Describing Motion Around Us
NCERT Class 9 Science • Chapter 4
Quick Answer
This Class 9 physics chapter introduces the description of motion: distance and displacement, speed and velocity, uniform and non-uniform motion, and acceleration. Students learn to interpret distance-time and velocity-time graphs and apply the equations of motion to objects moving in a straight line.
↓ Free PDF, NCERT solutions, summary, key terms, FAQs below.
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Key Terms
- Distance
- The total length of the path travelled by an object, regardless of direction. It is a scalar quantity.
- Displacement
- The shortest straight-line distance from the starting point to the final position of an object, with direction. It is a vector quantity.
- Speed
- The rate at which an object covers distance, calculated as distance divided by time. It is a scalar quantity.
- Velocity
- The rate of change of displacement, meaning speed in a specified direction. It is a vector quantity.
- Acceleration
- The rate of change of velocity with time. It occurs when an object speeds up, slows down, or changes direction.
- Uniform Motion
- Motion in which an object covers equal distances in equal intervals of time, so its speed remains constant.
- Equations of Motion
- A set of formulas relating displacement, initial and final velocity, acceleration, and time for uniformly accelerated straight-line motion.
Frequently Asked Questions
What is the difference between distance and displacement?▾
Distance is the total path length travelled and has no direction, making it a scalar. Displacement is the straight-line distance between the start and end points in a given direction, making it a vector.
What is the difference between speed and velocity?▾
Speed is how fast an object moves and is a scalar with only magnitude. Velocity is speed in a specific direction and is a vector, so it changes if direction changes even at constant speed.
What is acceleration?▾
Acceleration is the rate at which an object's velocity changes with time. An object accelerates when it speeds up, slows down (negative acceleration), or changes its direction of motion.
What does a distance-time graph tell us?▾
A distance-time graph shows how distance changes over time. A straight sloping line indicates uniform motion, and the slope of the line gives the speed of the object.
What is uniform and non-uniform motion?▾
In uniform motion an object covers equal distances in equal time intervals at constant speed. In non-uniform motion the distances covered in equal times are unequal, so the speed keeps changing.
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