# CBSE – 7th Standard Science ## CBSE – 7th Standard Science

### Chapter 13 – Motion and Time

Chapter 13

Motion and Time

• The distance covered by an object in a unit time as the speed of the object.
• Determination of speed of a given object once we can measure the time taken by it to cover a certain distance.
• The distance moved by objects in a given interval of time tells faster or slower speed.
• A higher speed seems to indicate that a given distance has been covered in a shorter time, or a larger distance covered in a given time
• Motion could be along a straight line it could be circular or periodic
• One of the most well-known periodic motions is that of a simple pendulum.
• If the speed of an object moving along a straight line keeps changing, its motion is said to be non-uniform.
• On the other hand, an object moving along a straight line with a constant speed is said to be in uniform motion, the average speed is the same as the actual speed. • Speed is the total distance covered divided by the total time taken.

Thus, Speed = Total distance covered /Total time taken • The to and fro motion of a simple pendulum is an example of a periodic or an oscillatory motion. • Time taken by the pendulum to complete one oscillation is called its time period.
• The basic unit of time is a second. Its symbol is s.
• Larger units of time are minutes (min) and hours (h).the speed is distance/time, the basic unit of speed is m/s. it could also be expressed in other units such as m/min or km/h.
• The symbols of all units are written in singular.  Different units of time are used depending on the need.
• The smallest time interval that can be measured with commonly available clocks and watches is one second.
• Some clocks can measure time intervals as small as one millionth or even one billionth of a second.
• One microsecond is one millionth of a second. A nanosecond is one billionth of a second
• Formula : Distance covered = Speed × Time and Time taken = Distance/Speed
• A speedometer records the speed directly in km/h.
• There is also another meter that measures the distance moved by the vehicle, is known as odometer. • The types of graph, Bar graph, for graphical use a pie chart is used and line graph.
• The distance-time graph is a line graph. Two lines are drawn perpendicular to each other x-axis and y-axis. • The point of intersection quantities between which the graph is drawn are shown along these two axes.
• Draw the positive values on the x-axis along similarly, positive values on the y-axis are shown along.
• Joining of all the points on the graph makes a straight line. This is the distance-time graph
• If the distance-time graph is a straight line, it indicates that the object is moving with a constant speed.
• However, if the speed of the object keeps changing, the graph can be of any shape.
• Scales should be chosen to make graph by i) the difference between the highest and the lowest values of each quantity and ii) the intermediate values of each quantity.
• With the scale chosen it is convenient to mark the values on the graph, and to utilise the maximum part of the paper on which the graph is to be drawn.
• Distance-time graphs provide a variety of information about the motion when compared to the data presented by a table.
• The distance moved by an object in a unit time is called its speed. • Speed of objects help us to decide which one is moving faster than the other.
• The speed of an object is the distance traveled divided by the time taken to cover that distance. Its basic unit is metre per second (m/s).
• Periodic events are used for the measurement of time. Periodic motion of a pendulum has been used to make clocks and watches. • Motion of objects can be presented in pictorial form by their distance-time graphs.
• The distance-time graph for the motion of an object moving with a constant speed is a straight line.
• Different units of time are used depending on the need.
• The time taken in saying aloud “two thousand and one” is nearby one second.
• The pulse of a normal healthy adult at rest beats about 72 times in a minute that is about 12 times in 10 seconds.
• This rate may be slightly higher for children.
• The smallest time interval that can be measured with commonly available clocks and watches is one second.
• However, now special clocks are available that can measure time intervals smaller than a second.
• Some of these clocks can measure time intervals as small as one millionth or even one billionth of a second.
• You might have heard the terms like microsecond and nanosecond.
• One microsecond is one millionth of a second. A nanosecond is one billionth of a second.
• Clocks that measure such small time intervals are used for scientific research.
• The time measuring devices used in sports can measure time intervals that are one tenth or one hundredth of a second.
• On the other hand, times of historical events are stated in terms of centuries or millenniums.
• The ages of stars and planet are often expressed in billions of years. Can you imagine the range of time intervals that we have to deal with?