ABLENewton's Laws of Motion
Newton's First Law (Inertia)
An object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless acted upon by a resultant force. This is also known as the law of inertia. Inertia is the tendency of an object to resist changes in its motion.
Newton's Second Law (F = ma)
The resultant force on an object is equal to its mass multiplied by its acceleration. The greater the force, the greater the acceleration. The greater the mass, the smaller the acceleration for the same force.
Newton's Third Law (Action-Reaction)
For every action, there is an equal and opposite reaction. The two forces act on different objects, are the same type of force, and are equal in magnitude but opposite in direction.
Resultant Forces
When multiple forces act on an object, they can be combined to find the resultant (net) force. If forces are balanced (resultant = 0), the object is in equilibrium — it remains stationary or moves at constant velocity.
Key Points
- 1st Law: No resultant force → no change in motion
- 2nd Law: F = ma — force causes acceleration
- 3rd Law: Action-reaction pairs act on different objects
- Balanced forces → constant velocity or stationary
- Unbalanced forces → acceleration
Exam Tips
- For Newton's 3rd Law pairs, always name the two objects and the type of force
- Remember: F = ma requires the resultant force, not just any single force
- Draw free-body diagrams to identify all forces before solving