External forces are influences that originate from outside an object or a defined system, capable of causing a change in its state. These forces play a fundamental role in the physical world. Understanding external forces helps explain why objects move, stop, or change their shape. They are common in everyday life, from a gentle breeze moving leaves to the powerful impact of a collision.
Distinguishing External and Internal Forces
In physics, a “system” is a collection of objects chosen for analysis; everything outside is its surroundings. External forces act on the system from these outside surroundings. For example, using a car as a system, the force of wind pushing against it is an external force.
Internal forces, in contrast, originate and act within the system, between its components. Using the car example, the forces between its engine parts or the tension within a seatbelt are internal forces. A key distinction is that only net external forces can alter the motion of a system, causing it to accelerate or decelerate. Internal forces, while causing interactions within the system, do not change the system’s overall motion because they cancel each other out due to Newton’s third law.
Common Types of External Forces
Gravitational force is an external force, the attraction between objects with mass. On Earth, this is often seen as the planet’s pull on objects, causing them to fall downwards. For instance, an apple falling from a tree is under the influence of Earth’s gravitational force.
The normal force arises when a surface supports an object, acting perpendicular to that surface. When a book rests on a table, the table exerts an upward normal force on the book, preventing it from falling through. This force adjusts to counteract the weight and any other forces pressing the object into the surface.
Frictional force opposes motion or attempted motion between surfaces in contact. When you push a box across the floor, friction acts in the opposite direction of your push, resisting the movement. This force depends on the texture of the surfaces and how firmly they are pressed together.
An applied force is a direct push or pull exerted on an object by another object or person. Kicking a ball or pushing a door open are examples where an applied force causes a change in motion.
Tension force is a pulling force transmitted along a flexible medium like a rope, cable, or string. When you pull a wagon with a rope, the rope transmits tension from your hand to the wagon. This force always acts along the length of the medium and pulls away from the attached objects.
Air resistance, also known as drag, is a type of frictional force that opposes an object’s motion through the air or other fluids. As a car moves, air molecules collide with its surface, creating a resistive force that slows it down. This force increases with the object’s speed and its surface area facing the fluid.
How External Forces Influence Objects
External forces primarily influence objects in two main ways: by changing their motion and by causing deformation. A net external force acting on an object can cause it to accelerate, meaning its speed or direction changes. If you push a stationary shopping cart, it begins to move, showing a change in motion. The greater the net external force, the greater the acceleration an object will experience.
Beyond changing motion, external forces can also alter an object’s shape or structure. When you squeeze a sponge, the applied external force causes it to deform. Similarly, a strong external impact might bend a metal rod or even break it, depending on the material’s properties and the force’s magnitude.