Summary
Speed is the distance covered by an object in a unit time, calculated as total distance divided by total time taken. Its basic unit is metre per second (m/s).
Class 7 Science Chapter 9 covers speed, time measurement, and types of motion. Speed equals total distance covered divided by total time taken, with the basic unit being metre per second (m/s); it can also be expressed in km/h or m/min. An object moving along a straight line at constant speed is in uniform motion; if speed keeps changing, the motion is non-uniform. Time is measured using periodic events — a simple pendulum consists of a bob suspended by a thread, and the time for one complete oscillation is its time period, which remains constant for a given pendulum regardless of small changes in initial displacement. Galileo Galilei (A.D. 1564–1642) established this. Modern clocks use quartz crystals for greater accuracy. A speedometer measures speed in km/h; an odometer measures distance. Distance-time graphs represent motion visually — a straight line indicates constant speed.
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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