Mass is a fundamental property of matter, representing the amount of matter an object contains. It also measures an object’s resistance to changes in its state of motion. Velocity, in contrast, describes an object’s speed in a specific direction. For instance, “5 meters per second east” is a velocity, combining speed and direction. A common question is whether an object’s mass influences its velocity.
Mass and Falling Objects
Heavier objects do not fall faster than lighter ones in a vacuum. Galileo Galilei demonstrated that, in the absence of air resistance, all objects fall at the same rate, regardless of their mass. This occurs because the acceleration due to gravity is constant for all objects near Earth’s surface, approximately 9.8 m/s². Gravitational force is proportional to mass, but a more massive object also has greater resistance to acceleration. These effects counteract each other, resulting in the same acceleration for all objects in a vacuum.
In everyday situations, air resistance influences how objects fall. Air resistance, a form of drag, opposes an object’s motion through the air. This force depends on factors like an object’s shape, surface area, and speed. Objects with a larger surface area relative to their mass, like a feather, experience greater air resistance, falling more slowly than denser objects. Thus, while mass does not affect inherent acceleration due to gravity, air resistance causes variations in observed falling speeds in an atmosphere.
Mass and Changes in Motion
While mass does not dictate how fast an object falls in a vacuum, it directly affects how easily an object’s motion can be changed. This resistance to changes in motion is known as inertia. An object with greater mass possesses more inertia, meaning it is more difficult to start moving, stop, or change its direction. This principle is described by Newton’s Second Law of Motion.
Newton’s Second Law states that the acceleration of an object is directly proportional to the net force applied to it and inversely proportional to its mass. Simply put, achieving the same acceleration requires a greater force for a larger mass. For example, pushing an empty shopping cart is easier than a loaded one; the full cart requires a stronger push for the same speed change. Similarly, stopping a heavy vehicle moving at a certain speed requires more braking force than a lighter one at the same speed. This relationship highlights mass as a measure of an object’s resistance to changes in its velocity.
Mass and Momentum
Momentum combines an object’s mass and velocity. It is calculated as the product of an object’s mass and its velocity (p = mv). Momentum measures the “quantity of motion” an object possesses. Since velocity is a vector quantity with both magnitude and direction, momentum is also a vector, sharing the same direction as the object’s velocity.
An object’s momentum increases with either greater mass or greater velocity. For instance, a large, slow-moving object like a heavy truck can have significant momentum. Conversely, a small, fast-moving object like a bullet also possesses considerable momentum due to its high velocity. Momentum is relevant in impacts and collisions. In such events, the total momentum of a closed system remains constant. This means momentum can be transferred between objects, influencing their resulting velocities after interaction.