Class 8 Basic Science (English Medium) Chapter 09 Motion - Questions and Answers
Class 8 Physics - Questions and Answers
Chapter: 09 - Motion
* Motion
If the position of an object changes with respect to time, the body is said to be in motion. In order to state whether a body is in motion or at rest, we need a reference body. Reference body is the object with respect to which the state of rest or motion of an object is described.
An object which is in motion for one person may be at rest for another person.
This can be explained with a simple example. Consider a passenger in a running train. Suppose he is looking at his co-passenger who is sitting in the opposite seat. According to him the co – passenger’s position remains the same even though the train is running fast. That is, he is at rest with respect to the train. But according to a man who is standing on the platform, the co-passenger is in motion. That is, when one considers the train as a reference body, the passenger is at rest. But the other one, then considers the platform as the reference body, the passenger is in motion.
* Distance and displacement.
Distance is the length of the path. Displacement is the straight-line distance
between an initial position and the final position.
Unit of distance and displacement is metre (m).
Here path length is AB + BC = 3+4 = 7m
So the distance is 7 m.
But the straight line distance from initial position A and final position C is only 5m.
Therefore displacement of the object is 5m.
Displacement never is greater than distance.
If an object moves in the same direction, its distance and displacement will be equal.
Example: Consider a stone falling from a height of 10 m. When it reaches the ground, both the distance and displacement of the stone will be 10 m.
* Vector quantities and scalar quantities.
Physical quantities having both direction and magnitude are called vector quantities. Displacement is a vector quantity.
Physical quantities having only magnitude are called scalar quantities. Distance is a scalar quantity.
* Speed and Velocity.
Speed is the distance travelled in unit time (in one second).
Speed = distance/time. Speed is a scalar quantity.
Velocity is the displacement in unit time.
Velocity = displacement/time.
Velocity is a vector quantity. Unit of speed and velocity is the same and is m/s.
Let us consider Figure.1 above. If the object takes 10 seconds to travel from A to C,
Its speed = 7/10 = 0.7 m/s and Velocity = 5/10 = 0.5 m/s
If an object moves in the same direction, velocity and speed will be equal.
* Uniform speed and non-uniform speed.
If an object travels an equal distance in an equal interval of time, the speed is said to be uniform.
The distance travelled by car is depicted below.
is 10/2 = 5 m/s.
The movement of the hands of a clock is an example of an object having uniform speed.
If an object covers the different distances in equal intervals of time, it is said to have a non-uniform speed.
A ball rolls along the floor, freely falling body etc are examples of objects having non-uniform speed.
* Average speed.
In the case of a vehicle running along a road, its speed will be different at different times. It can be observed from the speedometer. In this situation, the average speed is calculated for getting an idea about the effective speed of the
vehicle.
Average speed = total distance travelled/total time.
Uniform velocity and non-uniform velocity.
If a body has equal displacement in the same direction in an equal interval of time, its velocity is said to be uniform.
Examples for the uniform motion:
1. The train travels along a straight track at the same speed.
2. Light travels through a vacuum.
If there is a change either in speed or direction, the velocity is non-uniform.
It is noted that velocity may not be uniform even if speed is uniform.
For example, the speed of the hands of a clock is uniform, but its velocity is non-uniform as its direction of motion keeps on changing.
* Acceleration.
The rate of change in velocity is acceleration. It is noted that acceleration is non-zero only if its velocity is non-uniform.
Acceleration, a = change in velocity/time = (final velocity – initial velocity)/ time = (v – u)/t
Acceleration is a vector quantity. Its unit is m/s².
Freely falling of an object is an example of accelerated motion. This acceleration is due to the gravitational force of the earth. Acceleration of a freely falling object is called acceleration due to gravity. It is represented by the letter ‘g’
Sample Question.:1. The velocity of a moving object at points A and B are marked below.
