| 1.2: MOTION |
|---|
Motion describes how objects change position over time.
Key Definition Speed, defined as the distance travelled per unit time, is a scalar quantity measured in m s⁻¹.
Speed is calculated using the equation:
v = s/t
Key Definition Velocity is speed in a stated direction and is a vector, so a change in direction counts as a change in velocity even if the speed stays the same.
Average speed accounts for the entire journey and equals total distance travelled divided by total time taken.
- The gradient equals the speed (a steeper gradient means a greater speed)
- A horizontal line indicates the object is at rest
- A curved line indicates that the speed is changing, i.e. acceleration or deceleration
Speed–time graphs
- A horizontal line indicates constant speed
- A sloping line indicates acceleration or deceleration
- The area under the graph gives the distance travelled
Distance–time graphs and speed–time graphs are essential tools for analysing motion.
The acceleration of free fall g for an object near the Earth's surface is approximately constant at 9.8 m s⁻².
Extended Extended students must also:
Key Definition Acceleration is the change in velocity per unit time; it is a vector measured in m s⁻².
Use the equation:
a = Δ v / Δ t
- Calculate acceleration from the gradient of a speed–time graph
- Recognise deceleration as a negative acceleration
Key Definition Terminal velocity is the constant maximum velocity reached by a falling object when the resultant force, and therefore the acceleration, is zero.
Falling objects experiencing air resistance accelerate at a decreasing rate until air resistance equals weight, at which point they reach terminal velocity.