Motion and its Graphs Model

Reading the two graphs

	Distance–time graph	Speed–time graph
At rest	horizontal line	line on the time axis (speed = 0)
Constant speed	straight sloping line	horizontal line above the time axis
Accelerating	curve getting steeper	line sloping upwards
Decelerating	curve getting less steep	line sloping downwards
Gradient	speed	acceleration
Area under the line	(no meaning)	distance travelled

How to write it in the exam

Speed is the distance travelled per unit time. Velocity is speed in a given direction.
Average speed = total distance travelled ÷ total time taken. The total time includes any time spent at rest.
The gradient of a distance–time graph is the speed. Use a large triangle on a straight part of the line: change in distance ÷ change in time.
A horizontal line on a distance–time graph means the object is at rest. A horizontal line on a speed–time graph means constant speed.
The area under a speed–time graph is the distance travelled. Split it into rectangles and triangles (area of a triangle = ½ × base × height).
Acceleration is the change in velocity per unit time, a = Δv / Δt. It is the gradient of a speed–time graph. A straight line means constant acceleration; a curve means changing acceleration.
A deceleration is a negative acceleration: Δv is negative, so a is negative.
Close to the Earth’s surface, g is roughly constant, at about 9.8 m/s².
As a falling object speeds up, the air resistance increases, so the resultant force and the acceleration decrease. When the air resistance equals the weight, the resultant force is zero and the object falls at its terminal velocity.