Mechanical Concepts · 3 min read
Newton's Three Laws of Mechanics
First Law: Law of Inertia
An object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless acted upon by an unbalanced external force.
Inertia is the tendency of an object to resist changes in its state of motion. The more mass an object has, the greater its inertia.
Practical implications: Heavy equipment requires more force to start moving and more braking force to stop. Unsecured loads continue moving when a vehicle stops suddenly. Tools left on elevated surfaces remain stationary until disturbed.
Exam applications: Questions may ask why objects slide forward when a vehicle brakes, why heavy objects are harder to push, or why seat belts are necessary.
Second Law: Force and Acceleration
The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.
Force = Mass × Acceleration (F = ma)
Units: Force is measured in newtons (N) in the metric system and pounds-force (lbf) in the imperial system. One newton is the force needed to accelerate one kilogram at one meter per second squared.
Key relationships:
Greater force produces greater acceleration (for constant mass)
Greater mass produces less acceleration (for constant force)
Doubling the force doubles the acceleration
Doubling the mass halves the acceleration
Practical applications: Determining engine power needed for equipment, calculating braking distances, sizing motors for conveyors, and understanding load-handling capacity.
Third Law: Action and Reaction
For every action, there is an equal and opposite reaction. When object A exerts a force on object B, object B exerts an equal force in the opposite direction on object A.
Important clarification: The action and reaction forces act on different objects. They do not cancel each other because they are applied to separate bodies.
Examples:
When you push against a wall, the wall pushes back against you with equal force
A rocket expels gas downward; the gas pushes the rocket upward
When walking, you push the ground backward; the ground pushes you forward
A hammer strikes a nail; the nail exerts an equal force back on the hammer
Exam applications: Questions may present scenarios involving collisions, propulsion, or force pairs and ask you to identify the reaction force or explain why objects move.
Balanced and Unbalanced Forces
When all forces acting on an object sum to zero, the forces are balanced, and the object remains at rest or continues at constant velocity. When forces do not sum to zero, they are unbalanced, and the object accelerates in the direction of the net force.
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