Q: Question 26: (a) What term is used to denote the change of velocity with time? (b) Give one word which means the same as 'moving with a negative acceleration'. (c) The displacement of a moving object in a given interval of time is zero. Would the distance travelled by the object also be zero? Give reason for your answer.

Answer: (a) Acceleration is used to denote the change in velocity with time. (b) Retardation is the same as ‘moving with a negative acceleration'. (c) Displacement is a vector quantity, so it can be zero for two reasons: 1. When the body doesn't move at all: In this case, the distance travelled will also be zero. 2. When the body comes back to its initial position: In this case, the distance travelled is non-zero, but the displacement is zero.

Q: Question 32: An aircraft travelling at 600 km/h accelerates steadily at 10 km/h per second. Taking the speed of sound as 1100 km/h at the aircraft's altitude, how long will it take to reach the 'sound barrier'?

Answer: Final velocity, v = 1100 km/h = 1100 × 5/18 = 305.55 m/s Initial velocity, u = 600 km/h = 600 × 5/18 = 166.66 m/s Acceleration = 10 km/h per second = 10 × 5/18 = 2.77 m/s2 Time taken by body, v = u + at t = (v - u)/a t = (305.55 -166.66 )/2.77 t = 50.14 sec t ≃ 50 sec

Q: Question 35: Define motion. What do you understand by the terms 'uniform motion' and 'non-uniform motion'? Explain with examples.

Answer: A body is said to be in motion when its position changes continuously with respect to a stationary point taken as the reference point. Uniform motion: A body is said to be in uniform motion if it travels equal distances in equal intervals of time in a particular direction, no matter how small these time intervals are. For example, a car running at a constant speed of 10 m/s towards east will cover the equal distance of 10 m every second towards east, so its motion will be uniform. Non-uniform motion: A body is said to be in non-uniform motion if it travels unequal distances in equal intervals of time. For example, motion of a freely falling ball from the roof of a tall building.

Q: Question 36: (a) Define speed. What is the SI unit of speed? (b) What is meant by (i) average speed, and (ii) uniform speed?

Answer: (a) Speed of a body is the distance travelled by it per unit time. The S.I. unit of speed is m/s. (b) (i) Average speed of a body is the total distance travelled by it divided by the total time taken by it to cover the given distance. Average speed =Total time takenTotal distance travelled (ii) A body has a uniform speed if it travels equal distance in equal intervals of time, no matter how small these time intervals are.

Q: Question 38: (a) What is meant by the term 'acceleration'? State the SI unit of acceleration. (b) Define the term 'uniform acceleration'. Give one example of a uniformly accelerated motion.

Answer: (a) Acceleration of a body is defined as the rate of change of its velocity with respect to time. It is a vector quantity. The S.I. unit of acceleration is (m/s2) . (b) A body has uniform acceleration if it travels in a straight line and its velocity increases by equal amounts in equal intervals of time. For example, a freely falling body has uniform acceleration.

Question 1

Question 21:
What type of motion, uniform or non-uniform, is exhibited by a freely falling body? Give reason for you answer.

Accepted Answer

Answer:
A freely falling body exhibits non-uniform motion. Its velocity increases at a constant rate, so it shows uniformly accelerated motion.

Question 2

Question 22:
State whether speed is a scalar or a vector quantity. Give reason for your choice.

Accepted Answer

Answer:
Speed is a scalar quantity. This is because it has magnitude, but it does not specify direction. It is the distance travelled by a body per unit time.

Question 3

Question 23:
Bus X travels a distance of 360 km in 5 hours whereas bus Y travels a distance of 476 km in hours. Which bus travels faster?

Accepted Answer

Answer:
We should first consider Bus X.
Distance travelled (d1) = 360 km
Time taken (d1) = 5 hr
So, we can calculate the speed as:
Speed = TimeDistance ​
Thus, the speed of Bus X is 72 km/h.     
Similarly,
For Bus Y:
Distance travelled  
Time taken (t2) = 7 hr
So, we can calculate the speed as:
 Speed = TimeDistance ​
Thus, the speed of Bus Y is 68 km/h.
Speed of Bus X is more than that of Bus Y. Hence, Bus X travels faster.

