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NCERT Class 9 Science Important Chapter 4 Describing Motion Around Us
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Describing Motion Around Us
Chapter – 4
| IMPORTANT QUESTION ANSWER |
Short Questions & Answers:
1. What is the simplest kind of motion?
Ans: Motion in a straight line (or linear motion) is the simplest kind of motion.
2. What is required first to describe the position of an object?
Ans: To describe the position of an object, we first need to specify a fixed reference point.
3. When is an object considered to be at rest?
Ans: An object is at rest if its position with respect to a reference point does not change with time.
4. Define displacement.
Ans: Displacement is the net change in the position of an object between two given instants of time.
5. What is the difference between scalar and vector quantities?
Ans: Scalar quantities require only a numerical value (magnitude) to be specified, whereas vector quantities require specifying both magnitude and direction.
6. What does it mean if an object is in “uniform motion”?
Ans: An object is in uniform motion if it moves in a straight line and travels equal distances in equal intervals of time.
7. How is average speed calculated?
Ans: Average speed is calculated by dividing the total distance travelled by the time interval taken to cover that distance.
8. What formula is used to calculate average velocity?
Ans: Average velocity is the change in position (displacement) divided by the time interval.
9. Under what condition is the magnitude of average velocity equal to average speed?
Ans: For motion in a straight line, average speed and the magnitude of average velocity are equal if the object moves in only one direction without turning back.
10. How is average acceleration defined?
Ans: Average acceleration is the change in velocity of an object divided by the time interval over which that change occurs.
11. If a vehicle is slowing down, what is the direction of its acceleration?
Ans: When the magnitude of velocity is decreasing, the average acceleration is in the opposite direction to the velocity.
12. What is the acceleration due to the gravitational force of the Earth?
Ans: The acceleration due to gravity is a constant value of 9.8 m s⁻² and is denoted by ‘g’.
13. On a position-time graph, what does a straight line parallel to the time axis represent?
Ans: It represents a stationary object (an object at rest) since its position is not changing with time.
14. What physical quantity is given by the slope of a position-time graph?
Ans: The slope of a line on a position-time graph gives the velocity of the object.
15. What physical quantity does the slope of a velocity-time graph represent?
Ans: The slope of a straight line on a velocity-time graph represents the acceleration of the object.
16. How can you find the displacement of an object from its velocity-time graph?
Ans: The displacement can be calculated by finding the area enclosed by the velocity-time graph line and the time axis.
17. What are kinematic equations?
Ans: They are a set of equations that relate displacement, time interval, initial velocity, final velocity, and constant acceleration to describe the motion of an object.
18. What is motion in a plane also known as?
Ans: Motion in a plane, like a vehicle overtaking or a satellite in a circular path, is called motion in two dimensions.
19. Define uniform circular motion.
Ans: When an object moves in a circular path with a constant (uniform) speed, its motion is called uniform circular motion.
20. Why is uniform circular motion considered an accelerated motion despite having a constant speed?
Ans: Because the direction of the object’s velocity continuously changes at every point along the circular path, which results in acceleration.
Fill in the Blanks:
1. Motion in a straight line is called __________ motion.
Ans: Linear.
2. Average speed is equal to the total __________ travelled divided by the total time.
Ans: Distance.
3. The slope of a position-time graph gives the __________ of an object.
Ans: Velocity.
4. The acceleration due to gravity is denoted by the symbol __________.
Ans: g.
5. Motion in a circular path with constant speed is called __________ circular motion.
Ans: Uniform.
True or False:
1. An object is at rest if its position does not change with respect to a reference point.
Ans: True.
2. Displacement is a scalar quantity.
Ans: False.
3. The area under a velocity-time graph represents the displacement of the object.
Ans: True.
4. In uniform circular motion, the direction of velocity keeps changing.
Ans: True.
5. Average acceleration is the total distance travelled divided by the total time.
Ans: False.
Long Questions & Answers:
1. Describe the significance of position-time graphs and velocity-time graphs. Specifically, explain what the slope and the area under the curve represent in these graphical representations of motion.
