Motion and Time Class 7

 📝 Motion and Time  

Motion

An object is said to be in motion if its position, with respect to its immediate surroundings, changes with time.    

Rest 

An object is said to be at rest if its position, with respect to its immediate surroundings, does not change with time. 

All Motion is Relative

A given object may be regarded as 'being at rest' by one observer and as 'being in motion' by another observer. For example, two persons, sitting next to each other in a moving train, are at rest with respect to each other. However, they are both in motion with respect to a person standing outside on the platform. 

Motion along a Straight Line

The simplest type of motion is motion along a straight line (linear motion). For example, the motion of a train on a straight track, an apple falling from a tree, etc. 

In all cases, the object nearly moving along a straight path. We call such motion rectilinear or simply linear motion. 

Describing Straight Line Motion

We need to specify the position of an object, at different times, to describe its motion along a straight path. 

Choose point 'O' as the point from where the car starts moving. This means that the car is at 'O' at 't = 0'. Let A, B, C represent the position of the car at different instants of time. 

In the first 10 minutes, the car moves from O to A. The distance covered is 60 m in these first 10 minutes. In the next 10 minutes, the car moves from A to B. The distance covered by it is 20 m in these 10 minutes. We can say that the car speeded up in the time interval `0-10` minutes and slowed down in the time interval `10 - 20` minutes.

Speed 

Speed is a measure of how fast an object is moving. We, therefore, regarded speed as an indicator of the rate/rapidity of a given object. Its SI unit is metre per second `(ms^-1)`.

Speed = Distance / Time 

Average Speed

The average speed of an object is the total distance moved divided by the total time taken to cover that distance.

Average Speed = Total distance / Total time

Uniform Motion

An object is said to be a uniform motion if it moves along a straight path and covers equal distances in equal intervals of time, however small these time intervals may be. 

Car moving along a straight road (Uniform Motion)

Its speed remains constant over these one-hour time intervals. We can therefore call the motion of this car a uniform motion