Two examples: motion of swing and simple pendulum. 

5. What is meant by the mean position of a simple pendulum?

Answer: When the pendulum is at rest , with the string vertical, it is said to be at its mean position. 

Long-Answer Questions

Q1. Explain the statement "all motion is relative

Answer: Motion is relative term. We usually judge the state of rest or of motion of a body bycomparing its position with respect to stationary landmarks in its surroundings.

Body at rest with respect to one set of objects at the same time it is in motion with respect to another setof objects. 

Q2. Explain translatory, rotatory and oscillatory motions with suitable examples. 

Answer: When a body moves from one point to another along a straight line or a smooth curve, itsmotion is called translatory. When ball is thrown up, ball as a whole move in curved path.

When an object turns or rotates or spins about an axis passing through itself, its motion is calledrotatory. Electric fan is the example. 

Oscillatory motion is the periotic motion of a body about its mean position. In Simple Pendulum  , bobmoves about its mean position. 

Q3. What is circular motion? How is it different from rotatory motion? When a body rotates, can parts of it be said to follow a circular path? 

Answer: When a body moves around a circular path , it's motion is called circular motion. In rotatorymotion, position of an object doesn't change but in circular motion, position of an object changescontinuously. 

Q4. Can a body have more than one type of motion at the same time? Explain with two examples.

Answer: Yes, a body can  have more than one type of motion at the same time.

Examples- 1 spinning top- rotatory and translatory 

                  2. Wheels of a car- translatory and rotatory 

Q5. What is one oscillation of a simple pendulum? (You can use a sketch to explain your answer.)

How does the time period of a simple pendulum depend on its length, the amplitude of itsoscillations and the mass of the bob?

Answer: One oscillation of the pendulum can be described as its motion

1. from one extreme position to the other and then back to its initial position, i.e., from A to B and back

to A 

2. between two consecutive crossings of the mean position while moving in the same

direction, i.e., from C to A to B to C.

Length of pendulum increases, time period increases 

Time period of a pendulum does not depend upon amplitude and mass of the bob.

Q6. What is meant by a seconds pendulum? Describe how such a pendulum can be set up.

Answer: A simple pendulum with time period  two seconds.

Seconds pendulum can be made by keeping the length of the string close to 99 cm.

Q7. Distinguish between speed and velocity. Give one example of a moving body whose speed remains constant, but velocity changes continuously.

Answer: Speed of a body - 

1. rate of change of its distance.

2. It is scalar quality 

Velocity of a body-

1. rate of change of its displacement 

2. It is vector quality. 

Example- body moving in a circular  path with speed of  5 m/s.

Speed - constant 

Velocity- changes continuously as direction changes at at every point on circular path. 

Objective Questions

Choose the correct option.

1. Which of the following is not an example of regular motion?

(a) A child on a swing

(b) An object moving in a circle

(c) A mosquito flying about in a room

(d) A swinging pendulum

2. A body starts moving from some point and returns to the same point. For this entire motion, its displacement is D and the distance covered by it is S.

(a) Both D and S are zero

(b) D is zero but S is not zero

(c) S is zero but D is not zero. 

(d) Neither D nor S is zero.

3. A particle moves from one point to another along a straight line at a constant speed..

(a) Its velocity is constant

(b) Its velocity may be constant..

(c) Its velocity cannot be constant.

(d) No conclusion can be drawn about its velocity.

4. The total time taken by a seconds pendulum to complete 20 oscillations is

 (a) 20 s

(b) 38 s

(c) 40 s

(d) 42 s

5. The time taken by a pendulum to swing from one extreme position to another is 0.5 s. Its time period is

(a) 0.5 s

(b) 1 s

(c) 2 s

(d) 1.5 s

6. Which of the following has a magnitude and a direction?

(a) Speed

(b) Distance

(c) Length

(d) Velocity

Fill in the blanks:

1. An object is said to be at rest when its position with respect  to its surroundings does not change 

2. The amplitude of a simple pendulum should not exceed 4⁰C during experiments.

3. The length of a seconds pendulum is close to 1m .

4. The distance from the point of support to the centre of the bob is taken as the length of a simple pendulum.

5. The word speed means distance covered per unit  time.

Write true or false:

1. Rectilinear and curvilinear motions are both examples of translatory motion. True

2. When an object moves in a circle with a constant speed, its velocity remains constant. False

3. The time period of a simple pendulum is the time in which it moves from one extreme position to the other extreme position. False

4. All repetitive motions are periodic. False

5. For a body moving from one point to another, the velocity and speed may be of equal magnitude. True

6. The displacement of a body moving from one point to another depends on the path it takes. False

Numericals:

Q1. Refer to Figure 2.9(b). An insect flying at a constant speed of 50 cm/s flies from A to B, B to C

and C to A. (a) What is its  displacement? (b) What is the total distance it covers? (c) How muchtime does it take t return to A?

Answer: Given speed = 50 cm/s

a) displacement =0

b) Distance = 12 m =1200 cm

c) Time = Distance ÷ speed

              = 1200 ÷ 50 = 24 seconds

Q2. A soldier marches at the speed of 2 m/s. What distance will he cover in one hour?

Answer: Given

Speed= 2m/s

Distance covered in one hour

In 1 s , Distance covered = 2m

In 3600 s, Distance covered = 2 x 3600 = 7200 m = 7.2 km

Q3. A car travels at 40 km/h for the first third of its journey, at 35 km/h for the second third of its journey and at 42 km/h for the final third of its journey. What is its average speed?

Answer: Average speed=( 40 + 35+ 42 ) /3 = 39 km/h

Q4. A bus travels at an average speed of 35 km/h between two towns that are 245 km apart. If it stops twice for half-hour breaks on the way, how long does it take to complete the journey?

Answer: time = distance ÷ speed

                      = 245 km ÷ 35 = 7 h

Total time taken = 7 h + 1/2 h + 1/2 h = 8 h

 

Q5. A simple pendulum completes 20 oscillations in 30 s. How much time does it take to move from an extreme position to the mean position?

Answer: Given 

Number of oscillations = 20

Time taken = 30 s

Time period = 30 ÷ 20 = 1.5 s.

Time taken to move from extreme position to mean position = 1.5 s ÷ 4= 0.375 s.

Link for 🌈 mind map of motion

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