Question

CLASS. 9
SUBJECT : PHYSICS [MOTION
Q.1
An object travels 10m first seconds 10m in next 3 seconds. What is its average speed?
0.2
A car travels. First 30km at a uniform speed of 20km/hr and next 30 km at a uniform
speed of 40 km/hr. what is its average speed?
Q.3
A bus decreases its speed from 72 km/hr to 54 km/hr in 10 seconds. Calculate the
acceleration of the bus?
Q.4
A scooter acquires a velocity of 30 km/hr in 10 seconds just after the staht. It takes in
two cases.
Q.5
An electric train is moving with a velocity of 20 km/hr. I low much distance will it
move in 30 seconds.
Q.6
Ahmed is moving in his car with a velocity of 45km/hr. how much distance will be
cover in one minute.​

Answer 1

An object has moved through a distance. Can it have zero displacement? If yes, support your answer with an example.
Answer: An object can have zero displacement even when it has moved a distance and shown some displacement. This takes place when the final position of an object coincides with its own initial position. Let's say, if a person moves around circular area and comes back to the place from where he started then the displacement will be zero.
2. A farmer moves along the boundary of a square field of side 10 m in 40 s. What will be the magnitude of displacement of the farmer at the end of 2 minutes 20 seconds from his initial position?
Answer:

Here, Side of the given square field = 10m
so, perimeter of a square = 4*side = 10 m * 4 = 40 m
Farmer takes 40 s to move along the boundary.
Displacement after 2 minutes 20 s = 2 * 60 s + 20 s = 140 seconds
since in 40 s farmer moves 40 m
Therefore, in 1s the distance covered by farmer = 40 / 40 m = 1m
Therefore, in 140s distance covered by farmer = 1 � 140 m = 140 m. Now, number of rotation to cover 140 along the boundary= Total Distance / Perimeter
= 140 m / 40 m = 3.5 round
Thus, after 3.5 round farmer will at point C of the field.

Thus, after 2 min 20 seconds the displacement of farmer will be equal to 14.14 m north east from initial position.
3. 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.
Answer: None of the above statements is actually true for displacement.
First statement is completely false because displacement can be zero in case of a circular path. Second statement is also false because displacement is less than or equal to the distance travelled by the object in case of motion in opposite direction.
INTEXT QUESTIONS Pg 102
1. Distinguish between speed and velocity.
Answer:
Speed Velocity
Speed is the distance travelled by an object in a given interval of time. Velocity is the displacement shown by an object in a given interval of time.
Speed = Distance / Time Velocity = Displacement / Time
Speed is scalar quantity and has only magnitude. Velocity is vector quantity and has both magnitude and direction.
2. Under what condition(s) is the magnitude of average velocity of an object equal to its average speed?
Answer: The magnitude of average velocity of an object is equal to its average speed, only in one condition when an object is moving in a straight line.
3. What does the odometer of an automobile measure?
Answer: The odometer of an automobile measures the distance covered by a vehicle or an automobile.
4. What does the path of an object look like when it is in uniform motion?
Answer: An object with uniform motion has a straight line path.
5. 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 × 108 m s-1.
Answer
Speed= 3 × 108 m s-1
Time= 5 min = 5 *60 = 300 seconds.
Distance= Speed * Time
Distance= 3 * 108 m s-1 * 300 secs. = 9 x 1010m
INTEXT QUESTIONS Pg 103
1. When will you say a body is in (i) uniform acceleration? (ii) Non-uniform acceleration?
Answer:
(i) A body is said to be in uniform acceleration if it travels in a straight line and its velocity increases or decreases by equal amounts in equal time intervals.
(ii) A body is said to be in non-uniform acceleration if the rate of change of its velocity is not constant, that is differs in different time intervals.
2. A bus decreases its speed from 80 km h-1 to 60 km h-1 in 5 s. Find the acceleration of the bus.
Answer:

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

Answer 2

Answer:54 km/hr = 54 x 5/18 = 15 m/s

36 km/hr = 36 x 5/18 = 10 m/s

18 km/hr = 18 x 5/18 = 5 m/s

1st Case

Speed = 15m/s

Time = 20 s

Distance = Speed x Time = 15 x 20 = 300 m

2nd Case

Speed = 10 m/s

Time = 30 sec

Distance = 10 x 30 = 300 m

3rd Case  

Speed = 5 m/s

Time = 10s

Distance = 5 x 10 = 50 m

Now Total Distance = 300 + 300 + 50 = 650 m

Total time = 20 + 30 + 10 = 60 s

Average Speed = 650/60 = 65/6 m/s

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