Motion

1. Aman felt the trees were moving backwards because he was in motion along with the moving car. With respect to the moving car, the trees outside kept changing their position, so they appeared to move backwards. Rest and motion are relative terms, and the trees appeared to move only because of Aman's point of observation.
2. The three types of motion they observed at the science park were circular motion (the rotating globe), periodic motion (the pendulum clock moving back and forth) and rectilinear motion (the person jogging on a straight track).
3. On the smooth surface, the toy car covered the same distance in each equal interval of time, which is uniform motion. On the rough patch, the distance varied in equal intervals of time, which is non-uniform motion.
4. Suman felt lighter while climbing the hill because weight depends on the gravitational pull, which decreases slightly at higher altitudes. Her mass remained constant, but as the gravity reduced, her weight became less, making her feel lighter.
5. The distance they travelled was 30 km, which is the total length of the actual path covered. Their displacement was zero because they returned to the same starting point, and displacement is the shortest distance between the initial and final positions.

1. Stationary object
   Reason — Circular, periodic and random are all types of motion, whereas a stationary object is at rest.
2. Weight
   Reason — Mass is measured in kilograms and remains constant, whereas weight depends on gravitational pull and may change from place to place.
3. Sitting
   Reason — Running, rotating and swinging are all forms of motion, whereas sitting represents a state of rest.
4. Time
   Reason — Direction, displacement and distance are related to position and movement, whereas time is a separate physical quantity.
5. Gravity
   Reason — Mass, volume and weight are physical quantities of a body, whereas gravity is the force of attraction of the earth.

1. Motion
2. Uniform motion
3. Average speed
4. Mass
5. Weight

1. When you see outside, the trees will appear to be moving backwards. But to a person standing on the roadside, they will appear stationary (at rest).
2. An object is said to be in curvilinear motion if it moves along a curved or circular path.
3. The position of the pendulum at rest is its mean position.
4. When a motion is oscillatory, the entire body of the object moves to and fro from the mean position.
5. Distance is the total length of the actual path covered by an object irrespective of its direction.

1. False
   Corrected Statement — Displacement is the shortest possible distance covered by an object from its initial position to its final position in a particular direction.
2. False
   Corrected Statement — A body is said to exhibit uniform motion if it covers equal distances in equal intervals of time.
3. True
4. True
5. False
   Corrected Statement — The more is the mass, the greater is the attraction between the body and the earth.

1. If a body exhibits both rotatory as well as translatory motion, its motion is called rolling motion.
2. The earth keeps on rotating on its axis as well as revolves around the sun in a curved (elliptical) path in a fixed time interval.
3. The vehicles move in a linear motion and their wheels rotate in a circular motion.
4. Some examples of rotatory motion are the movement of a fan, motion of swings like merry-go-round and giant wheel.
5. An object is said to be in curvilinear motion if it moves along a curved or circular path.

We see the trees move backwards when we sit in a moving car because we are in motion along with the car. With respect to the moving car, the position of the trees keeps changing, so they appear to move backwards. This happens because rest and motion are relative terms and depend on the point of observation.

Rest and motion are relative terms because an object may appear to be in motion with respect to one object and at rest with respect to another. Whether a body is at rest or in motion depends on the point of observation and its surroundings. For example, a passenger sitting in a moving car is at rest with respect to a fellow passenger but in motion with respect to a person standing on the road.

A child on a swing is in periodic motion because the swing moves to and fro about its mean position and repeats its motion after a fixed interval of time. Since the movement gets repeated regularly, it is an example of periodic motion.

Displacement can be zero even when distance is not, because displacement is the shortest distance between the initial and final positions, whereas distance is the total length of the path covered. When an object returns to its starting point, its initial and final positions are the same, so its displacement is zero, but the distance it actually covered is not zero.

Uniform motion means constant speed because in uniform motion a body covers equal distances in equal intervals of time. As the distance covered in each equal interval of time is the same, the speed of the body does not change and remains constant throughout.

An object is said to be in a state of rest if it does not change its position with respect to its surroundings with time. For example, trees, houses and buildings are at rest.

Rectilinear motion is the motion of a body moving along a straight line, such as a car moving on a straight road. Curvilinear motion is the motion of a body moving along a curved or circular path, such as a train moving along a curved track.

Oscillatory motion is the back and forth (to and fro) movement of a body about its mean position. For example, the motion of a pendulum and the motion of a swing are oscillatory motions.

A body is said to exhibit uniform motion when it covers equal distances in equal intervals of time. In uniform motion, the body moves with a constant speed.

Some examples of non-periodic motion are the beating of the heart, the movement of the body while dancing, the movement of hands while writing and a car moving on a busy road.

Rest and motion are relative terms because whether a body is at rest or in motion depends on the point of observation and its surroundings. The same object may appear to be in motion with respect to one observer and at rest with respect to another.

For example, imagine you are sitting near the window of a moving car. The trees outside will appear to be moving backwards. But to a person standing on the roadside, the trees will appear stationary. The person sitting next to you in the car is at rest with respect to you, but both of you will appear to be in motion to the person standing outside.

