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Newton's Second Law explains how an object's motion changes when a net force acts on it. It connects three central ideas in mechanics: force, mass, and acceleration. This law matters because it lets us predict how carts, cars, rockets, athletes, and falling objects speed up, slow down, or change direction. The compact equation Fnet = ma is one of the most useful tools in physics.
Key Facts
- Newton's Second Law: Fnet = ma.
- Acceleration is directly proportional to net force when mass is constant: a = Fnet/m.
- Acceleration is inversely proportional to mass when net force is constant: a decreases as m increases.
- The SI unit of force is the newton: 1 N = 1 kg·m/s^2.
- Net force is the vector sum of all forces acting on an object.
- If Fnet = 0, then a = 0, so the object keeps a constant velocity.
Vocabulary
- Net force
- The net force is the combined total of all forces on an object, including their directions.
- Mass
- Mass is a measure of how much matter an object has and how strongly it resists acceleration.
- Acceleration
- Acceleration is the rate at which an object's velocity changes over time.
- Newton
- A newton is the SI unit of force, equal to the force needed to accelerate 1 kg at 1 m/s^2.
- Free-body diagram
- A free-body diagram is a simplified drawing that shows all forces acting on a single object.
Common Mistakes to Avoid
- Using the applied force instead of the net force. This is wrong because friction, gravity, normal force, and other forces may change the total force that causes acceleration.
- Forgetting that force and acceleration are vectors. This is wrong because direction matters, so forces in opposite directions must be subtracted.
- Thinking a heavier object always has more force on it. This is wrong because force depends on both mass and acceleration, and the same mass can experience different net forces.
- Using grams instead of kilograms in Fnet = ma. This is wrong because the standard SI units are newtons, kilograms, and meters per second squared.
Practice Questions
- 1 A 4.0 kg cart is pushed with a net force of 12 N to the right. What is its acceleration?
- 2 A 10 kg box accelerates at 1.5 m/s^2 to the left. What is the net force on the box, including direction?
- 3 Two carts are pushed with the same net force. Cart A has a mass of 2 kg and Cart B has a mass of 6 kg. Explain which cart has the greater acceleration and why.