A mechanical force is a push or a pull that occurs when two objects physically interact. This direct contact distinguishes mechanical forces from others, like gravity, which can act over a distance. Pushing a grocery cart or pulling open a heavy door are examples where your body makes direct contact with an object to set it in motion. These actions illustrate a mechanical force causing a change in an object’s state of movement.
Common Types of Contact Forces
An applied force is a general term for a force exerted on an object by a person or another object. When you push a box across the floor or throw a ball, you are demonstrating an applied force. This force provides the initial push or pull needed to overcome the object’s stillness and initiate a change in motion.
An object resting on a surface, such as a book on a table, experiences a normal force. The table exerts an upward support force on the book, perpendicular to the surface, that counteracts the downward pull of gravity. This is why the book remains stationary and doesn’t fall through the table. The term “normal” in this context refers to the mathematical term for being perpendicular, highlighting the direction of the force.
As an object begins to move across a surface, a frictional force comes into play, opposing the motion. This force arises from the microscopic imperfections on the two surfaces that are in contact. The brakes on a car, for example, work by creating a large frictional force between the brake pads and the wheels, converting the car’s motion into heat and slowing it down.
Tension is a force transmitted through a flexible connector like a rope, cable, or string when it is pulled taut. In a game of tug-of-war, the force each team exerts is transmitted along the rope as tension. This force pulls equally on the objects at either end of the connector. This principle allows cranes to lift heavy objects and enables suspension bridges to support their own weight.
Objects moving through the air encounter a type of friction known as air resistance, which opposes the object’s motion through the atmosphere. A parachute dramatically increases air resistance, creating a large upward force that slows a person’s descent to a safe speed. A piece of paper falling to the ground is also slowed by the push of air particles against its surface.
Force and Its Effect on Motion
The relationship between force and motion is explained by Newton’s Three Laws of Motion, which are foundational to classical mechanics. The first law, the law of inertia, states an object remains at rest or moves at a constant velocity unless an unbalanced external force acts on it. An object’s tendency to resist changes to its motion is inertia. A soccer ball on a field will not move until kicked, and it would travel forever if not for forces like friction and air resistance.
Newton’s second law provides a quantitative description of how forces cause objects to accelerate. It states that the acceleration of an object is directly proportional to the net force applied and inversely proportional to its mass, summarized by the equation F=ma. This means that applying a greater force to an object results in greater acceleration. A more massive object requires more force to achieve the same acceleration, which is why it is easier to get a bicycle moving than a car.
The third law of motion describes a symmetry in forces: for every action, there is an equal and opposite reaction. This means when one object exerts a force on a second object, the second object simultaneously exerts a force of equal magnitude and opposite direction back on the first. A rocket propels itself forward by expelling hot gas backward. The force pushing the gas out creates an equal and opposite force that pushes the rocket up.
Quantifying and Representing Forces
The standard unit of force in the International System of Units (SI) is the Newton (N). One Newton is defined as the amount of force required to accelerate a 1-kilogram mass at a rate of 1 meter per second squared. To put it in relatable terms, the force of Earth’s gravity on a small apple is approximately one Newton.
Force is a vector quantity, meaning it possesses both magnitude (its size or strength) and direction. Vectors are often visualized as arrows where the length represents the magnitude of the force. The direction the arrow points indicates the direction in which the force is being applied. This representation helps in calculating the net force when multiple forces act on an object.
Mechanical Forces in Structures and Materials
Mechanical forces are also at play within stationary objects and structures. When an external force acts on a material, internal forces are generated between its particles; this internal force per unit area is known as stress. For instance, the weight of a bridge’s deck and the traffic on it creates compressive stress in the supporting pillars, squeezing them together.
In response to stress, a material will deform, and this deformation is called strain. Strain is a measure of the relative change in a material’s shape or size. For example, the steel cables holding up a suspension bridge are under immense tension, which causes them to stretch slightly. This elongation, relative to their original length, is the strain on the cables.
The relationship between stress and strain is a property engineers study to ensure the safety and stability of structures. For a material to be suitable for construction, it must withstand expected stresses without excessive strain or breaking. Understanding these internal mechanical forces allows for the design of everything from skyscrapers to electronic components, ensuring they can endure the physical loads they will encounter.