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Class 9–10 · Physics
Motion
Change it and see
Motion
Lessons in this chapter
Lessons
1Reference frames and types of motion
Motion is a change of position relative to a chosen reference. A passenger can be stationary relative to a bus and moving relative to the road. Straight-line, circular, rotational, periodic and oscillatory motion describe different features of a path or repetition.
Key relationship
specify origin + positive direction + clock
Worked example
Passenger relative to bus: position constant; relative to road: position changes.
Next lesson →2Scalars, vectors, distance and displacement
Scalars have magnitude; vectors also have direction. Distance is total path length; displacement connects the initial and final positions. Add vectors with directions, using a right triangle for perpendicular components.
Key relationship
Δx = x₂ − x₁; distance ≥ magnitude of displacement
Worked example
3 m east + 4 m north ⇒ distance 7 m; displacement √(3²+4²) = 5 m northeast
Next lesson →3Speed and velocity
Average speed uses total distance divided by total time; average velocity uses total displacement. Instantaneous values describe a moment. An arithmetic mean of two speeds is generally wrong unless the time intervals are equal.
Key relationship
average speed = total distance/total time; average velocity = displacement/time
Worked example
60 m out in 10 s, 60 m back in 20 s ⇒ speed 120/30 = 4 m/s; velocity 0
Next lesson →4Acceleration and deceleration
Acceleration is the rate of change of velocity. Speed increases when velocity and acceleration align and decreases when they oppose. A negative acceleration is not automatically slowing down; its meaning depends on the chosen axis and velocity direction.
Key relationship
a = (v − u)/t; unit m/s²
Worked example
u = +10 m/s, v = +4 m/s, t = 3 s ⇒ a = −2 m/s²
Next lesson →5Constant-acceleration equations
For constant acceleration, final velocity is u + at and mean velocity is (u+v)/2. Multiplying mean velocity by time gives the displacement equation. Choose an equation containing the known quantities and only one unknown.
Key relationship
v = u+at; s = ut+½at²; v² = u²+2as
Worked example
u = 0, a = 2 m/s², t = 5 s ⇒ v = 10 m/s; s = 25 m
Next lesson →6Motion graphs
The slope of a position–time graph is velocity. A velocity–time graph gives acceleration from slope and displacement from signed area. For distance, add the magnitudes of positive and negative areas.
Key relationship
slope of x–t = v; slope of v–t = a; area under v–t = s
Worked example
v rises linearly 0→10 m/s in 5 s ⇒ s = ½ × 5 × 10 = 25 m
Next lesson →7Free fall and upward projection
Near Earth's surface, neglecting air resistance, acceleration is downward with magnitude about 9.8 m/s². With upward positive, a = −g. At the top of an upward throw, velocity is momentarily zero but acceleration remains downward.
Key relationship
v = u−gt; h = ut−½gt²; hmax = u²/(2g)
Worked example
u = 19.6 m/s ⇒ time to top 2 s; maximum height 19.6 m
Next lesson →8Uniform circular motion
Constant speed around a circle still requires acceleration because velocity changes direction. Tension, friction or gravity can supply the net inward force. If that force disappears, the object initially travels along the tangent.
Key relationship
a = v²/r; F = mv²/r
Worked example
v = 4 m/s, r = 2 m ⇒ a = 8 m/s²
Next lesson →Check your understanding
Try answering first, then open the question to compare your reasoning.
1. Displacement after a complete circular lap?
0; initial position = final position
2. From rest at 3 m/s² for 4 s: final velocity?
v = 0 + 3 × 4 = 12 m/s
3. What is the slope of a velocity–time graph?
a = Δv/Δt
4. Velocity and acceleration at the top of an upward throw?
v = 0; a = −g, with upward positive
5. Equal distances at 10 and 20 m/s: average speed?
Choose 100 m each: total time 10+5 = 15 s; 200/15 ≈ 13.3 m/s
Sources and scope
Original GetReady teaching notes. The 13 core chapters follow the existing site sequence. Physics to save life is an explicitly labelled older-edition supplement. Check your current book and board instructions for examination scope. Illustrations are schematic. Unless stated otherwise, use g = 9.8 m/s². A physics teacher should review content and edition coverage before it is relied on.