Forces are fundamental to understanding how objects interact and behave. A force is a push or a pull exerted on an object. Force is a vector quantity, meaning it has both a magnitude (strength) and a direction. This dual characteristic allows forces to produce a wide range of outcomes.
How Forces Influence Motion
Forces are responsible for every change in an object’s state of motion, whether starting, stopping, speeding up, slowing down, or altering its path. An object’s motion will not change unless an external force acts upon it.
Consider kicking a soccer ball; the force from your foot causes the ball to accelerate from a standstill. The greater the force applied, the faster the ball will move. If another player applies a force to the moving ball, they can change its speed or direction, making it swerve or stop.
When a force is applied in the same direction as an object’s motion, it causes the object to speed up. Pushing a child on a swing provides an additional force that increases their speed. Conversely, applying a force in the opposite direction of motion causes an object to slow down or stop. This is evident when a car applies its brakes, where the friction force between the tires and the road acts against the car’s movement, bringing it to a halt.
Forces can also cause an object to change its direction of motion, even if its speed remains constant. For instance, a baseball bat striking a pitched ball dramatically alters its trajectory. Even without direct contact, forces like gravity continuously influence object motion, such as a ball thrown into the air that eventually falls back down.
How Forces Change Object Shape
Beyond influencing motion, forces also have the capacity to alter the physical form or shape of objects. This change can be temporary, where the object returns to its original shape once the force is removed, or permanent, leading to deformation or even breakage. The extent of the shape change depends on the material properties of the object and the magnitude of the applied force.
When you squeeze a soft sponge, the applied force causes it to compress and change shape, but it springs back when released. This demonstrates a temporary change. Similarly, stretching a rubber band elongates it, and it reverts to its initial length once the stretching force is no longer applied.
Permanent changes in shape occur when the applied force exceeds the material’s ability to recover. Bending a metal wire, for example, requires enough force to permanently deform its structure. Crushing an empty aluminum soda can with your hand results in a lasting alteration of its cylindrical form into a flattened, crumpled shape.
Forces can also cause objects to break or fracture if the stress surpasses their structural integrity. Kneading dough, for example, involves applying forces that continuously reshape it for baking.
Types of Forces in Daily Life
One familiar type is gravity, the force that pulls objects towards each other, notably drawing everything on Earth towards its center. This is why an apple falls from a tree or why you remain grounded.
Friction is another common force, opposing motion between surfaces in contact. It allows us to walk without slipping and enables a car to slow down when its brakes are applied.
An applied force refers to a direct push or pull on an object by a person or another object. Pushing a shopping cart or pulling a door open are both instances of applied forces.
When an object rests on a surface, the surface exerts an upward push known as the normal force. This force acts perpendicular to the surface and prevents the object from falling through it. For example, a book lying on a table experiences an upward normal force from the table.
Lastly, tension is a pulling force transmitted through a string, rope, cable, or similar object when it is pulled taut. When you pull on a rope in a tug-of-war, the force transmitted along the rope is tension. It is the force that keeps a suspended object from falling.