Moving Objects – Maharashtra Board Class 9 Solutions for Science and Technology (English Medium)
- The shortest distance between initial and final points of movement of the body is called displacement of the body.
- In case of uniform motion, an object covers equal distance in equal intervals of time.
- Retardation means negative acceleration.
- The first kinematical equation gives relation between velocity and time.
- When body is performing uniform circular motion, its velocity changes at every point.
- A body is said to be in motion if it changes its position with respect to the surroundings. Otherwise, the body is said to be at rest.
- When we travel by a bus, we observe that the trees and the buildings by the side of the road move backwards, i.e. in the opposite direction. However, a person standing by the side of the road observes these trees and buildings to be at rest.
- When a person is sitting in a moving train, the fellow passenger sitting opposite him is at rest according to him. However, for an observer standing outside on the railway platform, the passenger in a moving train is in motion.
- These two examples show that an object may appear to be moving for one person and stationary for the other. It means that motion is a relative concept.
The examples of uniform circular motion are
- Motion of the electron around the nucleus of an atom.
- Motion of the Earth around the Sun.
- Motion of the Moon around the Earth.
- Motion of a vehicle along a curved path.
- The motion of the blades of a fan (when the speed of the fan is not changed).
The following examples show that the motion of a body cannot be perceived but can be experienced:
- The flow of blood through our veins and arteries.
- The motion of air cannot be perceived but can be experienced by wind.
- The rotational motion of the Earth around its axis and the revolution of the Earth around the Sun cannot be perceived but can be experienced by the occurrence of day and night and of different seasons.
- Distance is the length of the actual path followed by a body between the points under consideration. It can be greater than the length of the straight line joining the two points. It does not have direction, so it is a scalar quantity.
- Displacement is the shortest distance from the initial point to the final point of movement of a body. It has direction, so it is a vector quantity.
- Hence, we can say that distance and displacement are different concepts.
- An object is said to be in uniform motion when it has constant speed.
- For an object at rest, its speed is zero all the time, i.e. it has a constant speed. Therefore, an object at rest can be considered to have uniform motion.
- When a body falls freely to the ground, the only force acting on it is that due to gravity.
- This force acting on the body is uniform near the Earth’s surface. Hence, a body falling freely to the ground has a uniform acceleration.