Overview
This lecture reviews Newton's three laws of motion, explains their meaning, presents key equations, and solves practical problems applying each law.
Newton’s First Law of Motion (Law of Inertia)
- An object at rest stays at rest unless acted on by a net (unbalanced) force.
- An object in motion continues moving at constant velocity unless acted on by a net force.
- Balanced forces (e.g., gravity and normal force) result in zero net force and no acceleration.
- Friction and air resistance are examples of forces that can stop motion; in space, motion continues due to lack of these forces.
- If an object moves at constant velocity, net force and acceleration are zero.
Newton’s Second Law of Motion
- The net force on an object equals mass times acceleration: F = m × a.
- Acceleration occurs if net force is not zero; otherwise, velocity is constant.
- For a constant force, increasing mass decreases acceleration and vice versa.
- Force is also the rate of change of momentum: F = Δp / Δt (where p is momentum).
- Impulse (force × time) equals change in momentum: Impulse = Δp.
Newton’s Third Law of Motion
- For every action, there is an equal and opposite reaction: forces come in pairs with equal magnitude and opposite directions.
- If you exert a force on an object, it exerts an equal and opposite force back on you.
- The object with less mass will experience greater acceleration for the same force.
Example Problems and Applications
- Net force is zero for objects at rest or moving at constant velocity.
- To find acceleration: a = F / m.
- For changing speed, use v_f = v_i + a × t.
- For displacement with constant acceleration: d = v_i × t + ½ a t².
- Applied force equals net force plus friction if present.
- When two bodies interact (e.g., astronaut and package), their accelerations are inversely proportional to their masses.
Key Terms & Definitions
- Net Force (F) — The total (vector sum) of all forces acting on an object.
- Inertia — The tendency of objects to resist changes to their state of motion.
- Acceleration (a) — The rate of change of velocity.
- Momentum (p) — Product of mass and velocity; p = m × v.
- Impulse — Change in momentum, equal to force times the interval during which it acts.
- Normal Force — The perpendicular force exerted by a surface to support the weight of an object.
Action Items / Next Steps
- Review key formulas: F = m × a, F = Δp / Δt, d = v_i × t + ½ a t².
- Practice solving problems involving the three laws.
- Memorize definitions and relationships between mass, force, acceleration, and momentum.