Overview
This lecture introduces kinematics in AP Physics 1, focusing on five motion variables, their graphical interpretation, and solving for unknowns—including projectile motion.
Scalar and Vector Quantities
- Scalars have only magnitude (e.g., time, speed, distance).
- Vectors have both magnitude and direction (e.g., velocity, displacement, acceleration).
- Of the five motion variables, only time is a scalar; the other four are vectors.
The Five Motion Variables
- Initial velocity (v₀), final velocity (v), displacement (Δx), acceleration (a), and time (t) are the main kinematic variables.
- Displacement is distance in a specific direction.
- Velocity is displacement divided by time.
- Acceleration is change in velocity divided by time.
Graphs in Kinematics
- Position-time graphs: Position on the vertical axis, time on the horizontal.
- Slope of a position-time graph gives velocity.
- Horizontal line = object at rest; straight line = constant velocity; curved line = acceleration.
- To find slope/velocity at a point on a curve, use change in y over change in x.
- Velocity-time graphs: Slope gives acceleration; area under the curve gives displacement.
- On AP Physics 1, velocity-time graphs are not curved.
Kinematic Equations
- Five main kinematic equations relate the motion variables.
- Each equation links four variables and omits the fifth.
- The acceleration formula: a = (v - v₀) / t.
- If you know any three variables, you can find the remaining two using the equations.
Projectile Motion
- Applies to objects in freefall with known downward acceleration (9.8 m/s² on Earth).
- For projectiles launched at an angle, split velocity into x (horizontal) and y (vertical) components using trigonometry: vx = v·cos(θ), vy = v·sin(θ).
- Time in air depends on vertical velocity; range depends on horizontal velocity and time.
- Solving projectile motion problems tests understanding of all kinematic concepts.
Key Terms & Definitions
- Kinematics — Study of motion without regard to its causes.
- Scalar — A quantity with magnitude only.
- Vector — A quantity with magnitude and direction.
- Displacement — Change in position with direction.
- Velocity — Displacement per unit time.
- Acceleration — Change in velocity per unit time.
- Projectile motion — Motion of an object in freefall, influenced only by gravity.
Action Items / Next Steps
- Review and memorize the five kinematic equations.
- Practice analyzing and interpreting position-time and velocity-time graphs.
- Solve example problems on projectile motion, including two-dimensional cases.