Overview
This lecture introduces basic physics concepts including motion (distance, displacement, speed, velocity, acceleration), projectile motion, and Newton’s three laws of motion. Key definitions, formulas, and example applications are covered.
Distance vs. Displacement
- Distance is how far an object travels; displacement is the change in position with direction considered.
- Distance is a scalar (magnitude only); displacement is a vector (magnitude and direction).
- Displacement = final position − initial position; can be positive or negative.
- Distance is always positive.
Scalars and Vectors
- Scalar quantities have magnitude only (e.g., distance, speed).
- Vector quantities have magnitude and direction (e.g., displacement, velocity).
Speed and Velocity
- Speed = distance traveled per unit time; always positive, scalar.
- Velocity = displacement per unit time; can be positive or negative, vector.
- Formula: average speed = total distance / total time.
- Formula: average velocity = total displacement / total time.
Calculating Speed and Velocity
- Example: Object moves 12 m east, 20 m west in 4 s. Total distance = 32 m; average speed = 8 m/s.
- Net displacement = −8 m (since 12 east + (−20) west), average velocity = −2 m/s.
Acceleration
- Acceleration = change in velocity / change in time.
- Positive acceleration: object speeds up; negative acceleration: object slows down.
- Formula: a = (v_final − v_initial) / t.
- If acceleration and velocity have same sign, object speeds up; opposite signs, it slows down.
Gravitational Acceleration
- On Earth, gravitational acceleration g = −9.8 m/s² (downward).
- Gravitational acceleration only affects vertical (y) component, not horizontal (x) component.
- Projectile’s vertical velocity changes by −9.8 m/s each second; horizontal velocity remains constant if no other force acts.
Projectile Motion
- Projectile motion involves both x (horizontal) and y (vertical) directions.
- x-velocity is constant; y-velocity changes due to gravity.
- At the top of trajectory, vertical velocity (v_y) = 0; horizontal velocity (v_x) remains constant.
- v_x = v·cos(θ), v_y = v·sin(θ) for projectiles launched at an angle.
Newton’s Three Laws of Motion
- First Law: Objects at rest stay at rest, and objects in motion stay in motion unless acted on by a net force.
- Second Law: Net force = mass × acceleration (F = ma).
- Third Law: (Not covered yet in detail in this part.)
Key Terms & Definitions
- Distance — Total path length traveled by an object (scalar).
- Displacement — Change in position with direction (vector).
- Scalar — Quantity with only magnitude.
- Vector — Quantity with magnitude and direction.
- Speed — Rate of distance traveled (scalar).
- Velocity — Rate of change of displacement (vector).
- Acceleration — Rate of change of velocity.
- Projectile — Object moving under only gravity’s influence.
- Force — A push or pull on an object.
- Tension — Force transmitted through a rope or string.
- Friction — Force opposing motion between surfaces.
Action Items / Next Steps
- Practice problems: Calculate displacement, speed, velocity, and acceleration for sample motions.
- Read about Newton’s third law for next session.