An object changes its motion due to the application of external forces. According to Newton's first law of motion (also known as the law of inertia), an object at rest tends to stay at rest, and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an external force.
When an external force is applied to an object, it can cause a change in its motion in different ways:
Acceleration: If the applied force is unbalanced, meaning there is a net force acting on the object, it will cause the object to accelerate. Acceleration is the rate of change of velocity, which includes changes in speed, direction, or both. The object will experience a change in its velocity over time due to the applied force.
Deceleration or Retardation: If the applied force opposes the object's existing motion, it can cause the object to decelerate or slow down. Deceleration is simply negative acceleration, and it results in a reduction of the object's velocity.
Change in Direction: If the applied force acts perpendicular to the object's current motion, it can cause a change in its direction without altering its speed. This change in direction is a result of the force causing the object to deviate from its original path and follow a new trajectory.
It's important to note that changes in motion are governed by Newton's second law of motion, which states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. The relationship is expressed as F = ma, where F is the net force, m is the mass of the object, and a is the acceleration produced.
In summary, external forces are responsible for causing changes in an object's motion by either accelerating, decelerating, or altering its direction.