In the absence of external forces acting horizontally, the horizontal motion of an object remains constant.
Understanding Horizontal Motion Without Forces
When an object moves horizontally and there are no external forces influencing its motion in that direction (such as friction or air resistance), its state of motion will not change. This principle is a fundamental concept in physics, often described by Newton's First Law of Motion, which states that an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced external force.
Based on the provided reference:
"Horizontal motion of an object has no external forces acting upon it (with the exception of air resistance but this is generally not accounted for). Due to this absence of horizontal forces, a projectile remains in motion with a constant horizontal velocity, covering equal distances over equal periods in time."
This means that if an object is already moving horizontally, and no horizontal force is present, it will continue to move at the exact same speed and in the exact same direction horizontally. Its horizontal velocity does not increase, decrease, or change direction.
Key Implications
The absence of horizontal forces leads to several key characteristics of the object's horizontal motion:
- Constant Horizontal Velocity: The speed and direction of the object's motion along the horizontal axis do not change.
- Zero Horizontal Acceleration: Since velocity is constant, there is no acceleration in the horizontal direction. Acceleration is the rate of change of velocity.
- Equal Distances in Equal Times: Because the velocity is constant, the object will cover the same amount of horizontal distance in every equal time interval.
Consider an object sliding across a perfectly smooth, frictionless surface. If you give it an initial push, it will move horizontally. Once your hand is no longer pushing it, and assuming no air resistance, there are no horizontal forces acting on it. The object will then continue to slide forever at the constant velocity it had the moment your hand stopped pushing.
This contrasts with motion influenced by forces, like vertical motion under gravity, where velocity changes over time due to the constant downward force of gravity.