Ans: (i) Position-Time Graphs: A position-time graph shows how the position of an object changes with time. A straight-line graph indicates that the object is moving with a constant velocity, whereas a curved line indicates that the velocity is changing (accelerated motion). The steepness, or the slope of the line on a position-time graph, gives information about the rate of change of position, which represents the magnitude of the object’s velocity.
(ii) Velocity-Time Graphs: A velocity-time graph shows how an object’s velocity changes over time. If the graph is a straight line parallel to the time axis (X-axis), it indicates that the velocity is constant and acceleration is zero. If the graph is a sloped straight line, it indicates a constant acceleration.
(iii) Slope and Area in Velocity-Time Graphs: The slope of the line on a velocity-time graph tells us how fast the velocity is changing, which represents the acceleration of the object. Furthermore, the area enclosed by the velocity-time graph and the time axis for a specific time interval is equal to the displacement of the object during that time interval.
2. Describe the step-by-step procedure to plot a position-time graph. What do a straight line and a curved line on this graph indicate about the nature of an object’s motion?
Ans: To plot a position-time graph, you must follow these steps:
(i) Set up the axes: On a graph paper, draw a horizontal line called the X-axis and a vertical line called the Y-axis. The point where they intersect is the origin (O).
(ii) Choose the quantities and scale: Time is typically plotted along the X-axis, and position is plotted along the Y-axis. Choose a suitable scale for both axes to fit the data effectively on the paper (for example, 5 divisions = 1 s on the X-axis, and 5 divisions = 20 m on the Y-axis).
(iii) Mark values and plot points: Mark the numerical values for time and position along their respective axes. For each data pair (e.g., at 1 s, the position is 20 m), find the time on the X-axis and move parallel to the Y-axis to match the corresponding position value. Mark the intersecting point.
(iv) Draw the graph: Connect all the plotted points to create the graph.
Interpretation of the shape:
(i) If the resulting graph is a straight line, it indicates that the object is moving with a constant velocity (it covers equal displacements in equal time intervals).
(ii) If the graph is a curved line, it indicates that the velocity is not constant, meaning the object is in accelerated motion.
3. Explain the three different motion scenarios that can be represented by a straight line on a velocity-time graph. What specific physical quantities do the slope and the area under the curve represent?
Ans: A straight line on a velocity-time graph can represent three specific scenarios of constant acceleration:
(i) A horizontal line parallel to the X-axis (time axis): This indicates that the velocity is completely constant over time. Because the velocity does not change, the acceleration is zero.
(ii) A line sloping upwards: This indicates that the velocity is increasing by equal amounts in equal time intervals. The object has a constant positive acceleration acting in the same direction as the velocity.
(iii) A line sloping downwards: This indicates that the velocity is decreasing uniformly over time. The object has a constant negative acceleration acting in the opposite direction to its velocity.
Slope and Area:
(i) The slope (steepness) of the straight line on a velocity-time graph represents the object’s acceleration (the rate of change of velocity).
(ii) The area enclosed by the velocity-time graph and the time axis represents the displacement of the object during that specific time interval.
4. Contrast the motion of an athlete running on a rectangular track with one running on a circular track. Why does a marble spinning inside a circular ring suddenly travel in a straight line if the ring is lifted?
Ans: (i) Rectangular vs. Circular Track: When an athlete runs on a rectangular track, they can maintain a uniform speed and a constant direction along the straight sections, only needing to sharply change their direction four times (at the corners). However, if the track has an infinite number of sides, it becomes a circle. On a circular track, even if the athlete runs at a constant speed, the direction of their velocity is continuously changing at every single point. Because a change in direction means a change in velocity, the motion on a circular track is always considered accelerated motion.
(ii) The Marble Experiment: When a marble moves inside a ring, the ring’s boundary constantly forces it to change direction, keeping it in a circular path. The velocity of the object at any point in a circular path is directed along the tangent to the circle at that point. If the ring is suddenly lifted, the physical force changing its direction is removed. Consequently, the marble will simply continue moving in the exact direction it was heading at that specific instant, causing it to shoot off in a straight tangential line.
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