Class 7 Science Chapter 13 – Motion and Time: Detailed Notes and Questions & Answers
Introduction: Motion and Time is a foundational chapter in Class 7 Science that introduces how we describe and measure movement. Understanding motion helps us make sense of everyday activities — from walking to driving, from the swinging of a pendulum to the motion of vehicles and the rotation of Earth. This page contains thorough, student-friendly notes followed by a comprehensive question bank: 7 MCQs, 7 very short answer questions, 7 short answer questions, and 7 long answer questions with model answers. Use the notes for quick revision and the Q&A for exam practice.
📑 Notes — Motion and Time (Expanded)
What is Motion?
In physics, motion means a change in the position of an object with respect to time. For example, when a bus moves from one bus stop to another or when a ball rolls down a slope, these are motions. Motion must be described relative to a reference point (a tree, road marker, or building).
Frame of Reference
A frame of reference is the point or object we use to decide whether something is moving. A person sitting inside a moving train feels at rest relative to the train but in motion relative to the ground. Always mention the reference point when describing motion in answers.
Types of Motion
- Rectilinear Motion: Motion along a straight line. Example: A car on a straight highway.
- Circular Motion: Motion along a circular path. Example: A merry-go-round or the hands of a clock.
- Periodic Motion: Motion that repeats itself at equal intervals of time. Example: Swing of a pendulum or the oscillation of a clock pendulum.
- Non-uniform Motion: Motion in which an object covers unequal distances in equal intervals of time, e.g., a vehicle in city traffic.
Distance vs Displacement (Short Note)
Distance is the total path covered and is a scalar (only magnitude). Displacement is the straight-line distance from start to end in a specific direction and is a vector. Class 7 focuses mainly on distance and speed, but knowing the difference helps build future concepts.
Speed — Definition and Formula
Speed = Distance ÷ Time. If a cyclist covers 30 metres in 5 seconds, speed = 30 ÷ 5 = 6 m/s. Common units: metre per second (m/s) and kilometre per hour (km/h). Conversions: 1 m/s = 3.6 km/h and 1 km/h = 5/18 m/s. Always write units with your numeric answers.
Uniform and Non-uniform Motion
In uniform motion, an object covers equal distances in equal intervals of time — speed is constant. In non-uniform motion, distances covered in equal times are different — speed varies. Example: A train cruising at 60 km/h (uniform) versus a car frequently slowing and accelerating in traffic (non-uniform).
Distance–Time Graphs
A distance–time graph plots time on the x-axis and distance on the y-axis. For uniform motion the graph is a straight line; slope = speed. For faster motion the line is steeper. A horizontal line means no motion (object at rest). Curved lines indicate changing speed (non-uniform motion).
Measuring Time — Past to Present
Humans measured time using natural signs (sunrise, seasons), sundials, water clocks, and hourglasses. Mechanical clocks improved accuracy. Today, atomic clocks define the second with extreme precision. In school and labs we use stopwatches and digital timers for experiments.
Pendulum — Example of Periodic Motion
A simple pendulum consists of a bob suspended on a string. When displaced and released, it swings back and forth — one back-and-forth is one oscillation. The period (time for one oscillation) depends mainly on the length of the string. For small swings, the period is approximately proportional to the square root of the length (shorter length → shorter period).
- Speed = Distance ÷ Time
- Distance = Speed × Time
- Time = Distance ÷ Speed
- Conversions: 1 m/s = 3.6 km/h
Real-life Examples and Conversions
Typical speeds: walking ~ 5 km/h, cycling ~ 15 km/h, city car ~ 30–50 km/h, train ~ 80–120 km/h, airplane ~ 800–900 km/h. Example conversion: A runner covers 400 m in 50 s → speed = 400 ÷ 50 = 8 m/s → 8 × 3.6 = 28.8 km/h.
Short Classroom Experiment (How to measure average speed)
Mark a 5 m straight track. Start a stopwatch when the toy car or student starts and stop when they cross the 5 m mark. Record time for 3 trials, calculate average time, then speed = 5 m ÷ average time. Repeat and discuss errors (reaction time, surface friction).
✍️ Questions & Answers — Motion and Time
Multiple Choice Questions (7)
MCQ 1: The SI unit of speed is:
(a) km/h (b) m/s (c) cm/s (d) hour
Answer: (b) m/s
MCQ 2: A horizontal line on a distance–time graph indicates:
(a) increasing speed (b) decreasing speed (c) object at rest (d) circular motion
Answer: (c) object at rest
MCQ 3: Which instrument measures the instantaneous speed of a moving vehicle?