Initial velocity u = 8 m/s
Final velocity, v = 20 m/s
Time = 4 s
Acceleration, a = (v-u)/t = (20 – 8)/4 = 12/4 = 3 m/s²
Sample Question. 2. A food packet dropped from a helicopter reaches the ground in 5 s. If the velocity with which it hits the ground is 50 m/s, what is its acceleration?
Initial velocity u = 0
Final velocity v = 50 m/s
Time t = 5 s
Acceleration, a = (v-u)/t = (50 – 0)/5 = 10 m/s²
* Retardation.
Negative acceleration is called retardation. When the velocity of an object decreases with time, its acceleration will be negative.
Situations for negative acceleration:
i. A ball rolling on level ground.
ii. Motion of a vehicle when the brake is applied.
iii. A stone projected up.
* Two situations related to motion are given below. Find out the Frame of reference in each case
A passenger in a running bus is at rest
A crow sitting on a cow grazing in a field
What do you mean by a frame of reference
Ans.
A passenger in a running bus is at rest - Frame of reference BUS
A crow sitting on a cow grazing in a field - Frame of reference field
Definition -Reference body is the object with respect to which the state of rest or motion of an object is described.
* The figures of two methods in which a person started the journey from A and reached C through B are given below
a) In figure1, What is the total distance travelled? What is displacement?
b) In figure2, What is the total distance travelled? What is its displacement?
c) Comparing the measurements, in which situation, the distance and displacement are equal?
Ans.
a) In figure 1, Total distance travelled = 100m
displacement = 80m
b) In figure 2, total distance travelled is 100m, displacement = 100m
c) when an object travelling in a straight line, its distance and displacement will be equal * Find out whether the acceleration in the following situation is positive or negative?
a. A ball rolling on a smooth floor
b. A train started a journey from a station
c. An object falling freely due to the attraction of the earth
Ans.
a. A ball rolling on a smooth floor - Negative acceleration
b. A train started a journey from a station -Positive acceleration
c. An object falling freely due to the attraction of earth - Positive acceleration
* A path AB has a distance of 30m. An object travels from A to B and then B to A. It took 4 seconds to travel from A to B and 6 seconds for B to A
a) Find out the velocity when it reaches from A to B?
b) Find out the average speed of the object?
Ans.
a) velocity of the object when it reaches from A to B = 30/4 = 7.5 m/s
b) Average speed = total distance/ total tiem = 60 / 10 = 6 m/s
* A boy started from A and reaches C through B in 10 seconds
a) What is the distance travelled by the boy? What is his displacement?
b) What is the velocity of the boy?
c) Write a situation in which speed and velocity are equal.
Ans.
a) Distance travelled by the boy is 120 m and displacement is 80 m
b) velocity of the boy = displacement / time = 60m /10s = 6 m/s
c) When a body travels in a straight line in the same direction
* Tabulate the following statements into Speed and Velocity
a) Displacement in unit time
b) Distance travelled in unit time
c) Scalar quantity
d) Vector quantity
Ans.
a) Displacement in unit time - Velocity
b) Distance travelled in unit time - Speed
c) Scalar quantity - Speed
d) Vector quantity - Velocity
* A car started from rest and travels in uniform acceleration and its velocity becomes 40m/s in 10 seconds
a) What is the initial velocity of the car?
b) Find out the acceleration of the car?
Ans.
a) initial velocity of the car = 0
b) acceleration of the car = 40 m/s - 0 m/s / 10 s
= 4 m/s²
* Fill in the blanks suitably
a) distance = m
velocity = ....................
Ans. velocity = m/s
* Analyse the following statements and write whether it is Uniform velocity or Non-uniform velocity
a) magnitude of velocity does not change but direction changes
b)Neither magnitude nor direction of velocity changes
c) the magnitude of velocity changes but its direction is not changing
d) the direction and magnitude of velocity changes
Ans.
a) magnitude of velocity doesn't change but direction changes -Non-uniform velocity
b)Neither magnitude nor direction of velocity changes - Uniform velocity
c) the magnitude of velocity changes but its direction is not changing - Non-uniform velocity
d) the direction and magnitude of velocity changes - Non-uniform velocity
* A car travels with a velocity of 10m/s. When the brake is applied the car came to rest in 4 seconds
a) What is the change in velocity of the car?
b)What is its retardation?