This shows that an object may appear in motion with respect to some objects and at rest with respect to others. Hence, rest and motion are relative terms.

When a body exhibits more than one type of motion at the same time, such a motion is called multiple motion.

Some examples of multiple motion are:

1. Motion of the earth: The earth rotates on its axis and at the same time revolves around the sun in a curved (elliptical) path.
2. Drawing water from a well: The bucket shows linear motion while the pulley on which the rope runs exhibits circular motion.
3. Vehicle moving on a straight road: The wheels of a vehicle rotate in a circular motion while the vehicle itself moves in a linear motion.
4. Motion of a drill: While making a hole in wood, a drill rotates (rotatory motion) and at the same time gets pushed forwards into the wood (translatory motion).

In a distance-time graph, uniform motion is represented by a straight line, because the body covers equal distances in equal intervals of time.

1. Pakhi felt that the trees were moving backwards because she was in motion along with the moving train. With respect to the moving train, the trees kept changing their position, so they appeared to move backwards. This happens because rest and motion are relative terms.

2. Two types of motion Pakhi observed were:

   • Rectilinear motion: The man walking straight on the platform moved along a straight line.
   • Periodic motion: The hanging toy repeated its rhythmic motion after regular intervals of time.

3. Distance is the total length of the actual path covered by an object irrespective of its direction, whereas displacement is the shortest distance between the initial and final positions in a particular direction. Distance is a scalar quantity, while displacement is a vector quantity.

4. Pakhi experienced uniform motion in the cable car because it moved at a steady speed, covering equal distances in equal intervals of time. In the car on the hill road, the speed kept changing as it moved slowly uphill and quickly downhill, covering unequal distances in equal intervals of time, which is non-uniform motion.

1. The different types of motion are translatory motion (rectilinear and curvilinear), rotatory motion, circular motion, oscillatory motion, vibratory motion, periodic motion, non-periodic motion, multiple motion, rolling motion and random motion.

2. The difference between oscillatory and vibratory motion is:

3. Rolling motion is both translatory and rotatory because when a body rolls, it rotates about its own axis (rotatory motion) and at the same time moves forward from one place to another (translatory motion). For example, when a ball rolls on the ground, it spins as well as moves ahead.

4. Some types of motion observed around us are:

   • Rectilinear motion: a car moving on a straight road.
   • Circular motion: a child sitting on a merry-go-round.
   • Rotatory motion: the blades of a ceiling fan.
   • Oscillatory motion: a swing moving to and fro.
   • Periodic motion: the hands of a clock.

(a) Total distance covered = 3 km + 4 km = 7 km

(b) The bus first moves 3 km, then turns right and moves 4 km. The two paths are at right angles, so the displacement is the shortest straight-line distance from the starting point to the final point. Using the Pythagoras' theorem:

$\text{Displacement} = \sqrt{3^2 + 4^2} = \sqrt{9 + 16} = \sqrt{25} = 5\ \text{km}$

Therefore, the total distance covered is 7 km and the displacement of the bus is 5 km.

Both persons can be correct because rest and motion are relative terms and depend on the reference point (point of observation).

The person standing on the moving escalator is at rest with respect to the escalator, because his position does not change with respect to the escalator. So, he says he is at rest.

The person at the bottom of the escalator sees that the position of the standing person keeps changing with respect to the ground. So, with respect to the ground, the person on the escalator is in motion.

Since both are observing from different reference points, both statements are correct.

Given,
First part: distance = 60 km, time = 1 h
Second part: distance = 30 km, time = 30 min = $\frac{1}{2}$ h

Total distance travelled = 60 km + 30 km = 90 km
Total time taken = 1 h + $\frac{1}{2}$ h = $\frac{3}{2}$ h = 1.5 h

$\text{Average speed} = \frac{\text{Total distance travelled}}{\text{Total time taken}} = \frac{90\ \text{km}}{1.5\ \text{h}} = 60\ \text{km/h}$

In the first part, the train covers 60 km in 1 hour (60 km/h), and in the second part it covers 30 km in 30 minutes (60 km/h). Since the train covers equal distances in equal intervals of time, the motion is uniform.

Therefore, the average speed of the train is 60 km/h and the motion is uniform.

Given,
Mass of the object, m = 10 kg
Gravity on Earth, g = 9.8 m/s²
Gravity on Moon, g = 1.6 m/s²

We know that, Weight, W = m × g

Weight on Earth = 10 kg × 9.8 m/s² = 98 N

Weight on Moon = 10 kg × 1.6 m/s² = 16 N

Therefore, the weight of the object is 98 N on Earth and 16 N on the Moon. The mass remains the same (10 kg) at both places, but the weight changes because gravity is different.

Aim: To show the difference between uniform and non-uniform motion using a toy car.

Materials required: A toy car, a ruler or measuring tape, a stopwatch, a smooth surface and a rough surface (such as a carpet).

