CLASS: IX NCERT (CBSE)
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
