When observing an object’s movement, any change to its state of motion—whether starting, stopping, speeding up, slowing down, or turning—depends entirely on the forces acting upon it. Understanding this condition explains the physics behind all movement in the universe, from the trajectory of a baseball to the orbit of a planet.
Defining Balanced and Unbalanced Forces
A force in physics is defined as an interaction, a push or a pull, that can change an object’s state of motion. When multiple forces act on an object simultaneously, their combined effect is the net force.
If all the forces acting on an object cancel each other out, the resulting net force is zero, and the forces are considered balanced. An object subjected to balanced forces will not change its movement; it remains at rest or continues moving at a constant speed in a straight line. For example, a book resting on a table experiences a downward pull from gravity and an equal upward push from the table, resulting in a net force of zero.
The condition required to cause motion is the presence of an unbalanced force, meaning the combined net force is greater than zero. This occurs when opposing forces are unequal in magnitude, such as in a game of tug-of-war where one team pulls harder than the other.
How Unbalanced Forces Cause Acceleration
The direct consequence of an unbalanced force acting on an object is acceleration, which is a change in velocity. Since velocity describes both the speed and the direction of an object, acceleration can mean speeding up, slowing down, or changing the path of movement. The object will always accelerate in the direction of the greater, unbalanced force.
The relationship between the unbalanced force, the object’s mass, and the resulting acceleration is precise. A larger unbalanced force produces a proportionately larger acceleration. Conversely, if the same force is applied to two different objects, the one with greater mass will experience a smaller acceleration.
This demonstrates that an object’s mass directly resists the change in motion caused by the applied force. For instance, a small push might cause a light shopping cart to accelerate quickly, but applying that same push to a heavy, fully loaded cart results in a much smaller increase in speed.
The Role of Inertia in Motion
The necessity of an unbalanced force to cause motion is rooted in inertia, the property of matter that resists changes in its state of motion. This inherent resistance applies whether the object is at rest or already moving at a constant velocity.
An object resting on the ground will remain there because its inertia resists any force attempting to set it in motion. To overcome this resistance, an applied force must be strong enough to create an unbalanced condition against opposing forces, such as static friction.
Similarly, a moving object will continue its path unless an unbalanced force, like friction or air resistance, acts to slow it down or change its direction. The measure of an object’s inertia is directly related to its mass; objects with more mass have greater inertia. This explains why a large truck is much harder to start moving and stop compared to a bicycle.
Everyday Instances of Unbalanced Forces
Unbalanced forces are responsible for nearly every dynamic action observed in the physical world. When a person kicks a stationary soccer ball, the force from the foot overcomes the ball’s inertia and friction, causing it to accelerate forward.
The process of a car braking also involves an unbalanced force, where the friction between the tires and the road creates a net force opposite to the car’s motion, causing deceleration.
The force of gravity provides many examples of unbalanced motion, such as an apple falling from a tree. While the apple hangs on the branch, the forces are balanced. Once it detaches, the downward pull of gravity becomes the unbalanced force, causing the apple to accelerate toward the ground. Even when a rocket launches, the engine thrust must be an unbalanced force, exceeding the pull of gravity and air resistance to propel the vehicle upward.