Physics - Motion - Class - 7 - Selina Solution - ICSE.

3. Short/long answer questions:

1. Explain the meaning of the terms rest and motion.

Rest - a body is said to be at rest if it does not change its position with respect to its immediate surroundings.

Motion - a body is said to be in motion if it changes its position with respect to its immediate surrounding.

2. Comment on the statement 'rest and motion are relative terms'. Give an example.

         Rest and motion are the relative terms because they depend on the observer's frame of reference. If an observer is at rest in his or her own frame of reference, but he may be moving in other observer's frame of reference.

For example - when a person travels by a car a person who is outside the car sees the person inside the car in motion. But the person inside the car feels himself to be at rest with car as the frame of reference.

3. Fill in the blanks using one of the words: at rest, in motion.

(a) a person walking in a compartment of a stationary train is in motion (person) relative to the compartment and is in motion (person) relative to the platform.

(b) a person sitting in a compartment of a moving train is in rest relative to the other person sitting by his side and is. In motion relative to the platform.

4. Name five different types of motion you know.

Explanation: following are the five different types of motion:

1.  Vibratory motion.

2.  Linear motion.

3.  Oscillatory motion.

4.  Circular motion.

5.  Rotatory motion.

5. What do you mean by translatory motion? Give one example.

         A motion in which all the points of the object move through the same distance in equal intervals of time is called translatory motion. Example - motion of a person walking on a road is an example of translatory motion.

6. Explain the meanings of (i) rectilinear motion, and (ii) curvilinear motion. Give one example of each.

Rectilinear motion - if the motion of a body is along a straight line, it is known as rectilinear motion. Rectilinear motion is also called linear motion.

An apple falling from height falling on earth and a car moving in a straight path are examples of rectilinear motion.

Curvilinear motion - if the motion of a body is along a curved path, it is known as curvilinear motion. The direction of motion is constantly changing in this motion.

The cyclist taking a turn and the path of a stone thrown in the sky are examples of curvilinear motion.

7. What is rotatory motion? Give two examples.

Rotatory motion - a body is said to be in rotatory motion if it moves about a fixed axis.

A spinning wheel is an example of rotatory motion. It moves in a circular path about a fixed axis. Similarly, the rotating fan is another example of rotatory motion.

8. What is meant by circular motion? Give one example.

Circular motion - the motion of a body along a circular path is known as circular motion. The direction of motion is constantly changing in circular motion. Example - when a person whirls a stone tied by a string, the motion of the stone is an example of circular motion.

9. How does rotatory motion differ from circular motion?

The main difference between these types of motion is that circular motion is a special case of rotational motion, where the distance between the body’s Centre of mass and the axis of rotation remains fixed.

In rotatory motion, a body rotates on its own axis (passing through it) while in circular motion, anybody/particle moves on a circular path with centre outside the body.

Rotational motion is based around the idea of rotation of a body about its Centre of mass. In rotational motion, the axis of rotation and centre of mass could change whereas in circular motion, the axis of rotation and centre of mass does not change.

10. Explain oscillatory motion by giving one example.

Oscillatory motion - Oscillatory motion - the to and fro motion of a body about a fixed point is called oscillatory motion.it is a type of periodic motion.

Examples - Motion of a pendulum is the most common example of oscillatory motion.

The bob of a pendulum is always in a to and fro motion about a fixed point.

11. What is vibratory motion? Give one example.

Solution

Vibratory motion - Vibratory motion: it is a motion in which some part of the body always remains fixed, and the rest part moves TO and FRO about its mean position.

It generally takes place in an elastic body or medium and the shape and size of the body changes during vibratory motion. 

Examples - While playing guitar, the strings of the guitar when plucked undergo vibratory motion. The sound waves are generated by this vibration which feels like music to our ears.

12. Differentiate between periodic and non-periodic motions by giving an example of each.

Periodic motion	Non - periodic motion
Any motion which gets repeated after regular intervals of time.	Motion which does not get repeated after regular intervals of time.
Ex.- motion of pendulum. It completes one oscillation every two seconds.	Ex.- the motion of a cricket ball hit by a batter. The motion of the ball doesn’t get repeated.

13. What is random motion? Give an example.

Random motion: The motion in which an object changes its direction frequently is called random motion. It is also called zig-zag motion or irregular motion.

Example:

1.  The motion of flying mosquito.     2.  The motion of smoke particle.

 14. Name the type/types of motion being performed by each of the following:

(a) Vehicle on a straight road - rectilinear motion.

