When you drive across a bridge, you expect it to remain stable. You also expect to speed up or slow your car in response to traffic changes. In both cases, you deal with forces. The forces on the bridge are in equilibrium, so it stays in place. In contrast, the force produced by your car engine causes a change in motion. Isaac Newton discovered the laws of motion that describe these situations. Forces affect every moment of your life. Your body is held to Earth by force and held together by the forces of charged particles. When you open a door, walk down a street, lift your fork, or touch a baby’s face, you are applying forces. Zooming in deeper, your body’s atoms are held together by electrical forces, and the core of the atom, called the nucleus, is held together by the strongest force we know—strong nuclear force.
- 4.2: 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).
- 4.3: 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.
- 4.4: 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.
- 4.5: 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.
- 4.6: 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.
- 4.7: 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.
- 4.8: 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.
Thumbnail:The Golden Gate Bridge, one of the greatest works of modern engineering, was the longest suspension bridge in the world in the year it opened, 1937. It is still among the 10 longest suspension bridges as of this writing. In designing and building a bridge, what physics must we consider? What forces act on the bridge? What forces keep the bridge from falling? How do the towers, cables, and ground interact to maintain stability?
Contributors and Attributions
Samuel J. Ling (Truman State University), Jeff Sanny (Loyola Marymount University), and Bill Moebs with many contributing authors. This work is licensed by OpenStax University Physics under a Creative Commons Attribution License (by 4.0).