The chapter on Motion introduces how things move and the language to describe that movement. It starts by explaining that whether something is moving or still depends on what you’re comparing it to. Different kinds of motion exist, like straight-line movement, circles, spinning, back-and-forth, and random paths.
To describe motion accurately, we use terms like distance (how much ground is covered) and displacement (the straight-line change in position with direction). Similarly, speed tells us how fast something is going, while velocity includes the direction of that speed. Some motion is steady (uniform), and some is constantly changing (non-uniform). We also learn about average speed and velocity for journeys with varying motion. Finally, the chapter emphasizes using the correct standard units to measure these quantities and solving simple math problems related to motion.
Test yourself
A. Objective Questions
1. Write true or false for each statement
(a) Two trains going in opposite directions with the same speed are at rest relative to each other.
Ans: False.
(b) A ball is thrown vertically upwards. Its motion is uniform throughout.
Ans: False.
(c) The motion of a train starting from one station and reaching at another station is non-uniform.
Ans: True.
(d) A motion which repeats itself after a fixed interval of time is called periodic motion.
Ans: True.
(e) A ball thrown by a boy from a roof-top has oscillatory motion.
Ans: False.
(f) Mass has both magnitude and direction.
Ans: False.
(g) .Weight always acts vertically downwards.
Ans: True
(h) Mass varies from place to place but weight does not.
Ans: False
2. Fill in the blanks
(a) Two boys cycling on the road with the same speed are _________ relative to each other.
Ans: at rest
(b) The motion in a _________ is rectilinear motion.
Ans: straight line
(c) One to and fro motion of a clock pendulum takes time = _______
Ans: 2 s
(d) 36 km h-1 = _________
Ans: 10 m s-1
(e) Total distance travelled = ________________ × total time taken.
Ans: average speed
(f) The weight of a girl is 36 kgf. Her mass will be _________ .
Ans: 36 kg.
(g) The weight of a body is measured using a ______________.
Ans: spring balance.
Activity
3. Match the following
4. Select the correct alternative
(a) A book lying on a table is an example of
- a body at rest
- a body in motion
- a body neither at rest nor in motion
- none of these
(b) The motion of a pendulum is
- rotatory
- oscillatory
- curvilinear
- rectilinear
(c) A car moving on a straight road is an example of
- rotatory motion
- rectilinear motion
- oscillatory motion
- periodic motion
(d) A ball falls down vertically. Its motion is
- periodic
- linear
- circular
- vibratory
(e) If a body covers equal distance in equal interval of time, the motion is said to be
- uniform
- non-uniform
- oscillatory
- rotatory
(f) A boy goes from his house to school by bus at a speed of 20 km h-1 and returns back through the same route at a speed of 30 km h_1. The average speed of his journey is
- 24 km h-1
- 25 km h-1
- 30 km h-1
- 20 km h-1
(g) The earth attracts a body of mass 1 kg with a force of 10 N. The mass of a boy is 50 kg. His weight will be
- 50 kg
- 500 N
- 50 N
- 5 N
B. Short/Long Answer Questions
Question 1.
Explain the meaning of the terms rest and motion.
Ans:
Rest signifies that an object maintains a constant position relative to its immediate surroundings as time progresses. A stationary object, like a parked car or a book on a shelf, exemplifies rest because its location doesn’t change with respect to nearby fixed points.
Motion, conversely, describes a state where an object continuously changes its position with respect to its surroundings over a period of time. A moving bicycle or a flying bird are in motion because their location constantly shifts relative to stationary objects around them. It’s crucial to remember that the determination of rest or motion is relative to the chosen frame of reference.
Question 2.
Comment on the statement ‘rest and motion are relative terms’. Give an example.
Ans:
The idea that ‘rest and motion are relative’ means an object’s state of being still or moving isn’t absolute but depends on what you’re looking at it compared to. Something can be still for one person but moving for another, all at the same time.
For instance, think of someone on a moving bus. To another person sitting on the same bus, they are at rest. But to someone standing on the sidewalk watching the bus go by, that same person is in motion. So, whether we say something is at rest or in motion depends entirely on our point of view or what we’re using as a reference.
Question 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 relative to the compartment and is relative to the platform.
(b) A person sitting in a compartment of a moving train is relative to the other person sitting by his side and is relative to the platform.
