Motion and Time
Overview
How fast did the vehicle go? How accurate is the clock? How many times does a pendulum swing? This chapter is about measuring motion — speed, time, and distance-time graphs — the first calculations of physics.
Typed Notes
Speed — How Fast
Speed = distance ÷ time. If a vehicle covers 100 km in 2 hours — speed = 50 km/h. SI unit: metre/second (m/s). Converting km/h to m/s: × 5/18 (72 km/h = 20 m/s). Uniform motion — constant speed: equal distance in equal time (cruising on a highway). Non-uniform — stopping and moving in traffic. Average speed = total distance ÷ total time (count the time spent stopped as well).
Time and Its Measurement
SI unit of time — second. Earlier, people used the sun and moon (sundial, water clock) — then the pendulum clock (Galileo observed: every pendulum has its own fixed rhythm). Pendulum clock: the swinging of the bob — one oscillation (A→B→back to A) — its time = time period. Time period changes with length (longer = slower), not with mass (heavy bob or light bob — same!). Today, quartz clocks: quartz crystal vibration (32768 times every second) — very accurate. Digital clocks, phones — all based on this.
Reading a Distance-Time Graph
Time on the x-axis, distance on the y-axis. A straight line (sloping upward) = uniform speed (the slope of the line = speed — steeper the line, faster the speed). A curve = speed is changing (becoming steeper — getting faster). A flat (horizontal) line = the vehicle is stopped (time is increasing, distance is not). From the graph: you can find the distance at any point in time, and the speed from the slope — both can be determined.
Instruments in a Vehicle
Speedometer — shows instantaneous speed (km/h) — how fast right now. Odometer — total distance (how far the vehicle has travelled in its lifetime) — used for service records. Tachometer — engine RPM. Every vehicle has all these readings for the driver. On trails, GPS also gives distance and speed.
Key Concepts
Revision ke liye ek nazar — exam se pehle yahi padhna hai.
- 01Speed = distance/time. Units: m/s (SI), km/h. Conversion: km/h → m/s × 5/18.
- 02Uniform motion — constant speed (equal distance in equal time); Non-uniform — speed keeps changing (city traffic). Average speed = total distance/total time.
- 03SI unit of time: second (s). Clocks: quartz (crystal vibration), pendulum clock.
- 04Simple pendulum: one complete swing of the bob (A→B→A) = one oscillation; its time = time period. Every pendulum has its own fixed time period (changes with length — not with mass!).
- 05Distance-time graph: straight line (uniform speed — slope = speed); curve (speed is changing). Speedometer — instantaneous speed; odometer — total distance.
Questions & Solutions
Pehle khud try karo — phir solution kholo. Yahi tariqa topper banata hai.
Write the formula for speed and its SI unit.
A vehicle covers 150 km in 3 hours — what is its average speed?
What is one oscillation of a simple pendulum?
What does a straight sloping line on a distance-time graph mean?
Write the separate functions of a speedometer and an odometer.
What changes the time period of a pendulum — a heavier bob or a longer rod?
Speed = distance/time (m/s, km/h — convert ×5/18). Uniform = constant speed; average = total D/total T. Pendulum: time for one oscillation = time period (depends on length, not mass). Distance-time graph: straight line = uniform speed (slope = speed). Speedometer shows instantaneous speed, odometer shows total distance.
Chapter-end quiz
4 MCQsOption tap karo — turant pata chalega sahi ya galat, explanation ke saath.
Q1.What is the SI unit of speed?
Q2.72 km/h = how many m/s?
Q3.What does the time period of a pendulum depend on?
Q4.What does a horizontal line on a distance-time graph mean?
Mark karo — dashboard par progress dikhega.