(a) Odometer (b) Speedometer (c) Stopwatch (d) Altimeter
Answer: (b) Speedometer
MCQ 4: Motion of the hands of a clock is an example of:
(a) rectilinear motion (b) circular motion (c) random motion (d) vibrational motion
Answer: (b) circular motion
MCQ 5: If a car travels 180 km in 3 hours, its average speed is:
(a) 30 km/h (b) 45 km/h (c) 60 km/h (d) 90 km/h
Answer: (c) 60 km/h
MCQ 6: Periodic motion repeats after:
(a) random intervals (b) equal intervals of time (c) changing intervals (d) no intervals
Answer: (b) equal intervals of time
MCQ 7: A pendulum’s period mainly depends on:
(a) mass of bob (b) length of string (c) colour of bob (d) shape of room
Answer: (b) length of string
Very Short Answer Questions (7)
V1: Define motion.
Ans: Motion is the change in position of an object with time.
V2: State the SI unit of time.
Ans: Second (s).
V3: Give an example of uniform motion.
Ans: A car moving at constant speed on a straight road.
V4: What does a steep line on a distance–time graph mean?
Ans: Higher speed.
V5: Convert 5 m/s into km/h.
Ans: 5 × 3.6 = 18 km/h.
V6: Name one ancient timekeeping device.
Ans: Sundial.
V7: Define periodic motion in one line.
Ans: Motion that repeats itself after equal intervals of time.
Short Answer Questions (7)
S1: Differentiate uniform and non-uniform motion with one example each.
Ans: Uniform motion covers equal distances in equal times (e.g., a moving escalator). Non-uniform motion covers unequal distances in equal times (e.g., a car in traffic).
S2: A bike goes 24 km in 1.5 hours. Calculate speed in km/h.
Ans: Speed = 24 ÷ 1.5 = 16 km/h.
S3: Explain how to read speed from a distance–time graph.
Ans: Pick two points, find change in distance (Δd) and change in time (Δt), then speed = Δd ÷ Δt. The slope = speed.
S4: Describe how a stopwatch and odometer differ in function.
Ans: A stopwatch measures time intervals (seconds), while an odometer records total distance travelled by a vehicle.
S5: Why is it important to write units in answers?
Ans: Units give meaning to numerical values and avoid ambiguity (e.g., 50 could be km/h or m/s).
S6: Give two uses of periodic motion in everyday items.
Ans: Pendulums in old clocks for timekeeping, and vibrations in mobile phones for alerts.
S7: A jogger runs 5 km in 30 minutes. Find speed in km/h and m/s.
Ans: Time = 0.5 h. Speed = 5 ÷ 0.5 = 10 km/h = 10 × (5/18) = 50/18 m/s ≈ 2.78 m/s.
Long Answer Questions (7)
L1: Explain how to draw and interpret a distance–time graph for two vehicles and determine which is faster at different intervals.
Ans: Put time on x-axis and distance on y-axis. Plot distances for both vehicles at same time intervals and join points. The steeper line at any time means the vehicle is faster at that instant. Compare slopes to decide which is faster overall or during certain intervals. Use numerical slope calculations for precise comparison.
L2: Describe the working of a simple pendulum, state factors affecting its period, and mention one use.
Ans: A pendulum swings due to gravity restoring it towards equilibrium. The time period depends mainly on length and gravitational acceleration; mass has negligible effect. Used in pendulum clocks for regular timekeeping.
L3: A car covers 150 km in 2 hours and then 50 km in 1 hour. Calculate average speed. Show all steps.
Ans: Total distance = 150 + 50 = 200 km. Total time = 2 + 1 = 3 h. Average speed = 200 ÷ 3 ≈ 66.67 km/h.
L4: Why are standard units important in science? Give examples where wrong units caused problems.
Ans: Standard units prevent confusion and mistakes. Example: NASA lost a $125 million spacecraft due to a mix-up between metric and imperial units. Standard units ensure everyone interprets measurements the same way.
L5: Explain different methods used historically to measure time and how accuracy improved over time.
Ans: Sundials and water clocks were first. Mechanical clocks improved precision with gears and escapements. Pendulum clocks gave regular intervals. Quartz and atomic clocks further increased accuracy; atomic clocks are used for GPS and scientific standards.
L6: Suggest a detailed classroom activity to compare uniform and non-uniform motion using toy cars and ramps.
Ans: Use two ramps: one straight and smooth, other with slight obstacles. Release identical cars from same height and record distances at equal times using stopwatch. Plot distance–time graphs; the smoother ramp yields a straighter line (more uniform), the other shows varying slope. Discuss friction, incline, and errors.
L7: How do modern technologies like GPS help in measuring motion? Provide three examples and their benefits.
Ans: GPS gives accurate position and speed (navigation), sports trackers measure runners’ speeds and routes (training), and vehicle telematics helps fleet managers monitor speeds and optimize routes (fuel efficiency and safety).
Practice regularly — write answers in your own words, show units, and practice converting units for speed problems.
Related Reading: Explore more Class 7 Science Chapters
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