Ans:
These scenarios illustrate the core concept of Relative Motion in physics, which states that the description of an object’s motion (its velocity, position, and acceleration) depends entirely on the frame of reference from which it is being observed.
(a) Walking in a Stationary Train
The motion of the person is described differently depending on the chosen observer:
- Frame of Reference: The Train Compartment
- Relative to the walls, floor, and other stationary objects inside the compartment, the person is clearly moving. The person is changing their position with respect to these immediate surroundings.
- Frame of Reference: The Platform
- Since the train itself is stationary (not moving relative to the platform), the person’s movement inside the train translates directly to their motion relative to the platform. Therefore, the person is also moving with respect to the platform.
(b) Sitting in a Moving Train
The description of motion changes completely because the primary frame of reference (the train) is in motion:
- Frame of Reference: The Adjacent Person/Seat
- Relative to the person sitting beside them or their own seat, the person’s position is fixed. The distance between the two people or the person and the seat does not change. Therefore, the person is at rest (not moving) relative to their traveling companion.
- Frame of Reference: The Platform
- Relative to an observer standing on the platform, the person is moving. Although the person is stationary relative to the train, the entire train, including the person, is undergoing continuous displacement over the ground. The person’s velocity relative to the platform is equal to the velocity of the train itself.
Question 4.
Name five different types of motion you know.
Ans:
Here are five distinct categories of movement:
- Straight-line Motion: This involves an object traversing a direct path without any curves or turns, such as a train moving along a straight track.
- Circular Trajectory: This describes movement along a curved path that forms a circle, like a bicycle wheel turning on its axle.
- Spinning Motion: This occurs when an object rotates around its internal axis, exemplified by a ceiling fan in operation.
- Back-and-forth Motion: This involves repetitive movement between two extreme positions, similar to the swinging of a grandfather clock’s pendulum.
- Irregular Movement: This characterizes motion with no predictable pattern or direction, such as the flight of a butterfly in a garden.
Question 5.
What do you mean by translatory motion ? Give one example.
Ans:
Translatory motion occurs when every point on a moving object travels the same distance in the same amount of time, and in the same direction. In simpler terms, the object moves from one place to another without rotating. All parts of the object follow identical paths.
Think of it like carrying a tray straight across a room without tilting or turning it. Every point on the tray moves the same distance, at the same speed, and in the same direction.
Example: A car moving along a straight road is undergoing translatory motion (assuming it’s not turning or the wheels are just rotating without the car changing its overall orientation). Every part of the car (the headlights, the doors, the roof) moves the same distance in the same direction over the same time interval.
Question 6.
Explain the meanings of (i) rectilinear motion, and (ii) curvilinear motion. Give one example of each.
Ans:
(i) Rectilinear Motion:
Rectilinear motion, also known as straight-line motion, is a type of translatory motion where an object moves along a straight path. During rectilinear motion, the direction of the object’s movement remains constant.
Example: A ball rolling freely on a perfectly level and straight track is undergoing rectilinear motion. Its path is a straight line, and its direction of movement doesn’t change unless an external force acts upon it.
(ii) Curvilinear Motion:
Curvilinear motion is another type of translatory motion where an object moves along a curved path. In this type of motion, the direction of the object’s movement continuously changes, even if its speed remains constant.
Example: A ball thrown at an angle into the air follows a curved path (a parabola) due to the force of gravity. This curved path indicates curvilinear motion, as the direction of the ball’s velocity is constantly changing as it moves upwards and then downwards.
Question 7.
What is rotatory motion ? Give two examples.
Ans:
Rotatory motion, also known as rotational motion or circular motion about an axis, is the movement of an object where every point of the object follows a circular path around a fixed central axis. This axis can be internal (passing through the object) or external. The object itself doesn’t necessarily change its overall position in space; instead, it spins or revolves around this axis.
Here are two examples of rotatory motion:
- The spinning of a ceiling fan: The blades of a ceiling fan rotate around a central rod (the axis of rotation) that is fixed in place. Each point on the fan blade travels in a circle centered on this rod. The fan itself generally stays in the same position while its parts undergo rotatory motion.
- The Earth rotating on its axis: Our planet Earth spins around an imaginary line passing through the North and South Poles, called its axis of rotation. This rotation causes day and night. Every point on Earth (except the poles) follows a circular path as the Earth completes one rotation approximately every 24 hours. While Earth also revolves around the Sun (another type of motion), its spinning on its own axis is a clear example of rotatory motion.
