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Forces & Newton's Laws

Understanding forces, friction, and Newton's three laws of motion.

Newton's Three Laws

An object at rest stays at rest, and an object in motion stays in motion, unless acted upon by a net external force.

ΣF = 0 ⟹ a = 0

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

F = ma → a = F/m

For every action there is an equal and opposite reaction.

F₁₂ = -F₂₁

Friction is a force that opposes relative motion between surfaces in contact.

Prevents motion. Maximum value before sliding begins.

f_s ≤ μ_s · N

Acts while object is sliding. Constant value.

f_k = μ_k · N

Net ForceF_net = ma

WeightW = mg

Normal Force (horizontal)N = mg

Normal Force (inclined)N = mg·cos θ

Frictionf = μN

Incline accelerationa = g(sin θ − μcos θ)

Solve randomized force and friction problems.