8. Motion
(Science - IX)
Motion: when an object changes its position respect to its surrounding with time, it is said to be the object is in motion.
Example: motion of flying birds, motion of car, motion of blood in arteriesand veins etc.
Most motions are complex. Some objects may move in a straight line, others may take a circular path. Some may rotate and a few others may vibrate. There may be situations involving a combination of these.
Origin: we need to specify a reference point that describes the position of an object is called the origin.
Distance: This is total path covered by the body between its initial and final position is called distance.
Displacement: The shortest distance measured from the initial to the final position of an object is known as the displacement.
Magnitude: The numerical value of a physical quantity is its magnitude.
Physical Quantities: Quantities in physics which can be measured are called physical quantities like mass, time, area, velocity, area, density, volume, force etc.
scalars and vectors quantities:
Scalars quantities: The quantities that require magnitudes only to specify them are called scalars quantities or scalars. e.g Mass, time, area, speed, distance, temperature, density, volume etc.
Vector quantities: Quantities that require both magnitudes and direction to specify them are called vector quantities or vectors. e.g Displacement, velocity, force, momentum, weight etc.
Differences between Displacement and Distance.
Displacement |
Distance |
(i) It is vector quantity. (ii) It can be negative and positive. (iii) It can be zero. (iv) It is the shortest distance between two points. |
(i) It is scalar quantity. (ii) It cannot be negative. (iii) It cannot be zero. (iv) It is total path covered between to point. |
Use of distance and displacement:
(i) The displacement and distance are used to describe the overall motion of an object.
(ii) They are used to locate its final position with reference to its initial position at a given time.
Motion along a straight line:
The simplest type of motion is the motion along a straight line.
When an object moves on a straight path, such a motion is called motion along a straight line or Linear motion.
Examples:
- A bus moving on a straight road.
- A train moving on straight road.
- A ball rolling along a straight path.
Type of Motion:
(i) Uniform Motion: As the object covers equal distance in equal intervals of time it is said to be in uniform motion.
Example:
- Motion of car.
- Flying of a bird.
- Motion of a sparing.
(ii) Non-uniform Motion: As the object does not covers equal distance in equal intervals of time it is said to be in uniform motion.
Uniform Motion:
When an object covers equal distance in equal intervals of time, it is said that the object is in uniform motion.
e.g: See table
Time | Distance travelled by object A in m | Distance travelled by object B in m |
9:30 am 9:45 am 10:00 am 10:15 am 10:30 am 10:45 am 11:00 am |
10 20 30 40 50 60 70 |
12 19 23 35 37 41 44 |
Object A covers equal distance in equal time interval, while object B does not cover equal distance in equal time interval. Therefore it is said that object A is in uniform motion and object B is in non-uniform motion.
Finding rate of motion:
Rate of motion is found by the speed of an object.
Speed: The distance travelled by the object in per unit time is called speed.
The SI unit of speed is metre per second. symbolically it is written as m s-1 .
Avarage speed: The average speed of an object is obtained by dividing the total distance travelled by the total time taken.
If an object travels a distance s in time t then its speed v is,