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Newton’s 1st Law

Newton’s 2nd Law

Balanced vs Unbalanced Forces

Newton’s 3rd Law

You may have come across velocity before in other math or science courses, but just as a quick refresher, velocity is a vector quantity which represents the rate of change of an object's position with respect to time.  Acceleration is the rate of change of velocity or how much the velocity of an object changes every second.  This means that if something is speeding up, slowing down, or changing direction, it is accelerating.  One of the most important things to understand in physics is that acceleration does not occur unless there are unbalanced forces acting on an object.  This makes acceleration the bridge between kinematics and forces, and once it is in play, either type of problem is fair game.


Newton's first law of motion, often referred to as the law of inertia, states that an object will remain at rest, or continue to move at a constant velocity unless acted upon by an external force. Inertia is the tendency of objects to resist changes in their state of motion.  If an object is stationary, it will remain stationary unless a force is applied to it.  Similarly, if an object is already in motion with a constant velocity, it will continue to move in the same direction and at the same speed unless an external force intervenes.  In other words, changes in an object's state of motion require the application of force.  Knowing the rate at which something is accelerating, or the forces applied to it helps predict the behavior of its motion.


Newton's second law of motion states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. Mathematically, this law is expressed as F = ma, where F represents the net force applied to the object, m is the mass of the object, and a is the acceleration produced. Essentially, this law quantifies the relationship between force, mass, and acceleration, indicating that the greater the force applied to an object, the greater its acceleration will be. Additionally, if the mass of the object increases, its acceleration decreases for the same amount of force applied. Conversely, if the force acting on an object increases while keeping its mass constant, its acceleration will increase proportionally.


The term “net” comes up a lot when discussing Newton’s 2nd Law and it refers to all of the forces that act on an object on a particular axis.  For example, if you are sitting in your chair studying, the force of gravity is pulling you down on the y-axis, and the normal force is pushing you up, also along the y-axis.  The net force on the y-axis is the sum of these two forces.  If there are any perpendicular forces acting on you, they would make up a separate net force acting on the x-axis.

When the net forces acting on an object are equal and opposite, the net force on the object is zero.  This results in a state of equilibrium where the object either remains at rest, or moves with a constant velocity.  For example, a book resting on a table experiences balanced forces; the force of gravity pulling it downward is balanced by the normal force exerted by the table, resulting in no net force and thus no acceleration.

Conversely, when the forces acting on an object are unbalanced, the net force is nonzero, causing an object to accelerate.  It will either speed up, slow down, or change it’s direction.  For instance, when a car accelerates, the force exerted by the engine is greater than the frictional and air resistance forces acting against it, resulting in a net forward force and acceleration in the direction of motion.

Overall, whether an object remains at rest, moves with constant velocity, or accelerates depends on the balance or imbalance of the forces acting upon it.


Newton's third law of motion states that for every action, there is an equal and opposite reaction. Although it often seems counter-intuitive, this law says that when one object exerts a force on a second object, the second object simultaneously exerts a force of equal magnitude but in the opposite direction on the first object. In other words, forces always occur in pairs, and they act in opposite directions along the same line. This law holds true for interactions between any two objects, regardless of their masses or the nature of the forces involved. For instance, if a motorcycle and a semi-truck collide head on, the motorcycle exerts the same force on the semi-truck that the truck does on the motorcycle. 

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