Ans.
a) 10m/s - 0 m/s - 10m/s
b) 0m/s - 10m/s / 4s
=2.5m/s²
* Find out the wrong statements from the following and correct it
a) Velocity is the displacement in unit time,
b) if the direction of motion of a body changes its, velocity changes
c) acceleration is a scalar quantity
Ans. acceleration is a vector quantity
* Match the columns A, B, C
|
A |
B |
C |
|
Speed |
m/s2 |
displacement/time |
|
acceleration |
m/s |
mass/volume |
|
density |
m3 |
distance / time |
|
|
kg/m3 |
change in
velocity / time |
Ans.
|
A |
B |
C |
|
Speed |
m/s |
distance /time |
|
acceleration |
m/s2 |
change in
velocity/ time |
|
density |
kg/m3 |
mass/volume |
|
|
|
|
* Find out the odd one
(velocity, acceleration, speed, displacement)
Ans. speed = it is a scalar quantityMOTION: PRACTICE QUESTIONS & ANSWERS.
1. The object with respect to which the state of rest or motion of an object is described is called .........
Answer: Reference body.
2. Acceleration of a body having uniform velocity is ........
Answer: Zero.
3. Initial velocity of a freely falling body is ......
Answer: Zero.
4. A stone that was thrown vertically up returned to the hand after raising 10 m. Find distance and displacement.
Answer: Distance = 10+10 = 20 m Displacement = 0
5. “Distance is always greater than displacement" Comment to this statement.
Answer: The statement is wrong. If an object moves in the same direction, the magnitude of distance and displacement will be equal.
6. When has the distance and displacement become equal?
Answer: Distance and displacement will be equal if the object moves without changing its direction.
7. An object travels from P and reaches R via Q with uniform speed. If it takes 10 s for the travel,
b. What is its displacement? c. Find its speed and velocity.
d. Is the velocity of the object uniform? Justify your answer.
Answer:
a. Distance =140 m. b. Displacement = 110 m
c. i.Speed = distance/time = 140/10 = 14 m/s
ii. Velocity = displacement/time = 110/10 = 11 m/s
d. No. As the direction of motion changes at Q, the velocity is not uniform.
8. Give a few examples for vector quantities.
Answer: Displacement, velocity, acceleration.
9. Write down one difference and one similarity between velocity and speed.
Answer:
i. Similarity:- The unit of both velocity and speed is m/s.
ii. Difference: Velocity is a vector quantity but speed is a scalar quantity.
10. Give two examples for uniform speed.
Answer:
i.The movement of the tip of the hand of a watch.
ii. Speed of light travels through a vacuum.
11. A car is travelling along a circular path with uniform speed. Is its velocity uniform? Why?
Answer: The velocity of the car is not uniform. Because its direction of motion keeps on changing.
12. If the displacement of a person travelling from A to B is 100 m, which of the following is not likely to be his distance. Why? 80m, 100 m, 150 m, 160 m.
Answer: 80 m. Because distance never be less than displacement.
13. Check whether the following statements are TRUE or FALSE. Correct the false ones.
a. Distance never be greater than displacement.
b.If an object keeps on moving in the same direction, distance and displacement will be equal.
c. If velocity is uniform, speed will definitely be uniform.
d. The velocity of a body may not be uniform even if it moves with uniform speed.
e. The magnitude of distance and displacement never be equal.
f.Units of velocity and speed are the same.
g.The magnitude of velocity and speed of an object will be equal if it moves without changing its direction.
h. The movement of an object along a circular path with uniform speed is an accelerated motion.
