Motion Book Back Questions 9th Science Lesson 2

9th Science Lesson 2

2] Motion

Book Back Questions with Answer and Do You Know Box Content

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Problem 1: An object travels 16 m in 4 s and then another 16 m in 2 s. What is the average speed of the object?

Solution: Total distance travelled by the object = 16 m + 16 m = 32 m

Total time taken = 4s + 2s = 6s

Average speed = Total distance travelled / Total time taken = 32m/6s = 5.33 ms^-1

Therefore, the average speed of the object is 5.33 ms^-1.

Problem 2: A sound is heard 5 s later than the lightning is seen in the sky on a rainy day. Find the distance of location of lightning? Given the speed of sound = 346 ms^-1

Solution: Speed = Distance/Time

Distance = Speed x Time = 346 x 5 = 1730 m

Thus, the distance of location of lightning is 1730 m.

The speedometer of an automobile measures the instantaneous speed of the automobile. In a uniform motion in one dimension, the average velocity is equal to instantaneous velocity. Instantaneous velocity is also called velocity or instantaneous speed or simply speed.

Problem 3: The brakes applied to a car produce an acceleration of 6 ms^-2 in the opposite direction to the motion. If the car takes 2 s to stop after the application of brakes, calculate the distance travelled during this time.

Solution: We have been given a = -6 ms^-2, t = 2s and v = 0

From the equation of motion,

V = u + at

0 = u + (-6 x 2)

0 = u – 12 ⸫ u = 12 ms^-1

S = ut + ½ at²

= (12 x 2) + ½ (-6 x 2 x 2) = 24 – 12 = 12 m

Thus, the car will move 12 m before it stops after the application of brakes.

Can a body have zero velocity and finite acceleration? Yes, when a body is thrown vertically upwards in space, at the highest point, the body has zero velocity but it has acceleration due to the gravity.

Problem 4: A 900 kg car moving at 10 ms^-1 takes a turn around a circle with a radius of 25 m. Determine the acceleration and the net force acting upon the car.

Solution: When the car turns around circle, it experiences centripetal acceleration, a = v²/r

a = (10) ²/25 = 100/25 = 4 ms^-2

Net force acting upon the car, F = m a = 900 x 4 = 3600 N

Any force like gravitational force, frictional force, magnetic force, electrostatic force etc., may act as centripetal force.

When you go for a ride in a merry-go-round in amusement parks, you will experience an outward pull as merry-go round rotates about vertical axis. This is due to centrifugal force.

Choose the best answers:

1. The area under velocity – time graph represents the

(a) Velocity of the moving object

(b) Displacement covered by the moving object

(c) Speed of the moving object

(d) Acceleration of the moving object

2. Which one of the following is most likely not a case of uniform circular motion?

(a) Motion of the Earth around the Sun

(b) Motion of the toy train on a circular track

(c) Motion of a racing car on a circular track

(d) Motion of hours’ hand on the dial of the clock

3. Which of the following graph represents uniform motion of a moving particle?

4. The centrifugal force is

(a) A real force

(b) the force of reaction of centripetal force

(c) A virtual force

(d) directed towards the centre of the circular path

Fill in the blanks:

1. Speed is a _________________ quantity whereas velocity is a ______________ quantity.

2. The slope of the distance – time graph at any point gives ____________.

3. Negative acceleration is called _____________.

4. Area under velocity – time graph shows _______________.

State true or false. If false, correct the statement:

1. The motion of a city bus in a heavy traffic road is an example for uniform motion.

2. Acceleration can get negative value also.

3. Distance covered by a particle never becomes zero but displacement becomes zero.

4. The velocity – time graph of a particle falling freely under gravity would be a straight line parallel to the x axis.

5. If the velocity – time graph of a particle is a straight line inclined to X-axis then its displacement – time graph will be a straight line.

Consider the statements given below and choose the correct option:

(a) If both assertion and reason are true and reason is the correct explanation of assertion

(b) If both assertion and reason are true but reason is not the correct explanation of assertion

(c) If assertion is true but reason is false

(d) If assertion is false but reason is true

1. Assertion: The accelerated motion of an object may be due to change in magnitude of velocity or direction or both of them.

Reason: Acceleration can be produced only by change in magnitude of the velocity. It does not depend the direction.

2. Assertion: The speedometer of a car or a motor-cycle measures its average speed.

Reason: Average velocity is equal to total displacement divided by total time taken.

3. Assertion: Displacement of a body may be zero when distance travelled by it is not zero.

Reason: The displacement is the shortest distance between initial and final position.

Match the following:

List I List II

1. Motion of a body covering equal distances in equal interval of time

-a)

2. Motion with non uniform acceleration

-b)

3. Constant retardation

-c)

4. Uniform acceleration

-d)

Answers

Choose the best answers:

1. acceleration of the moving object 2. Motion of the Earth around the Sun.

3. 4. a virtual force

Fill in the blanks:

1. Scalar, vector 2. Speed 3. Retardation or deceleration 4. Displacement

State true or false. If false, correct the statement:

1.False

Correct answer: The motion of a city bus in a heavy traffic road is an example for non- uniform motion.

2.True

3.True

4.False

Correct answer: The velocity – time graph of a particle falling freely under gravity would be a curved line

5.True

Consider the statements given below and choose the correct option:

1.If assertion is true but reason is false

2.If assertion is false but reason is true

3.If both assertion and reason are true and reason is the correct explanation of assertion

Match the following:

1.d

2.c

3.a

4.b