Question 4

Question 24:
Arrange the following speeds in increasing order (keeping the least speed first):

(i) An athlete running with a speed of 10 m/s.
(ii) A bicycle moving with a speed of 200 m/min.
(iii) A scooter moving with a speed of 30 km/h.

Accepted Answer

Answer:
We have three different moving bodies. To compare their speeds, we need to express them in similar S.I. units.
(i) An athlete running with a speed of 10 m/s.
(ii) A bicycle moving with a speed of 200 m/min. Converting into S.I. units, the speed will be:
=60200​=3.33 m/s
(iii) A scooter moving with a speed of 30 km/h. Converting into S.I. units, the speed will be:
=3600(30)(1000)​=8.33 m/s
On arranging the speeds in ascending order, we get:
(ii) < (iii) < (i)

Question 5

Question 26:
(a) What term is used to denote the change of velocity with time?
(b) Give one word which means the same as 'moving with a negative acceleration'.
(c) The displacement of a moving object in a given interval of time is zero. Would the distance travelled by the object also be zero? Give reason for your answer.

Accepted Answer

Answer:
(a) Acceleration is used to denote the change in velocity with time.
(b) Retardation is the same as ‘moving with a negative acceleration'.
(c) Displacement is a vector quantity, so it can be zero for two reasons:
1. When the body doesn't move at all: In this case, the distance travelled will also be zero.
2. When the body comes back to its initial position: In this case, the distance travelled is non-zero, but the displacement is zero.

Question 6

Question 32:
An aircraft travelling at 600 km/h accelerates steadily at 10 km/h per second. Taking the speed of sound as 1100 km/h at the aircraft's altitude, how long will it take to reach the 'sound barrier'?

Accepted Answer

Answer:
Final velocity, v = 1100 km/h = 1100 × 5/18 = 305.55 m/s
Initial velocity, u = 600 km/h = 600 × 5/18 = 166.66 m/s
Acceleration = 10 km/h per second = 10 × 5/18 = 2.77 m/s2
Time taken by body,
v = u + at
t = (v - u)/a
t  = (305.55 -166.66 )/2.77
t = 50.14 sec 
t ≃ 50 sec

Question 7

Question 35:
Define motion. What do you understand by the terms 'uniform motion' and 'non-uniform motion'? Explain with examples.

Accepted Answer

Answer:
A body is said to be in motion when its position changes continuously with respect to a stationary point taken as the reference point.

Uniform motion: A body is said to be in uniform motion if it travels equal distances in equal intervals of time in a particular direction, no matter how small these time intervals are.
For example, a car running at a constant speed of 10 m/s towards east will cover the equal distance of 10 m every second towards east, so its motion will be uniform.

Non-uniform motion: A body is said to be in non-uniform motion if it travels unequal distances in equal intervals of time.
For example, motion of a freely falling ball from the roof of a tall building.

Question 8

Question 36:
(a) Define speed. What is the SI unit of speed?
(b) What is meant by (i) average speed, and (ii) uniform speed?

Accepted Answer

Answer:
(a) Speed of a body is the distance travelled by it per unit time. The S.I. unit of speed is m/s.
(b) (i) Average speed of a body is the total distance travelled by it divided by the total time taken by it to cover the given distance.
           Average speed =Total time takenTotal distance travelled​
      (ii) A body has a uniform speed if it travels equal distance in equal intervals of time, no matter how small these time intervals are.

Question 9

Question 38:
(a) What is meant by the term 'acceleration'? State the SI unit of acceleration.
(b) Define the term 'uniform acceleration'. Give one example of a uniformly accelerated motion.

Accepted Answer

Answer:
(a) Acceleration of a body is defined as the rate of change of its velocity with respect to time. It is a vector quantity.    The S.I. unit of acceleration is  (m/s2) .
(b) A body has uniform acceleration if it travels in a straight line and its velocity increases by equal amounts in equal intervals of time.
For example, a freely falling body has uniform acceleration.

Chapter 1 : Motion