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Newton's Laws: Crash Course Physics #5

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Science and Nature

Scientific Concepts, Discoveries, and Nature Phenomena

Newton’s Laws of Motion (1687, Principia)

First Law (Inertia)

  • Objects resist changes in their motion.
    • An object at rest stays at rest.
    • An object in motion stays at constant velocity unless acted on by a net force.

Second Law (Net Force)

  • Net force relates to acceleration:

    • [ F_{\text{net}} = ma ]

  • Key idea: net force means the vector sum of all forces (some forces may cancel or add).

Equilibrium (Related to Forces)

  • Net force = 0
  • An object can still be moving (with constant velocity) as long as acceleration is zero.

Gravitational Force (Weight)

  • Gravitational acceleration near Earth:

    • [ g = 9.81\,\text{m/s}^2 ]

  • Weight (gravitational force):

    • [ F_g = mg ]

  • Units:

    • Weight is measured in newtons (N).
    • Mass is measured in kilograms (kg).

Newton’s Third Law (Action–Reaction)

  • When two objects interact, they exert forces on each other in:
    • equal magnitude
    • opposite direction
  • Normal force
    • Always perpendicular to the surface (for everyday contact surfaces like tables or ramps).
    • Its magnitude adjusts to balance other forces (e.g., weight on a surface).

Why objects can move despite action–reaction

  • Motion depends on:
    • the presence of additional forces
    • and which object is more able to accelerate
  • Example concept: reindeer + sleigh
    • The reindeer can push against the ground.
    • The ground pushes forward more strongly than the sleigh pulls back on the reindeer.

Forces and Problem-Solving Method

Free-Body Diagram Method (explicitly described)

  1. Draw a rough outline of the object.
  2. Place a dot at the center.
  3. Draw arrows for each force acting on the object.
  4. Choose a positive direction (example: up).
  5. Use Newton’s laws to set up equations from the force balance / net force.

Tension Force in Ropes

  • Rope is modeled as:
    • massless
    • unbreakable
  • Therefore, tension is treated as uniform along the rope in the examples.
  • Tension magnitude adjusts to support the forces/weights involved.
  • Example: A suspended box with net force = 0
    • Gravity is balanced by tension.

Elevator/Counterweight Dynamics (Multi-Body System with Algebra)

Setup

  • Lift mass (including rider): 1000 kg
  • Counterweight: 850 kg
  • Controlled by a pulley with tension forces.
  • Goal: determine the downward acceleration of the lift.

Approach

  • Draw separate free-body diagrams for:
    • the lift
    • the counterweight
  • Write two Newton’s second-law equations (each involving tension and acceleration).
  • Use algebra to eliminate tension (treat it as a shared unknown).

Result

  • Acceleration of the lift:

    • [ a \approx 0.795\,\text{m/s}^2 ]

    • downward (as implied by context)

Researchers or Sources Featured (at the End)

  • Isaac Newton — credited with the three laws and Philosophiae Naturalis Principia Mathematica / Principia (1687)
  • PBS Digital Studios
  • Doctor Cheryl C. Kinney
  • Thought Cafe (graphics team)

Original video