Question 8.
What is meant by circular motion ? Give one example.
Ans:
Circular motion describes the movement of an object tracing a circular path. The defining characteristic is that the object stays the same distance from a central point as it moves. Although the speed might be steady, the object’s velocity is always in flux because its direction of travel is constantly shifting along the curve.
Example: Consider a stone tied to a string being swung in a horizontal circle. The stone maintains a consistent distance from your hand (the center), and its direction of movement perpetually changes as it completes the circular trajectory.
Question 9.
How does a rotatory motion differ from the circular motion?
Ans:
Rotatory motion is when an entire object spins around its own axis, like a top or the Earth. Different parts of the object move in circles around this central line. The object itself might stay in the same spot while it rotates.
Circular motion, on the other hand, is when a single object (or something we treat as one point) travels along a circular path around a fixed point. Think of a ball on a string being swung in a circle. The ball moves along the circular path, and its distance from the center stays the same. So, rotatory motion is about an object spinning, while circular motion is about an object traveling in a circle.
Question 10.
Explain oscillatory motion by giving one example.
Ans:
Oscillatory motion is a type of repetitive back-and-forth movement of an object about a central or equilibrium position. The object moves to one extreme point, then returns through the equilibrium position to the opposite extreme point, and then repeats this cycle. This to-and-fro movement occurs over a fixed interval of time.
Example: A simple pendulum is a classic example of oscillatory motion. When you pull the bob of a pendulum to one side and release it, it swings back towards its resting (equilibrium) position. However, due to inertia, it overshoots the equilibrium and swings to the opposite side. It then reverses direction again, swinging back towards the equilibrium. This continuous back-and-forth movement about the central resting point is oscillatory motion. The pendulum repeats this swing with a relatively consistent time period.
Question 11.
What is vibratory motion ? Give one example.
Ans:
Vibratory motion is a kind of back-and-forth movement, a subset of oscillatory motion, but characterized by its small range and often high speed. Think of it as a rapid shaking or trembling around a central point.
Example: Consider the tight string of a guitar when it’s plucked. The string doesn’t swing widely like a pendulum; instead, it moves rapidly up and down a tiny amount around its resting position. These quick, small oscillations create the sound we hear. This rapid back-and-forth movement of the guitar string is vibratory motion.
Question 12.
Differentiate between periodic and non-periodic motions by giving an example of each.
Ans:
Periodic motion is movement that repeats itself consistently over a set amount of time. Think of the Earth orbiting the Sun; it follows the same path and takes roughly the same time each year. This regular repetition is the hallmark of periodic motion.
Non-periodic motion, on the other hand, is movement that doesn’t follow a regular, repeating pattern. A kite flying in the wind is a good example; its path and speed change unpredictably, and there’s no set time when it will return to the same spot with the same movement. The lack of a consistent cycle distinguishes non-periodic motion.
Question 13.
What is random motion. Give one example.
Ans:
Random motion is the movement of an object that does not follow any predictable pattern or fixed direction. The object changes its speed and direction erratically and unpredictably over time. There’s no set path or cycle that the object follows.
Example: The movement of dust particles suspended in the air is a classic example of random motion, often called Brownian motion. If you observe dust motes dancing in a beam of sunlight, you’ll notice they jiggle and move in all sorts of irregular ways. This is because they are constantly being bombarded by tiny air molecules moving at high speeds in random directions. The dust particles themselves don’t have a specific direction they intend to travel; their movement is a consequence of these countless random collisions.
Question 14.
Name the type/types of motion being performed by each of the following:
(a) Vehicle on a straight road
(b) Blades of an electric fan in motion
(c) Pendulum of a wall clock
(d) Smoke particles from chimney
(e) Hands of a clock
(f) Earth around the sun
(g) A spinning top.
Ans:
(a) Vehicle on a straight road: Primarily rectilinear motion (a type of translatory motion). The vehicle moves along a straight line. The wheels also exhibit rotatory motion.
(b) Blades of an electric fan in motion: The blades spin around a fixed axis. Each point on the blade follows a circular path.
(c) Pendulum of a wall clock: Oscillatory motion. The bob of the pendulum swings back and forth about its equilibrium position. This motion is also periodic.
