Summary
Chapter 9 of the Class 7 Science NCERT textbook, "Motion and Time", explains that speed is the distance covered by an object in a unit time, calculated as total distance divided by total time taken, with its basic unit being metre per second (m/s).
- Speed and types of motion — Speed compares distance covered to time taken, measured in m/s. When an object keeps a constant speed along a straight path its motion is uniform, but if the speed keeps changing the motion is called non-uniform.
- Measuring time with periodic events — Time is tracked using events that repeat. A simple pendulum's swing has a fixed time period unaffected by small displacement changes, a fact established by Galileo, while modern quartz clocks measure time far more accurately.
- Instruments and graphs of motion — A speedometer reads a vehicle's speed and an odometer records the distance it travels. Motion can also be pictured on a distance-time graph, where a straight line means constant speed and any other shape means it is changing.
Key points & formulas
- 01Speed = Total distance covered ÷ Total time taken; basic unit is m/s.
- 02Uniform motion: constant speed along a straight line. Non-uniform motion: speed keeps changing.
- 03A simple pendulum's bob makes one oscillation when it travels from its mean position to one extreme, to the other extreme, and back; the time for this is the time period.
- 04The time period of a given pendulum is constant — a slight change in initial displacement does not affect it (established by Galileo Galilei, 1564–1642).
- 05Distance covered = Speed × Time; Time taken = Distance ÷ Speed.
- 06A speedometer on a vehicle records speed in km/h; an odometer records the distance moved.
- 07A straight line on a distance-time graph means the object is moving with constant speed; any other shape means speed is changing.
- 08Basic unit of time is the second (s); larger units are minutes (min) and hours (h). Modern quartz clocks are more accurate than earlier pendulum-based clocks.
Frequently asked questions
01What is speed and how is it calculated?
Speed is the distance covered by an object in a unit time. It is calculated as: Speed = Total distance covered ÷ Total time taken.
02What is the basic unit of speed?
The basic unit of speed is metre per second (m/s). It can also be expressed in m/min or km/h.
03What is the difference between uniform and non-uniform motion?
An object moving along a straight line with a constant speed is in uniform motion. If the speed of an object moving along a straight line keeps changing, its motion is non-uniform.
04What is a simple pendulum?
A simple pendulum consists of a small metallic ball or a piece of stone (called the bob) suspended from a rigid stand by a thread.
05What is one oscillation of a pendulum?
The pendulum completes one oscillation when its bob, starting from its mean position O, moves to extreme position A, then to extreme position B, and comes back to O. It also counts as one oscillation when the bob moves from one extreme A to the other extreme B and returns to A.
06What is the time period of a pendulum?
The time taken by a pendulum to complete one oscillation is called its time period.
07Does changing the initial displacement of a pendulum change its time period?
No. A slight change in the initial displacement does not affect the time period of a pendulum.
08Who discovered that the time period of a given pendulum is constant?
Galileo Galilei (A.D. 1564–1642) discovered this. While sitting in a church he noticed a lamp swinging from the ceiling and found that his pulse beat the same number of times during each oscillation. He then experimented with various pendulums to verify this.
09What does a straight line on a distance-time graph mean?
A straight line on a distance-time graph indicates that the object is moving with a constant speed. If the speed keeps changing, the graph will be of some other shape.
10What is the difference between a speedometer and an odometer?
A speedometer is a meter fitted on vehicles that records speed directly in km/h. An odometer is another meter on vehicles that measures the total distance moved by the vehicle.
11What were ancient time-measuring devices before pendulum clocks?
Sundials, water clocks, and sand clocks were used in different parts of the world before pendulum clocks became popular.
12What is the basic unit of time?
The basic unit of time is a second, with symbol s. Larger units are minutes (min) and hours (h).
13What are quartz clocks and why are they more accurate?
Nowadays most clocks and watches have an electric circuit with one or more quartz cells; these are called quartz clocks. The time measured by quartz clocks is much more accurate than that by earlier clocks.
14How is a distance-time graph plotted?
Draw two perpendicular axes — time along the x-axis and distance along the y-axis. Choose a suitable scale for each axis. Mark the data points for each set of time and distance values, then join all the points. The resulting line is the distance-time graph.
15Is the NCERT PDF for Class 7 Science Chapter 9 free to download?
Yes — the NCERT PDF for Class 7 Science Chapter 9 Motion and Time is free to read and download. No sign-up or payment is required.
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