- Books Name
- CBSE Class 7 Science Book
- Param Publication
- CBSE Class 7
Slow or fast :
As you know that some vehicles move faster than others. For example : A car moves faster than a cycle. How can one say that an object is moving faster or slower. The distance moved by objects in a given interval of time can help us to decide.
Uniform and Non-uniform motion :
1. Uniform Motion :
A body has a uniform motion if it travels equal distances in equal intervals of time, no matter how small these time intervals may be. For example, a car running at a constant speed of say, 10 m/s, will cover equal distances of 10 metre, in every second, so its motion will be uniform.
If it moves along a straigth line with a constant speed it is said to be in uniform motion.
2. Non-Uniform Motion :
A body has a non-uniform motion if it travels unequal distances in equal intervals of time. For example, if we drop a ball from the roof of a building, we will find that it covers unequal distances in equal intervals of time. It covers :
4.9 metre in the 1st second,
14.7 metre in the 2nd second,
24.5 metre in the 3rd second, and so on.
If the speed of an object is not moving along a straight line keeps changing, its motion is said to be non-uniform.
Uniform and Non-uniform motion can be shown graphically also
We observe different types of objects around us. Some of them are at rest for us and some of them are in motion. For instance, tree, houses, etc. are at rest while flying birds, moving cars etc. are in motion.
“Motion is change in position of an object with time.”
As we know that a motion could be along a straight line, circular or periodic.
The branch of physics which deals with the study of the motion of the object is known as Mechanics.It is further divided as.
(i) Statics: Which deals with the study of the objects at rest.
(ii) Kinematics: Which describe motion without going into the cause of motion.
(iii) Dynamics: Which deals with the study of the motion of the objects by taking into the account of the force which cause the motion in the objects.
Rest and motion :
When we look around, we find some objects are in motion whereas, some other objects are at rest. A moving bus, a moving car are in motion. To win a race, the athletes run. They are in motion.The water in a river is in motion. A book lying on the table, a lamp post, a table, a chair, a black board etc., all are at rest.When do we say that an object is in motion? When do we say that it is at rest?
An object in motion is the one that changes its position with respect to its surroundings whereas, an object which is at rest is the one that does not change its position with respect to surroundings.
The rest and motion are relative. An object at rest with respect to one observer may not be at rest with respect to another observer. The same can be said about motion. For a person inside a bus, the fellow passengers are at rest but the same passengers are in motion with respect to a person standing on the ground. If we closely observe the motion of different objects, we find them performing different kinds of motion. A car moving on a straight road, the rotation of blades of a table fan, the motion of a swing which moves to and fro are all different kinds of motion.
Types of Motion :
(i) Random Motion : Motion having no specific path.
Example : Motion of butterfly, Motion of kite.
(ii) Translational Motion : Motion in which all particles of a moving body describes identical trajectory.
(a) Rectilinear Motion : When a body moves in straight line is known as rectilinear motion. It is also called linear motion.
Examples : The motion of a moving car, a person running, a stone being dropped, motion of a train on a straight track
(b) Curvilinear Motion : When a body moves in a curve path, it is known as curvilinear motion.
Examples : Car moving on a curved road.
(c) Circular Motion : When a body moves in a circular path, its motion is known as circular motion. In circular motion, the object remains at the same distance from a fixed point.
Examples : Motion of the tip of the clock’s arm, motion of the earth around the sun, motion of blades of fan.
(iii) Rotational Motion : Motion in which a whole body move about an axis is called a rotational motion.
Examples : Motion of top, Rotation of earth
(iv) Periodic Motion : Motion in which an object repeats its motion after a fixed interval of time is called periodic motion.
(ii) Motion of a swing.
(a) Oscillatory Motion : The to and fro periodic motion of a body around a fix point is called oscillatory motion.
Example : The motion of a simple pendulum, a body suspended from a spring (also called to and fro motion).
Frame of Reference :
To locate the position of object we need a frame of reference. A convenient way to set up a frame of reference is to choose three mutually perpendicular axes and name them x-y-z axes. The coordinates (x, y, z) of the particle then specify the position of object w.r.t. that frame.
If any one or more coordinates change with time, then we say that the object is moving w.r.t. this frame.
Distance and displacement :
1. Distance :
Distance is the actual path travelled by a body in a given time. Consider a body travelling from A to B along any path between A and B. The actual length of the path that a body travels between A and B is known as the distance. Here if the body goes from A to B via C, the distance travel will be ACB.
The distance travelled will be different for different paths between A and B. It is a scalar quantity.
2. Displacement :
The distance travelled in a given direction is the displacement. Thus displacement is the shortest distance between the given points. It is a vector quantity. S.I. unit of distance and displacement is metre.
e.g. : A body starts from A and moves according to given figure.
The distance and displaemcent are as follows for different path.
(i) : If a body travels in such a way that it comes back to its starting position, then the displacement is zero. However, distance travelled is never zero.
Ex.1 A train starts from station A and moves towards station B along a straight path which is 100 km from A. Reaching B it turns back and stops midway between A and B. Find the distance and displacement of train. Neglect length of train.
Sol. Say the midpoint as C.
∴ Distance = AB + BC = 100 + 50 = 150 km
and Displacement = AC = 50 km or 50 km along east.
Difference between distance and displacement :