Newton's Second Law — F = ma
Overview
The net force on an object equals its mass times its acceleration: F_net = ma. This is a vector equation — direction of acceleration matches direction of net force. Steps: (1) Draw free-body diagram (FBD). (2) Resolve all forces. (3) Find F_net. (4) Apply F_net = ma to find the unknown.
F = ma — acceleration proportional to net force
Newton's 2nd Law: the net force on an object equals the product of its mass and acceleration: F_net = ma. The direction of acceleration is always the same as the direction of net force. Key insight: the larger the mass for the same force, the smaller the acceleration. The larger the force for the same mass, the greater the acceleration. The 'F' in F = ma is ALWAYS the NET force (sum of all forces), not just one force.
Applying Newton's 2nd Law step by step
Problem-solving strategy: (1) Draw a free-body diagram showing ALL forces on the object. (2) Choose a positive direction. (3) Write ΣF = ma where ΣF is the algebraic sum of all force components in your chosen direction. (4) Solve for the unknown. If the object accelerates upward, the net force is upward; if downward, net force is downward. Don't forget to include ALL forces — weight, normal, tension, friction.
- ⚠Using a single force instead of the NET force in F = ma
- ⚠Not drawing an FBD first — missing forces is the most common error
- ⚠Forgetting the direction of net force determines the direction of acceleration