A force is an interaction between two objects that manifests as a push or a pull. Force is a vector quantity, meaning its effect depends on both the strength (magnitude) and the direction in which it is applied. When multiple forces act on an object, they combine into a single total influence called the net force. An object’s future state of motion is determined entirely by this net force.
Initiating and Altering Movement
The most direct effect of a net force is a change in an object’s state of motion, which includes starting movement, speeding up, or slowing down. An object at rest or moving at a steady speed in a straight line will continue in that state unless an unbalanced force acts upon it. This resistance to a change in motion is known as inertia, and mass is a measure of this inertial property.
When an unbalanced force is applied, it causes the object to accelerate, defined as any change in its velocity. If you push a stationary box, the net force overcoming friction causes it to start moving. Conversely, pushing a moving object opposite its current motion causes deceleration, resulting in the object slowing down.
The amount of acceleration produced is directly proportional to the size of the net force applied. The resulting acceleration is also inversely proportional to the object’s mass. This means a given force applied to a light baseball will cause a much greater speed increase than the same force applied to a heavy bowling ball.
Because force and acceleration are vectors, the change in speed always occurs in the direction of the net force. If a car is moving forward and the braking force is applied, the net force acts backward, causing the forward speed to decrease.
Shifting an Object’s Trajectory
A force can change an object’s direction of travel, even if its speed remains constant. This change in direction is still considered acceleration because velocity includes both speed and direction. For an object to follow a curved path, a net force must constantly be directed inward toward the center of that curve.
This center-seeking force necessary for circular motion is called centripetal force. When a car turns a corner, friction between the tires and the road provides this force. Gravitational pull acts as the centripetal force for an orbiting satellite, keeping it from flying off in a straight line.
A force applied off-center can cause an object to spin, a rotational effect known as torque. Torque is the rotational analogue of linear force and depends on the force’s strength and the perpendicular distance to the axis of rotation, called the lever arm. Applying a force far from the hinges of a door generates greater torque, making the door easier to open.
Changing Physical Form
The application of a force can lead to an alteration of the object’s physical structure, known as deformation, instead of movement. This internal effect occurs when the force causes internal resistance, or stress, within the material. Forces are categorized by the type of deformation they induce: tension, compression, and shear.
Tension involves forces pulling a material apart, such as the force on a rope in a tug-of-war. Compression involves forces pushing a material together, like the weight of a building on its foundation columns. Shear occurs when forces act parallel to the surface in opposite directions, causing one part of the material to slide past another.
Deformations can be temporary or permanent, depending on the material and the force magnitude. Elastic deformation is temporary; the material returns to its original shape once the force is removed, such as stretching a rubber band. If the force exceeds the material’s elastic limit, the object undergoes plastic deformation, a permanent change where internal bonds are rearranged.