# NCERT Solutions for Class 9 Science Unit 8

## Motion Class 9

### Unit 8 Motion Exercise Solutions

Motion:- If an object changes its position with respect to a reference point with elapse of time, the object is said to be in motion.
Rest:- When an object does not change its position with respect to a reference point with elapse of time, the object is said to be in rest.
Example When a vehicle changes its position with respect to an electric pole (a reference point) with elapse of time, then vehicle is called in motion. And if the same vehicle does not change its position with respect to that electric pole, the vehicle is called in the state of rest.
Thus to observe the motion of any object; two things are necessary a reference point and time.

### Question : Solutions of Questions on Page Number : 100

Q1 :

A farmer moves along the boundary of a square field of side 10 m in 40s. What will be the magnitude of displacement of the farmer at the end of 2 minutes 20 seconds?

The farmer takes 40 s to cover 4 x 10 = 40 m.

In 2 min and 20 s (140 s), he will cover a distance Therefore, the farmer completes rounds (3 complete rounds and a half round) of the field in 2 min and 20 s.

That means, after 2 min 20 s, the farmer will be at the opposite end of the starting point.

Now, there can be two extreme cases.

Case I: Starting point is a corner point of the field.

In this case, the farmer will be at the diagonally opposite corner of the field after 2 min 20 s.

Therefore, the displacement will be equal to the diagonal of the field.

Hence, the displacement will be Case II: Starting point is the middle point of any side of the field.

In this case the farmer will be at the middle point of the opposite side of the field after 2 min 20 s.

Therefore, the displacement will be equal to the side of the field, i.e., 10 m.

For any other starting point, the displacement will be between 14.1 m and 10 m.

Q2 :

Which of the following is true for displacement?

(a) It cannot be zero.

(b) Its magnitude is greater than the distance travelled by the object.

(a) Not true

Displacement can become zero when the initial and final position of the object is the same.

(b) Not true

Displacement is the shortest measurable distance between the initial and final positions of an object. It cannot be greater than the magnitude of the distance travelled by an object. However, sometimes, it may be equal to the distance travelled by the object.

Q3 :

Distinguish between speed and velocity.

Q4 :

Under what condition(s) is the magnitude of average velocity of an object equal to its average speed?

Q5 :

What does the odometer of an automobile measure?

Q6 :

What does the path of an object look like when it is in uniform motion?

Q7 :

During an experiment, a signal from a spaceship reached the ground station in five minutes. What was the distance of the spaceship from the ground station? The signal travels at the speed of light, that is, 3 ÃƒÆ’Ã¢â‚¬” 108m s - '1.

Q8 :

When will you say a body is in (i) uniform acceleration? (ii) non-uniform acceleration?

Q9 :

A bus decreases its speed from 80 km h - '1to 60 km h - '1in 5 s. Find the acceleration of the bus.

Q10 :

A train starting from a railway station and moving with uniform acceleration attains a speed 40 km h - '1in 10 minutes. Find its acceleration.

Q11 :

What is the nature of the distance - 'time graphs for uniform and non-uniform motion of an object?

Q12 :

What can you say about the motion of an object whose distance - 'time graph is a straight line parallel to the time axis?

Q13 :

What can you say about the motion of an object if its speed - 'time graph is a straight line parallel to the time axis?

Q14 :

What is the quantity which is measured by the area occupied below the velocity - 'time graph?

Q15 :

A bus starting from rest moves with a uniform acceleration of 0.1 m s - '2for 2 minutes. Find (a) the speed acquired, (b) the distance travelled.

Q16 :

A train is travelling at a speed of 90 km h - '1. Brakes are applied so as to produce a uniform acceleration of - '0.5 m s - '2. Find how far the train will go before it is brought to rest.

Q17 :

A trolley, while going down an inclined plane, has an acceleration of 2 cm s-2. What will be its velocity 3 s after the start?

Q18 :

A racing car has a uniform acceleration of 4 m s - '2. What distance will it cover in 10 s after start?

Q19 :

A stone is thrown in a vertically upward direction with a velocity of5 m s - '1. If the acceleration of the stone during its motion is 10 m s - '2in the downward direction, what will be the height attained by the stone and how much time will it take to reach there?

### Exercise : Solutions of Questions on Page Number : 100

Q1 :

An artificial satellite is moving in a circular orbit of radius 42250 km. Calculate its speed if it takes 24 hours to revolve around the earth?

Q2 :

An athlete completes one round of a circular track of diameter 200 m in 40 s. What will be the distance covered and the displacement at the end of 2 minutes 20 s?

Q3 :

Joseph jogs from one end A to the other end B of a straight 300 m road in 2 minutes 30 seconds and then turns around and jogs 100 m back to point C in another 1 minute. What are Joseph's average speeds and velocities in jogging (a) from A to B and (b) from A to C?

Q4 :

Abdul, while driving to school, computes the average speed for his trip to be 20 km h-1. On his return trip along the same route, there is less traffic and the average speed is 40 km h-1. What is the average speed for Abdul's trip?

Q5 :

A motorboat starting from rest on a lake accelerates in a straight line at a constant rate of 3.0 m sÃƒÂ¢Ã‹â€ '2 for 8.0 s. How far does the boat travel during this time?

Q6 :

A driver of a car travelling at 52 km hÃƒÂ¢Ã‹â€ '1 applies the brakes and accelerates uniformly in the opposite direction. The car stops in 5 s. Another driver going at 3 km hÃƒÂ¢Ã‹â€ '1 in another car applies his brakes slowly and stops in 10 s. On the same graph paper, plot the speed versus time graphs for the two cars. Which of the two cars travelled farther after the brakes were applied?

Q7 :

Fig 8.11 shows the distance-time graph of three objects A, B and C. Study the graph and answer the following questions: (a) Which of the three is travelling the fastest?

(b) Are all three ever at the same point on the road?

(c) How far has C travelled when B passes A?

(d)How far has B travelled by the time it passes C?

Q8 :

A ball is gently dropped from a height of 20 m. If its velocity increases uniformly at the rate of 10 m s-2, with what velocity will it strike the ground? After what time will it strike the ground?

Q9 :

The speed-time graph for a car is shown is Fig. 8.12. Fig. 8.12

(a) Find out how far the car travels in the first 4 seconds. Shade the area on the graph that represents the distance travelled by the car during the period.

(b) Which part of the graph represents uniform motion of the car?

Q10 :

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.

(b) an object moving in a certain direction with an acceleration in the perpendicular direction.