(d) Smoke particles from chimney: Random motion (Brownian motion). The smoke particles move erratically in all directions due to collisions with air molecules.
(e) Hands of a clock: Circular motion. The tips of the hands move along a circular path around the center of the clock face. This motion is also rotatory about the central axis of the hand.
(f) Earth around the sun: Primarily curvilinear motion (specifically, elliptical motion, which is a type of circular motion in a broader sense). It also exhibits rotatory motion by spinning on its own axis.
(g) A spinning top: Primarily rotatory motion. The top spins about its axis. If the top is also moving across a surface, it would also exhibit translatory motion (likely curvilinear as it wobbles).
Question 15.
Give two examples to illustrate that a body can have two or more types of motion simultaneously.
Ans:
Objects often exhibit more than one type of motion at the same time. Consider a rolling bicycle wheel. It’s spinning around its center (rotatory motion), but the center itself is also moving forward along the road (translatory motion). Every point on the wheel experiences a combination of these two movements, resulting in the complex motion of rolling.
Another example is a spinning top that’s wobbling and moving across a table. The top is clearly rotating rapidly on its axis (rotatory motion). However, if you watch its base, you’ll likely see it tracing a curved path on the table’s surface (translatory motion, specifically curvilinear). The entire top is simultaneously spinning and moving from one place to another.
These examples show that motion isn’t always just one simple type. Often, the movement we observe is a combination of different fundamental types of motion occurring together. Understanding these combined motions helps us analyze and describe the movement of objects in the real world more accurately.
Question 16.
State the types of motion of the following :
(a) The needle of a sewing machine
(b) The wheel of a bicycle
(c) The drill machine
(d) The carpenter’s saw
Ans:
(a) The needle of a sewing machine: Primarily oscillatory motion (specifically, reciprocating motion, which is a linear back-and-forth movement). The needle moves up and down repeatedly. It might also have a slight rotational component at certain points, but the dominant motion is oscillatory.
(b) The wheel of a bicycle: Exhibits rotatory motion as it spins about its axle. When the bicycle is moving, the wheel also exhibits translatory motion (specifically, rectilinear motion if moving straight). The combination of these results in the wheel rolling.
(c) The drill machine: The rotating bit exhibits rotatory motion. If the drill is being used to bore a hole, the entire drill (and the bit) also exhibits translatory motion as it moves into the material.
(d) The carpenter’s saw: Primarily exhibits translatory motion, specifically reciprocating motion (a back-and-forth movement). The carpenter moves the saw forward and backward to cut the material. The teeth of the saw also have a very small vibratory motion as they interact with the wood.
Question 17.
Distinguish between uniform and non-uniform motions, giving an example of each.
Ans:
Question 18.
How do you determine the average speed of a body in non-uniform motion ?
Ans:
When an object’s speed varies during its travel, determining its average speed necessitates considering the entire journey. This involves measuring the total length of the path traversed by the object and the total time elapsed from the beginning to the end of its movement.
The average speed is subsequently obtained by dividing the accumulated distance traveled by the total time taken for the entire trip. This resulting single value represents a hypothetical constant speed that the object would have needed to maintain throughout the same duration to cover the exact same total distance, thereby providing an overall measure that accounts for all the accelerations and decelerations experienced during the actual non-uniform motion.
Question 19.
Define the term weight and state its S.I. unit.
Ans:
It’s essentially how hard gravity is pulling on something. The amount of weight an object has depends on how much stuff it’s made of (its mass) and how strong the gravitational pull is at its location.
The standard unit for measuring weight in the International System of Units (SI) is the Newton (N). This is because weight is a type of force, and the Newton is the universally accepted unit for measuring all forces.
Question 20.
How are the units of weight, kgf and newton related ?
Ans:
Weight is a force caused by gravity pulling on an object’s mass, and its standard unit is the Newton (N). The Newton is part of the SI system and is defined by mass and acceleration (N=kg⋅m/s2). It’s the scientifically preferred unit for measuring weight.
Kilogram-force (kgf) is another unit for weight, but it’s not part of the SI system. One kgf is the force exerted by gravity on a 1-kilogram mass at Earth’s surface under standard gravity. Since the standard acceleration due to gravity (g) is about 9.8m/s2, 1 kgf is approximately equal to 9.8 Newtons.
