a body at rest tends to stay at rest, and a body in motion tends to stay in motion, unless acted on by a net external force.
6.4: Other Examples of Forces
Weight is taken as the force on an object due to gravity, and is different than the mass of an object.
6.5: Problem-Solving
Basic problem-solving techniques can aid in the solution of problems involving motion (i.e., the laws of motion).
6.6: Vector Nature of Forces
Forces act in a particular direction and have sizes dependent upon how strong the push or pull is.
6.7: Further Applications of Newton’s Laws
Net force affects the motion, postion and/or shape of objects (some important and commonly used forces are friction, drag and deformation).
6.8: Prelude to Newton's Laws of Motion
6.9: Forces
Dynamics is the study of how forces affect the motion of objects, whereas kinematics simply describes the way objects move. Force is a push or pull that can be defined in terms of various standards, and it is a vector that has both magnitude and direction. External forces are any outside forces that act on a body. A free-body diagram is a drawing of all external forces acting on a body. The SI unit of force is the newton (N).
6.10: Newton's First Law
According to Newton’s first law (the law of inertia), there must be a cause for any change in velocity (a change in either magnitude or direction) to occur. Inertia is related to an object’s mass. If an object’s velocity relative to a given frame is constant, then the frame is inertial and Newton’s first law is valid. A net force of zero means that an object is either at rest or moving with constant velocity; that is, it is not accelerating.
6.11: Newton's Second Law
Newton’s second law of motion says that the net external force on an object with a certain mass is directly proportional to and in the same direction as the acceleration of the object. Newton’s second law can also describe net force as the instantaneous rate of change of momentum. Thus, a net external force causes nonzero acceleration.
6.12: Mass and Weight
Careful distinctions must be made between free fall and weightlessness using the definition of weight as force due to gravity acting on an object of a certain mass. Some upward resistance force from the air acts on all falling objects on Earth, so they can never truly be in free fall.
6.13: Newton’s Third Law
Newton’s third law of motion represents a basic symmetry in nature, with an experienced force equal in magnitude and opposite in direction to an exerted force. Action-reaction pairs include a swimmer pushing off a wall, helicopters creating lift by pushing air down, and an octopus propelling itself forward by ejecting water from its body. Choosing a system is an important analytical step in understanding the physics of a problem and solving it.
6.14: Common Forces
When an object rests on a nonaccelerating horizontal surface, the magnitude of the normal force is equal to the weight of the object. On an inclined plane, the weight of the object can be resolved into components that act perpendicular and parallel to the surface of the plane. When a rope supports the weight of an object at rest, the tension in the rope is equal to the weight of the object. The force developed in a spring obeys Hooke’s law.
6.15: Drawing Free-Body Diagrams
A free-body diagram is a useful means of describing and analyzing all the forces that act on a body to determine equilibrium according to Newton’s first law or acceleration according to Newton’s second law. To draw a free-body diagram, draw the object of interest, draw all forces acting on that object, and resolve all force vectors into x- and y-components.
6.16: Newton's Laws of Motion (Exercises)
6.17: Newton's Laws of Motion (Summary)
6.7: Further Applications of Newton’s Laws - Copy
Net force affects the motion, postion and/or shape of objects (some important and commonly used forces are friction, drag and deformation).

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