Understanding these pushes and pulls on objects helps explain why things move, or why they stay still. Often, multiple forces act on an object simultaneously, making the concept of an overall, or “net,” force important for predicting how an object will behave. This overall force determines an object’s motion.
What Forces Are
In scientific terms, a force is a push or a pull that can cause a change in an object’s motion. Forces arise from the interaction between two or more objects. For example, when you push a door open, you are exerting a force on the door, and the door interacts with your hand.
Forces possess both magnitude, which refers to their strength or size, and direction. Because of this dual characteristic, forces are considered vector quantities. The standard unit for measuring force in physics is the Newton (N). One Newton is the amount of force needed to accelerate a one-kilogram mass by one meter per second squared.
Gravity, for instance, is a force that pulls objects toward the center of the Earth, like an apple falling from a tree. Friction is another force that opposes motion, such as the force slowing down a sliding box. When you push a shopping cart, you apply a direct force to it.
Combining Forces
When multiple individual forces act on an object, their combined effect is known as the net force. Forces are combined based on their direction. Forces acting in the same direction add together, resulting in a larger net force. For example, if two people push a box in the same direction, their individual pushes combine to create a stronger overall push on the box.
Forces acting in opposite directions, however, subtract from each other. If two people push a box from opposite sides, the net force is the difference between their pushes. The box will move in the direction of the stronger push. If the opposing forces are equal in magnitude, they cancel each other out, resulting in a net force of zero.
When the net force on an object is zero, the forces are considered balanced. An object subjected to balanced forces will either remain at rest or continue moving at a constant velocity, meaning its speed and direction do not change. For example, a book resting on a table experiences a downward gravitational force balanced by an equal upward force from the table.
Conversely, if the net force on an object is not zero, the forces are unbalanced. Unbalanced forces cause an object to change its state of motion. This change can involve starting to move, speeding up, slowing down, or changing direction.
How Net Force Changes Motion
An unbalanced net force directly causes an object to accelerate, which means it changes its velocity. This change in velocity can manifest as an increase in speed, a decrease in speed (deceleration), or a change in the direction of movement. The acceleration always occurs in the same direction as the net force. For instance, if the net force on a soccer ball is forward, the ball will speed up in that direction.
The magnitude of this acceleration depends on two factors: the strength of the net force and the mass of the object. A larger net force produces a greater acceleration, assuming the object’s mass remains constant. This means pushing an empty shopping cart with a certain force will make it accelerate more than pushing a fully loaded cart with the same force.
Conversely, the more massive an object is, the less it will accelerate for a given net force. This relationship is described by Newton’s Second Law of Motion, which states that acceleration is directly proportional to the net force and inversely proportional to the object’s mass. This explains why it takes considerably more force to accelerate a heavy truck compared to a small car.
This principle, known as Newton’s First Law of Motion, highlights that a net force is necessary to alter an object’s existing state of motion. It states that an object at rest will remain at rest, and an object already in motion will continue to move at a constant speed in a straight line, unless acted upon by a net force.