Therefore, while both kgf and Newton measure weight (a force), the Newton is the standard SI unit. The kilogram-force is a non-SI unit related to the Newton through the acceleration due to gravity. In scientific work, it’s best to use Newtons for consistency and adherence to the SI system.
Question 21.
State three differences between mass and weight.
Ans:
Here are three key differences between mass and weight:
- Definition: Mass is a fundamental property of an object that measures the amount of matter it contains. It’s a measure of inertia, or an object’s resistance to changes in its state of motion. Weight, on the other hand, is the force exerted on an object due to gravity. It’s the measure of how strongly gravity pulls on that object’s mass.
- Dependence on Location: Mass is a scalar quantity and remains constant regardless of the object’s location in the universe. An object has the same amount of matter whether it’s on Earth, the Moon, or in space. Weight, however, is a vector quantity and depends on the gravitational field strength at the object’s location. The same object will have different weights on Earth and the Moon because the gravitational forces are different. In zero gravity, an object would have zero weight, but its mass would remain the same.
- Units of Measurement: The standard International System of Units (SI) unit for mass is the kilogram (kg). Other units like grams (g) are also used. The SI unit for weight (which is a force) is the Newton (N). Sometimes, kilogram-force (kgf) is used as a unit of weight in practical applications, where 1 kgf is approximately equal to 9.8 N on Earth’s surface.
Question 22.
Which quantity : mass or weight, does not change by change of place ?
Ans:
Mass is the quantity that does not change with a change of place.
Here’s why:
- Mass is a fundamental property of an object that measures the amount of matter it contains. This amount of matter remains constant regardless of where the object is located in the universe. Whether an object is on Earth, the Moon, or in deep space, it is still made up of the same number and type of atoms and molecules.
- Weight, on the other hand, is the force exerted on an object due to gravity. The strength of gravity varies depending on the mass of the celestial body and the distance from its center. Therefore, an object’s weight will change if it is moved to a location with a different gravitational pull. For example, an object will weigh less on the Moon than on Earth because the Moon’s gravitational force is weaker. In zero gravity, an object would have no weight at all, even though its mass remains the same.
Question 23.
State which of the quantities, mass or weight is always directed vertically downwards.
Ans:
The quantity that is always directed vertically downwards is weight.
Weight is the force exerted on an object due to gravity, and the direction of this force is always towards the center of the celestial body exerting the gravitational pull. For objects near the Earth’s surface, this direction is what we perceive as vertically downwards, towards the ground.
Mass, on the other hand, is a scalar quantity that measures the amount of matter in an object and has no inherent direction associated with it. While mass influences the magnitude of the weight, it does not have a direction itself. Therefore, weight is the quantity that is consistently directed vertically downwards due to the force of gravity.
C. Numericals
Question 1.
A car covers a distance of 160 km between two cities in 4 h. What is the average speed of the car ?
Ans:
To find the mean rate of motion for any object, the fundamental definition of average speed is applied: it’s the ratio of the total distance covered to the total time taken for the journey .
Average Speed=Total Time ElapsedTotal Distance Traversed
Calculation
In this scenario:
- Total Distance (D): 160 km
- Total Time (T): 4 hours
Substituting these parameters into the formula yields:
Average Speed=4 h160 km
Average Speed=40 km/h
The derived average velocity of the object across the specified segment is 40 kilometers per hour. This figure signifies the consistent pace at which the entire path would have to be traversed to complete the trip in exactly four hours.
Question 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?
Ans:
Speed = 60 km h-1
Distance covered = 300 km
Question 3.
A boy travels with an average speed of 10 m s-1 for 20 min. How much distance does he travel ?
Ans:
Given:
Average speed (s) = 10 m s⁻¹
Time taken (t) = 20 minutes
First, we need to convert the time from minutes to seconds to maintain consistent units with the speed.
So, 20 minutes = 20 × 60 seconds = 1200 seconds.
Now, we can calculate the distance traveled using the formula:
Distance = Average Speed × Time
Distance = 10 m s⁻¹ × 1200 s
Distance = 12000 m
To express the distance in kilometers, we can divide by 1000:
Distance in km = 12000 m / 1000 m/km
Distance in km = 12 km
So, the boy travels a distance of 12000 meters or 12 kilometers.
Final Answer: The final answer is 12000 m or 12 km
