NCERT Solutions for Class 9 Science Chapter 8 Motion: Motion is a relative term. That is if we are travelling in a bus a person sitting behind us is at rest with respect to us but for a person outside the bus, he is moving. We feel like trees are moving when we are travelling in a bus, this is due to relative motion. We can find such enormous example related to motion. The NCERT Solutions for Class 9 Science Chapter 8 starts with the concept of reference point. To specify the location of an object a reference point is necessary. For example, if a person tells my home is 60 Km north of the airport. Here the reference point is the airport. To specify the position of an object we need to choose a reference point and a direction. If in the previous case person tells my home is 60 Km from the airport then we can go 60 Km in any direction. To know the exact point specifying direction along with reference is also important.
Next topic of the chapter motion from NCERT Solutions for Class 9 Science is motion along a straight line. Here we will study the concept of distance and displacement. To understand the concept of distance and displacement mentioned in the NCERT Solutions let's consider an ant moving along a straight line. The ant starts from point O and moves a distance 50 cm and reaches point B, then it comes back a distance of 25 cm and reaches point A. We need to calculate the distance and displacement when the ant is at B and also when the ant is at A.
When the ant is at B the distance travelled = 50cm and displacement (shortest distance from the origin) = 50 cm. That is displacement = distance. When the ant comes back to A total distance travelled = OB+BA=50+25=75 cm but displacement = OA =25 cm. Here the displacement is less than distance. The main point to remember from this session of NCERT Solutions for Class 9 is that distance is always greater than or equal to displacement. The distance (path length) travelled cannot be zero but the displacement can be zero. In the above example of the ant comes back to O then displacement=0 but distance = 50+50 =100 cm.
The NCERT Solutions for Class 9 Science Chapter 8 Motion talks about uniform and nonuniform motion.
Uniform motion: A motion of an object is said to be uniform if it travels equal distance in equal interval of time. That is a 10 m during the first second, 10m during next second, 10 m during 3rd second and so on. That is in each second the object covers a distance of 10 m.
Nonuniform motion: When an object travels different distance in equal interval of time the motion is said to be nonuniform. A car moving in a high traffic road will be nonuniform motion.
Next concept of the NCERT Solutions for Class 9 Science Chapter 8 is speed and velocity. Certain points to remember from this topic are listed below
For example, if an ant moves along a rectangle as shown in figure 2 from point A to C through B in 5 seconds then what is the average speed and average velocity of the ant?
figure 2
Solution: The distance travelled by the ant = 3+4 =7m
average speed=7/5 = 1.4 m/s
The displacement = AC
AC is obtained using Pythagoras theorem
average velocity = displacement / total time taken= 5/5 =1 m/s
The next topic of the NCERT Solutions for Class 9 Science Chapter 8 Motion is acceleration:
The main topics of NCERT Solutions for Class 9 Science Chapter 8 Motion are listed below:
8.1 Describing Motion
8.1.1 Motion along a straight line
8.1.2 Uniform Motion and Nonuniform Motion
8.2.1 Speed with Direction
8.3 Rate of Change of Velocity
8.4 Graphical Representation of Motion
8.4.1 Distance Time Graphs
8.4.2 VelocityTime Graphs
8.5 Equations of Motion by Graphical Method
8.5.1 Equation for VelocityTime Relation
8.5.3 Equation for PositionVelocity Relation
8.6 Uniform Circular Motion
NCERT Solutions for Class 9 Science Chapter 8: Distance time graph of an object moving with uniform velocity
Figure 3
The distancetime graph for an object moving with uniform velocity is a straight line. The speed of the object can be determined as follows
Consider a small portion AB on the line. Draw a line parallel to xaxis from A and a line parallel to yaxis. Let both the lines meet at C, then and mark the corresponding points of A, B and C as shown in the above figure. The points are s1,s2,t1 and t2
NCERT Solutions for Class 9 Science Chapter 8 Motion: The velocitytime graph of an object moving with uniform velocity
figure 4
The graph of an object moving with uniform velocity is a straight line parallel to the xaxis. To find the distance travelled for a given time interval (say t2t1 as in the above figure) we have to calculate the area of the velocitytime graph. That is in the above figure the area ABCD gives the distance travelled in time t2t1.
NCERT Solutions for Class 9 Science Chapter 8 Motion: Velocitytime graph for a car moving with uniform acceleration
Figure 5
Velocitytime graph for a car moving with uniform acceleration is a straight line and the area of the graph gives the distance travelled during a time interval. In the above graph during t2t1 the distance travelled= area of triangle ADE+area of rectangle ABCD.
An important topic of the NCERT Solutions for Class 9 Science Chapter 8 is equations of motion for an object moving with uniform acceleration. If the initial velocity is u, the final velocity is v and t is the time taken and s is the distance travelled then the following are the three equations of motion.
NCERT Solutions for Class 9 Science Chapter 8: Uniform circular motion
Let's understand this through an example: If an ant moves along a circular path of radius 7 cm from point A to B through C and then to A through D as shown in figure 6 with a uniform speed of 1cm/sec
i)what is the distance travelled when the ant is at B. Is the distance equal to displacement?
Solution:
Displacement =AB=diameter =14cm. So distance not equal to displacement
ii)what is the total distance travelled and the final displacement?
Total distance travelled = circumference of the circle =
Displacement =0 as the initial and final position are the same
iii) What is the time taken by the ant to travel from A to B
The time = distance / speed = 22 / 1 = 22 sec
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NCERT Solutions for Class 9 Maths 
NCERT Solutions for Class 9 Science 
Q 9. State which of the following situations are possible and give an example for each of these:
(a) an object with a constant acceleration but with zero velocity
What is the nature of the distancetime graphs for uniform and nonuniform motion of an object?
Q 4. A motorboat starting from rest on a lake accelerates in a straight line at a constant rate of for 8.0 s. How far does the boat travel during this time?