(b) Blades of an electric fan in motion body - circular motion

(c) Pendulum of a wall clock periodic - Oscillatory motion, Periodic motion.

(d) Smoke particles from chimney - Non periodic or Random motion,

(e) Hands of a clock - circular motion.

(f) Earth around the sun - rotation and revolution and Periodic

(g) A spinning top - rotation

15. Give two examples to illustrate that a body can have two or more types of motion simultaneously.

1) Wheels of a moving car it has circular motion and rectilinear motion

2) A bob of a pendulum has linear and periodic motion

16. State the types of motion of the following:

(a) The needle of a sewing machine - oscillatory motion and Periodic Motion

(b) The wheel of a bicycle Mixed - rotatory and translatory motion.

(c) The drill machine - Translational motion and rotatory motion.

(d) The carpenter's saw Rotary - Translatory and Oscillatory motion

17. Distinguish between uniform and non-uniform motions, giving an example of each.

Uniform motion	Non-uniform motion
1. It is a motion characterized as the movement of the body, in which the body moves linearly and occupies equidistance at the same time duration, so its velocity does not vary with the line regardless of the length of time.	1. It is described as the motion of a body that travels at different velocities and does not cover the same distance at the same time duration, regardless of the length of the time duration.
2. Uniform motion has zero acceleration.	2. The non-uniform motion has non-zero acceleration.
3. Uniform motion travels an equal distance at an equal time duration.	3. non-uniform motion travels unequal distances at equal time duration.
4. The uniform motion is the same as the actual speed of the object.	4. The non-uniform motion is not the same as the actual speed of the object.
5. In uniform motion, the distance to time graph shows a straight line.	5. In non-uniform motion, the distance to time graph shows a curved line.
Examples: Movement of the Ceiling fan blade.	Examples: The movement of the train.

 

18. How do you determine the average speed of a body in non-uniform motion?

In a non-uniform motion, the average speed is the ratio of the distance travelled and the corresponding time interval.

We calculate it using,

Average speed = Total distance covered/Total time taken

19. Define the term weight and state its S.I. unit.

Solution

The weight of a body is the gravitational force with which earth attracts the body. ​

It is represented by the symbol W.

W= mg

The S.I. unit of weight is Newton (N).​

20. How are the units of weight, kgf and newton related?

The kgf is the unit of force according to the Gravitational metric system and Newton is the S.I. unit of force. ​

1 kgf is the force which produces an acceleration equal to g=10m/s² in the body of mass 1 kg and 1 N is the force which produces an acceleration of 1 m/s² in a body of mass 1kg.

So, 1 kgf = 10 N

21. State three differences between mass and weight.

Mass	Weight
It is the quantity of matter contained in a body.	It is the gravitational force with which the earth attracts the body.
Measured by using a beam balance.	Measured by using spring balance.
It is constant for a body, and it does not change by changing the place of the body.	With the variation in acceleration due to gravity at different places, the weight of the body changes.
S.I. unit of mass is kilogram (kg)	S.I. unit of weight is same as force i.e., Newton (N)

22. Which quantity: mass or weight, does not change by change of place?

Mass

23. State which of the quantities, mass or weight is always directed vertically downwards.

Weight

C. Numerical

1. A car covers a distance of 160 km between two cities in 4 h. What is the average speed of the car?

2. A train travels a distance of 300 km with an average speed of 60 km h ^ - 1 . How much time does it take to cover the distance?

3. A boy travels with an average speed of 10 m s ^ - 1 for 20 min. How much distance does he travel?

 

4. A boy walks a distance of 30 m in 1 minute and another 30 m in 1-5 minute. Describe the type of motion of the boy and find his average speed in m s ^ - 1

5. A cyclist travels a distance of 1 km in the first hour, 0.5 km in the second hour and 0.3 km in the third hour. Find the average speed of the cyclist in (i) km h ^ - 1 , (ii) m s ^ - 1 .

6. A car travels with speed 30 km h¹ for 30 minutes and then with speed 40 km h ^ - 1 for one hour. Find:

(a) the total distance travelled by the car

(b) the total time of travel, and

(c) the average speed of car

7. On earth the weight of a body of mass 1.0 kg is 10 N. What will be the weight of a boy of mass 37 kg in (a) kgf (b) N?

8. The weight of a body of mass 6-0 kg on moon is 10 N. If a boy of mass 30 kg goes from earth to the moon surface, what will be his (a) mass. (b) weight?