Key Concepts: Newton’s Second Law (F = ma)

Newton's Second Law

The net force on an object equals its mass times its acceleration:

ΣF = ma

• Greater force → greater acceleration (for the same mass).
• Greater mass → less acceleration (for the same force).
• Acceleration is always in the same direction as the net force.

Weight vs Mass

• Mass (kg): Amount of matter; does not change with location.
• Weight (N): The gravitational force on an object. W = mg, where g ≈ 9.8 m/s² on Earth.
• A 70 kg person weighs about 686 N on Earth but only ~114 N on the Moon.

Problem-Solving

• Draw a free-body diagram.
• Choose a coordinate system.
• Write ΣF = ma for each axis.
• Solve for the unknown.