Answer:
a. False. Displacement never is greater than distance.
b. True.
c. True.
d. True.
e. False. The magnitude of distance and displacement will be the same if it travels in the same direction.
f. True.
g. True.
h. True.
14. "Distance and displacement of an object moves along a straight line will always be equal.” Comment to this statement.
Answer: The distance and displacement in respect of a body moving along a straight line may or may not be equal.
For example, in the case of a body thrown vertically up and returned back, the magnitude of distance and displacement is different though it is moved along a straight line.
15. .... and .... are the devices that make changes in the velocity of vehicles. Answer: Accelerator, Brake.
16. Rate of change in velocity is called .....
Answer: Acceleration.
17. a. Write down the equation for finding out acceleration. b. What is the unit of acceleration?
Answer:
a. Acceleration = (final velocity – initial velocity)/time = (v – u)/t
b. The unit of acceleration is m/s².
18. Give two situations where acceleration occurs.
Answer:
i. Freely falling stone.
ii. A ball rolls down along an inclined plane.
19. A stone falls freely from the height reached the ground in 5 s. Calculate its acceleration if it hit the ground with a velocity of 50 m/s.
Answer:
Initial velocity of the stone ,u = 0
Final velocity v = 50 m/s
Time t = 5 s
Acceleration a = (v – u)/t = (50 – 0)/5 = 50/5 = 10 m/s²
20. If a body is thrown up, the velocity when it reaches the topmost position is ......
Answer: Zero.
21. What is retardation? Give a situation where retardation occurs.
Answer: The negative acceleration is called retardation.
Example:- The motion of a stone thrown up.
22. Name the device which makes retardation in a vehicle.
Answer: Brake.
23. What is the acceleration due to gravity? Which is the letter used to represent it?
Answer: The acceleration produced in a freely falling object due to the gravitational force of the earth is called acceleration due to gravity. It is represented by the letter 'g'.
24. If acceleration is negative, its speed and velocity .... (increases/decreases)
Answer: decreases.
25. Give four situations that cause road accidents.
Answer:
i. Over speed
ii. Mixing drink with drive.
iii. Using a phone while driving.
iv. Negligence in using indicators.
26. List out the precautions to be taken by the pedestrians for minimising road accidents.
Answer:
i. Walks only along the right side of the road.
ii. Crossroads only at the zebra crossing.
iii. Before crossing the road, make sure there is no vehicle coming from both sides.
iv. Walk only along the footpath.
27. Find out the strange one from the following. How is it different from others?
(speed, velocity, acceleration, displacement)
Answer: Speed. All others are vector quantities.
28. A lorry travelling with a velocity of 30 m/s came to rest in 10 s. What is its acceleration?
Answer: Acceleration, a = (final velocity – initial velocity)/time = (v – u)/t = ( 0 – 30)/10 = -3 m/s²
29. What is the displacement of the car in 30 s if it is travelling with a uniform velocity of 15 m/s?
Answer: Velocity = displacement/time
Or Displacement = velocity x time = 15x30 = 450 m.
30. A car travelled the first 400 m distance with a speed of 8 m/s, the next 1200 m with a speed of 10 m/s and the last 360 m with a speed of 12 m/s along a straight road.
a. What is the total distance covered by the car? b. How long did it take to complete the travel?
c. Calculate the average speed of the car.
Answer:
a. Total distance = 400 +1200+ 360 = 1960 m.
b. i. Time required for first 400 m travel = 400/8 = 50 s
ii. Time for the next 1200 m = 1200/10 = 120 s
iii. Time for the last 360 m distance = 360/12 = 30 s
Total time of travel = 50+120+30 = 200 s
c. Average speed of the car = 1960/200 = 9.8 m/s.
31.” All objects having uniform speed need not have uniform velocity.” Describe with the help of an example.
Answer: The statement is correct. For example, the speed of the hands of a watch is uniform. But its velocity is non-uniform as its direction keeps on changing.
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