A force is a push or a pull. Gravity and electromagnetic forces have unlimited range. All other forces exerted on macroscopic objects involve contact. Force is a vector quantity.
Vectors have magnitude and direction and are added according to special rules. Vectors are added graphically by drawing each vector so that its tail is placed at the tip of the previous vector. The sum is drawn as a vector arrow from the tail of the first vector
to the tip of the last.
To find the components of a vector: draw a right triangle with the vector as the hypotenuse and the other two sides parallel to the x - and y -axes. Then use the trigonometric functions to find the magnitudes of the components. The correct algebraic sign must be determined for each component. The same triangle can be used to find the magnitude and direction of a vector if its components are known.
To add vectors algebraically, add their components to find the components of the sum:
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The SI unit of force is the newton. 1.00 N = 0.2248 lb.
The net force on a system is the vector sum of all the forces acting on it:
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Since all the internal forces form interaction pairs, we need only sum the external forces.
Newton's first law of motion: If zero net force acts on an object, then the object’s velocity does not change. Velocity is a vector whose magnitude is the speed at which the object moves and whose direction is the direction of motion.
- A free-body diagram (FBD) includes vector arrows representing every force acting on the chosen object due to some other object, but no forces acting on other objects.
- Newton’s third law of motion: In an interaction between two objects, each object exerts a force on the other. These two forces are equal in magnitude and opposite in direction.
At the fundamental level, there are four interactions: gravity, the strong and weak interactions, and the electromagnetic interaction. Contact forces are large-scale manifestations of many microscopic electromagnetic interactions.
The magnitude of the gravitational force between two objects is
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where r is the distance between their centers. Each object is pulled toward the other's center.
The weight of an object is the magnitude of the gravitational force acting on it. An object's weight is proportional to its mass: W = mg [Eq. (2-9)], where g is the gravitational field strength. Near Earth's surface, g has an average value of 9.80 N/kg.
The normal force is a contact force perpendicular to the contact surfaces that pushes each object away from the other.
Friction is a contact force parallel to the contact surfaces. In a simplified model, the kinetic frictional force and the maximum static frictional force are proportional to the normal force acting between the same contact surfaces.
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The static frictional force acts in the direction that tends to keep the surfaces from beginning to slide. The direction of the kinetic frictional force is in the direction that would tend to make the sliding stop.
An ideal cord pulls in the direction of the cord with forces of equal magnitude on the objects attached to its ends as long as no external force tangent to the cord is exerted on it anywhere between the ends. The tension of an ideal cord that runs through an ideal pulley is the same on both sides of the pulley.
