Overview
This lecture covers the key concepts of motion in physics, including speed, velocity, acceleration, relevant graphs, and the effects of gravity and resistance on falling objects.
Speed and Velocity
- Speed is the distance traveled per unit time and is a scalar quantity (no direction).
- Velocity is speed in a given direction and is a vector quantity (has direction).
- Both speed and velocity use the formula: distance ÷ time.
- Average speed = total distance ÷ total time.
- If direction changes but speed remains the same, velocity changes while speed can stay constant.
Acceleration
- Acceleration is the rate of change of velocity, describing how much velocity changes per second.
- Formula: acceleration = (final speed − initial speed) ÷ time.
- Objects can accelerate even if speed is constant when direction changes (change in velocity).
Graphs for Motion
Distance-Time Graphs
- Show how far an object travels over time.
- Gradient (slope) shows speed; steeper means faster.
- Straight line: constant speed; horizontal line: stationary object.
- Curved line: changing speed (increasing or decreasing).
- To find speed at a moment, draw a tangent and calculate its gradient.
Speed-Time Graphs
- Show how speed changes with time.
- Gradient indicates acceleration; straight line: constant acceleration; flat line: constant speed.
- Curved line: changing acceleration.
- To find acceleration at a point, draw a tangent and calculate its gradient.
- The area under the graph gives the total distance traveled.
Motion in a Gravitational Field
- Objects falling in gravity experience weight (downward force) and possibly air resistance (upward force).
- With no air resistance (e.g., in space), only weight acts, so speed increases steadily (free fall).
- With air resistance (e.g., skydiving), resistance increases with speed until it balances weight, resulting in terminal velocity (constant speed).
- Opening a parachute increases air resistance, resulting in a slower terminal velocity before landing.
- Acceleration due to gravity on Earth is 9.8 m/s².
Key Terms & Definitions
- Speed — distance per unit time (scalar).
- Velocity — speed in a specific direction (vector).
- Acceleration — rate of change of velocity.
- Terminal velocity — constant speed when forces are balanced during free fall.
- Free fall — motion under gravity with no air resistance.
Action Items / Next Steps
- Practice drawing and interpreting distance-time and speed-time graphs.
- Review the calculation of areas under speed-time graphs for distance.
- Prepare for the next unit: Mass and Weight.