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Motion
¤ When a body covers equal distances in equal intervals of time then the body is said to describe uniform motion.
¤ When a body moves unequal distances in equal intervals of time or vice-versa, then the body is said to describe non-uniform motion.
¤ Speed can be defined as the distance covered by a moving object in unit time
Uniform Speed : An object is said to be moving with uniform speed if it covers equal distances in equal intervals of time.
Non-uniform : An object is said to be moving with variable speed or non-uniform speed if it covers equal distances in unequal intervals of time or vice-versa.
Average speed : When we travel in a vehicle the speed of the vehicle changes from time to time depending upon the conditions existing on the road. In such a situation, the speed is calculated by taking the ratio of the total distance travelled by the vehicle to the total time taken for the journey. This is called the average speed.
Instantaneous speed : When we say that the car travels at an average speed of 60 km/h it does not mean that the car would be moving with the speed of 60 km/h throughout the journey. The actual speed of the car may be less than or greater than the average speed at a particular instant of time.
The speed of a moving body at any particular instant of time, is called instantaneous speed.
Velocity is defined as the distance travelled in a specified direction in unit time. The distance travelled in a specified direction is displacement.
Therefore, velocity can be defined as the rate of change of displacement .
Velocity is defined as the distance covered by a moving object in a particular direction in unit time or speed in a particular direction.
¤ SI unit of velocity is m/s (metre/second). [
SI unit of distance is metre and that of time is second] ¤ Velocity is a vector quantity.
SI unit of distance is metre and that of time is second] ¤ Velocity is a vector quantity.
Acceleration : When the train starts from rest its speed increases from zero and we say that the train is accelerating. After sometime the speed becomes uniform and we say that it is moving with uniform speed that means the train is not accelerating. But as the train is nearing Mysore it slows down, which means the train is accelerating in negative direction. Again the train stops accelerating when it comes to a halt at Mysore .
Acceleration is defined as the rate of change of velocity of a moving body with time
¤ Acceleration = Rate of change of velocity with time
¤ The SI unit of velocity is m/s and time is s
¤ Acceleration is a vector quantity
Positive Acceleration If the velocity of an object increases then the object is said to be moving with positive acceleration. Example: A ball rolling down on an inclined plane.
Negative Acceleration : If the velocity of an object decreases then the object is said to be moving with negative acceleration. Negative acceleration is also known as retardation or deceleration. Example: (1) A ball moving up an inclined plane. 2) A ball thrown vertically upwards is moving with a negative acceleration as the velocity decreases with time
Zero Acceleration : If the change in velocity is zero, i.e., either the object is at rest or moving with uniform velocity, then the object is said to have zero acceleration. Example: a parked car, a train moving with a constant speed of 90 km/hr
Uniform Acceleration : If the change in velocity in equal intervals of time is always the same, then the object is said to be moving with uniform acceleration. Example: a body falling from a height towards the surface of the earth.
Non-uniform or Variable Acceleration : If the change in velocity in equal intervals of time is not the same, then the object is said to be moving with variable acceleration.
Uniform velocity : A body is said to be moving with uniform velocity if it covers equal distances in equal intervals of time in a specified direction.
Variable velocity A body is said to be moving with variable velocity if it covers unequal distances in equal intervals of time and vice-versa in a specified direction or if it changes the direction of motion.
Circular motion : Motion along circular track is called circular motion .
An object moving along a circular track with uniform speed is an example for a non - uniform motion because the direction of motion of the object goes on changing at every instant of time. Example - A car negotiating a curve with uniform speed
A circle can be considered as a polygon with infinite sides and hence motion along a circular path is classified as non-uniform motion.
Summary
MOTION Whenever a body changes its position continuously with respect to the position of other bodies around it then it is said to be in motion
Rest: An object which does not change its position with respect to its surroundings is said to be at rest
Distance: It is the actual length of the path covered by a moving object. Distance is a scalar quantity. SI unit of distance is metre.
Displacement: It is the shortest distance covered by a moving object from the point of reference, in a specified direction. Displacement is a vector quantity. SI unit of displacement is metre.
Uniform motion: When a body covers equal distances in equal intervals of time then the body is said to describe uniform motion
Non-uniform motion: When a body moves unequal distances in equal intervals of time or vice-versa, then the body is said to describe non-uniform motion
Speed: It is the distance covered by a moving object in unit time. Speed is scalar quantity.
SI unit of speed is m/s.
Instantaneous speed: It is the speed which the body possesses at a particular instant of time 
The speed of a moving object can be determined by calculating the slope of the S-t graph. The S-t graph tells us about the nature of motion. For example, if S-t graph is a straight line then it means that the object is moving with uniform speed.
Velocity is defined as the distance covered by a moving object in a particular direction in unit time. Velocity is a vector quantity. SI unit of velocity is m/s. Velocity-time graph is a straight line for a uniform motion.
Acceleration is the rate of change of velocity with time. Acceleration is positive if velocity is increasing and negative if velocity is decreasing with time. Negative acceleration is also known as retardation. SI unit of acceleration is m/s2.
A body moving with uniform velocity has no acceleration
Area below v-t graph gives the distance covered. Slope of v-t graph gives the acceleration The equations of motion are v = u + at
If the body moves with uniform velocity then a = 0 If the body starts from rest, then u = 0 If the body comes to rest, then v = 0
FAQs on Motion, Chapter Notes, Class 9, Science
| 1. What is motion? |  |
Ans. Motion is the change in the position of an object with respect to its surroundings in a given time frame. It is the movement of an object from one place to another.
| 2. What are the different types of motion? | |
Ans. There are three main types of motion:
- Translational motion: It is the motion in which an object moves along a straight line or a curved path without any rotation.
- Rotational motion: It is the motion in which an object spins or rotates around an axis.
- Vibrational motion: It is the motion in which an object oscillates or moves back and forth around its mean position.
| 3. What are the units used to measure motion? | |
Ans. The units used to measure motion depend on the aspect of motion being measured. Some commonly used units include:
- Distance: Kilometer (km), meter (m), centimeter (cm), etc.
- Speed: Meter per second (m/s), kilometer per hour (km/h), etc.
- Time: Second (s), minute (min), hour (h), etc.
- Acceleration: Meter per second squared (m/s²)
| 4. What is the difference between speed and velocity? | |
Ans. Speed and velocity both measure how fast an object is moving, but there is a key difference between the two:
- Speed: It is a scalar quantity that only considers the magnitude of the object's motion. It is the rate at which an object covers distance.
- Velocity: It is a vector quantity that considers both the magnitude and direction of the object's motion. It is the rate at which an object changes its position.
| 5. What is uniform motion? | |
Ans. Uniform motion refers to the motion of an object in which it covers equal distances in equal intervals of time. In other words, the object moves at a constant speed and in a straight line without any change in direction.
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