Class 7 Motion-Time
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Motion: Motion means a change in position of an object with respect to time.
Rectilinear motion: Motion in a straight line.
Speed: The distance travelled by a body in a given time.
Speedometer: A device used for measuring the speed of a vehicle.
Odometer: A device used for measuring the distance travelled by a vehicle.
Average speed: It is defined as the total distance covered by an object divided by total time was taken.
Uniform motion: An object is said to be in uniform motion if its speed remains constant during motion.
Non-uniform motion: If an object moving in a straight path changes its speed it has nonuniform motion.
Time: It is a moment of duration in which things occur.
Time period: A stretch of time.
Amplitude: Maximum displacement.
POINTS TO REMEMBER
• The object which seems to change their position time to time is said to be in motion.
• The speed of a moving object is the rate at which distance is covered and is always measured in terms of units of distance divided by a unit of time.
• The SI unit of speed is metre per second (m/s).
• An object is said to be moving at a uniform speed if it moves along a straight line and covers equal distances in equal intervals of time.
• A body is said to be moving at a non-uniform speed if it moves along a straight line but covers unequal distances in equal intervals of time.
• The average speed of a moving object is the total distance covered divided by the total time taken at the end of travel i.e,
• A graph has two axes, in a distance-time graph, the horizontal axis represents time, while the vertical axis represents distance. The distance-time graph of a body in uniform motion is straight. The steeper the line, the greater is the speed of the moving body.
• The physical quantity which measures how far an object moves during a fixed period of time is called speed.
• The events which occur regularly at equal intervals of time are called periodic and give an idea of time involvement.
• Time is a fundamental physical quantity. The SI unit of time is second (s).
• Stop clocks and stopwatches are frequently used to measure short-time intervals very accurately.
Class 7 Motion and Time Exercise
A. Multiple Choice Questions (MCQs)
1. Speed of a body is measured in
2. Which of the following cannot be a unit
3. The slope of a distance-time graph of a
moving object indicates:
(a) distance moved by the object
(b) time is taken by the object
(c) the speed of the object
(d) the position of the object
4. The regular vibrations of which of the following electrically driven crystals are used for measuring time very
(a) calcite crystals
(b) quadric crystals
(c) chrome crystals
(d) quartz crystals
5. The clocks and watches which are used for measuring time are based on:
(a) rectilinear motion
(b) circular motion
(c) periodic motion
(d) rotational motion
6. When the amplitude of vibrations of a simple pendulum is increased, then it’s time-period?
(c) remains the same
(d) first increases and then
7. An athlete attains a maximum speed of 36 km/h. This speed is equal to:
(a) 5 m/s
(b) 10 m/s
(c) 15 m/s
(d) 20 m/s
1. (c) 2. (b) 3. (c) 4. (d) 5. (b) 6. (b) 7. (b)
Class 7 Motion and Time Answers:
1. Reduce the length of the string which is tied to the stone. The time reduces because the longer the string is the longer the time is taken. The shorter the string is the lesser the time is taken.
2. We need to measure time for a variety of applications like to meet our clear-cut schedules, to know the rate of heart
beat per minute,to know when a plate or train arrived or left, for writing exams , the duration of a journey, if we want to wait for someone or watch something, to sleep, to wake up in time, to know whether it’s night or day, to know what time a computer takes to perform an operation etc……………..so the list is endless.
3. As we know that if anybody keeps on covering equal distances in equal intervals of time, its speed remains constant. We call the motion of such an object a uniform motion. Thus, here the air is moving along a straight
line path is said to have a uniform motion since its speed remains constant.