Class 7 - Science - Chapter 13 - Motion And Time

Motion And Time

 

Motion

• Motion is a change in position of an object with respect to time. If the position of a body is not changing with respect to time, the body is said to be at rest, motionless, immobile or stationary. Any moving body is said to be in motion. Some types of motions are show below:

• Motion is based on Frame of Reference. When a person standing outside the bus is considered as a reference, the bus is in motion. When a person sitting inside the bus is considered as a reference, the bus appears to be stationary.

Oscillatory Motion:

• An oscillatory motion is a motion where a body moves between two extreme positions.
• The center of these extreme positions is called mean position.
• Time taken to complete one oscillation is called Time Period.
• When an oscillatory motion repeats itself in equal intervals of time it is called Periodic Motion.
• All periodic motions could not be oscillatory, but all oscillatory motions are periodic in nature.

Uniform and Non-Uniform Motion: 

• An object having uniform motion travels equal distances in equal intervals of time and An object having non-uniform motion travels unequal distances in equal intervals of time.

Speed

• Speed is the rate of change of position of an object with time. The average speed of an object in an interval of time is the distance travelled by the object divided by the duration of the interval hence,

                              

• Unit of Distance is meter or Kilometer and that of time is second or hour. Common units of speed are therefore meter/second or m/s or Kilometer/Hour or km/h. Units are always written in singular, that is, km/h and not kms/hrs.

Fast and Slow Motion/Speed: 

• Speed can be fast or slow depending upon the distance covered by them in a given interval of time. Since a motorbike covers more distance in a given interval of time than a bicycle, the bicycle is said to be in slower motion and motorbike in faster motion.

Graphs

• A graph is a picture or diagram that shows relation between two or more variable quantities.
• A simple graph usually shows the connection between two numbers or measurements in the form of a grid.
• A graph or a chart can be of different types based on number of quantities and representation method.

Distance-Time Graph

• This is usually drawn as a line graph as it taken two variable quantities – Distance and Time. In a Distance-Time graph, Distance is considered on the Y-axis (Vertical) and Time is considered on the X-axis (Horizontal).

• Some of the points to be kept in mind while choosing the most suitable scale for drawing a graph are:
• Determine the maximum quantity to be taken on each axis.
• Divide this maximum quantity in smaller equal parts, so that it is easy to mark the points. The number of equal parts to be taken depends upon the data available.
• For eg, in the above diagram, distance quantity is 10 whereas time quantity is 20.

• Try to draw the graph so that it covers maximum paper space which will make it look clean.

Slope calculation using D-T Graph: Slope of a Distance-

• Time graph (From point A to F) is also the speed at every instant. Higher the slope greater is the speed of that object. Below is a diagram to calculate slope.

 Measurement Devices in Vehicles and other instruments

• Speedometer: Measures and displays instantaneous speed of the vehicle.

• Odometer: Measures and displays distance travelled by the vehicle.

• Tachometer: Measures and displays the revolutions per minute or the rotation speed of a shaft or disk.