Force is a fundamental concept in physics that describes an interaction which, when unopposed, changes an object’s motion. It is simply a push or a pull exerted on an object. Its presence is evident through the visible changes it brings about in the world around us.
Force in Action Around Us
Forces are at play in countless everyday situations, often without us consciously recognizing their influence. When you open a door, you apply a pushing or pulling force to its handle, causing it to pivot. Kicking a ball involves a pushing force from your foot, propelling it forward. Pulling a wagon requires a continuous pulling force to maintain its motion.
Gravity is a pervasive force, pulling objects toward the center of the Earth. This is why an apple falls from a tree, rain descends from the clouds, and we remain firmly on the ground. Friction is another common force that opposes motion when two surfaces slide. When a car brakes, friction between the tires and the road slows it down and stops it. Rubbing your hands together generates warmth due to friction, and walking without slipping is possible because of the frictional force between your shoes and the ground.
The normal force is exerted by a surface to support an object resting on it. For instance, a book resting on a table experiences an upward normal force, preventing it from falling through. Similarly, when a person stands on the ground, the ground exerts an upward normal force that counteracts their weight. Air resistance, also known as drag, is a type of friction that acts on objects moving through the air. A skydiver deploys a parachute to increase air resistance, which slows their descent. Wind pushing against a car as it drives down the road is another example of air resistance.
What Force Does to Objects
Forces actively change the state of objects. One primary effect is to initiate movement in a stationary object. A soccer ball, for example, remains at rest until a player applies a kicking force, setting it into motion. Conversely, a force can bring a moving object to a halt; applying the brakes in a car creates a frictional force that stops the vehicle.
Forces are also responsible for altering the path of moving objects. A tennis ball changes its trajectory dramatically after being struck by a racket, demonstrating how a force can redirect an object’s motion. Furthermore, a force can cause an object to either accelerate or decelerate. When you press the gas pedal in a car, an applied force increases its speed, while applying the brakes reduces it.
Beyond influencing motion, forces can also deform objects, causing them to change shape. Squeezing a sponge with your hand applies a compressive force that temporarily alters its form. Crushing an empty aluminum can requires a significant force to permanently reshape it. Stretching a rubber band illustrates how a pulling force can elongate an object, which then returns to its original shape once the force is released.