Procedure:

1. Mark a starting point and several equal distances (for example, every 1 metre) on a smooth, flat surface.
2. Place the toy car at the starting point and give it a gentle push. Using the stopwatch, note the time taken by the car to cover each equal distance.
3. If the car covers equal distances in equal intervals of time, it is in uniform motion.
4. Now repeat the same activity on a rough surface (carpet).
5. Note the time taken by the car to cover each equal distance.
6. If the car covers unequal distances in equal intervals of time, it is in non-uniform motion.

Measuring the motion: Use the ruler to measure the distance and the stopwatch to measure the time. Record the readings in a table and compare the distances covered in equal time intervals to decide whether the motion is uniform or non-uniform.

Scenario: Riya's day at school.

1. In the morning, Riya walks straight along the road to reach her school bus stop. — Rectilinear motion (motion along a straight line).
2. At school, she enjoys a ride on the merry-go-round in the playground, moving around a fixed point. — Circular motion (motion around a fixed point in a circular path).
3. In the evening, she swings on the swing in the park, moving to and fro and repeating the motion after a fixed interval of time. — Periodic motion (motion repeated after a fixed interval of time).

In this way, Riya experiences rectilinear, circular and periodic motion in a single day.

1. Palak thought she was moving because when the train passed by, she appeared to change her position with respect to the moving train. To an observer in the train, her position kept changing, so she seemed to be in motion.

2. Yes, it is possible for a person to be at rest and in motion at the same time. This is because rest and motion are relative terms. A person can be at rest with respect to one object and in motion with respect to another at the same time. For example, Palak was at rest with respect to the ground but in motion with respect to the moving train.

3. The observer's position decides the reference point from which motion is observed. With respect to one reference point, an object may appear at rest, while with respect to another it may appear in motion. So, whether a body is at rest or in motion depends on the position of the observer.

4. Palak appears to move from the point of view of someone on the train because her position keeps changing with respect to the moving train. As the train moves, the distance between Palak and the train keeps changing, so she seems to be in motion.

5. Another example: When we sit in a moving bus, the trees, poles and buildings outside appear to move backwards, even though they are actually at rest. They appear to move only because of the observer's position in the moving bus.

1. We can identify the type of motion by observing the path of the object. If the object moves along a straight line, it is showing rectilinear motion. If it moves around a fixed point along a circular path, keeping a constant distance from the centre, it is showing circular motion.

2. Yes, an object can show more than one type of motion at once; this is called multiple motion. For example, a moving vehicle shows linear motion while its wheels rotate in circular motion at the same time.

3. Circular motion can be identified when an object moves around a fixed point in a circular path, keeping a constant distance from the centre. Two examples are the motion of the moon around the earth and a child sitting on a merry-go-round.

4. Yes, an object can show periodic and circular motion together. For example, the motion of the earth around the sun is circular as well as periodic, because it moves in a nearly circular path and repeats its motion after a fixed interval of time (one year).

5. The pendulum of a wall clock shows oscillatory (periodic) motion, as it moves to and fro about its mean position and repeats its motion after a fixed interval. Yes, it can be measured by finding its time period, that is the time taken to complete one oscillation, using a stopwatch.

1. Preeti used two types of surfaces (smooth floor and carpet) to compare how the surface affects the motion of the toy car. On the smooth surface the car covered equal distances in equal time (uniform motion), while on the rough carpet the distances varied (non-uniform motion). This helped her study the difference between uniform and non-uniform motion.

2. By swinging the key on a string, Preeti wanted to check whether it came back to the same position at regular intervals of time. This was to demonstrate oscillatory (periodic) motion, like that of a simple pendulum.

3. Preeti marked a zigzag path and walked through it. The total length of the zigzag path she walked was the distance, and the shortest straight line from the start to the end point was the displacement. By comparing the two, she showed the difference between distance and displacement.

4. Preeti used two tools because mass and weight are measured by different instruments. The kitchen scale (beam balance type) measures the mass of the brick, while the spring balance measures its weight (the force with which the earth attracts it). This helped her compare mass and weight.

5. It is important to change only one thing at a time in an experiment so that we can clearly understand the effect of that single change. If many things are changed at once, we cannot tell which change caused the result, and the experiment would not give reliable conclusions.

1. The same displacement can result from different distances because displacement depends only on the shortest distance between the starting and ending points, not on the path taken. Saniya walked straight while Rohit took a winding path, so they covered different distances, but since both ended at the same point 100 m from the start, their displacement was the same.

2. Yes, two people can have the same distance but different displacements. For example, if one person walks 100 m in a straight line and another walks 100 m along a curved path, both cover the same distance, but the straight walker has a larger displacement than the one who curved back towards the start.

3. If they both end at the same point, their displacement is the same because displacement depends only on the initial and final positions. Only the distance is different because distance depends on the actual path covered, and Rohit's winding path was longer than Saniya's straight path.

4. New example: Aarav walks 4 m east, then 3 m north (distance = 7 m, displacement = 5 m). Ishaan walks 7 m straight in one direction (distance = 7 m, displacement = 7 m). Both cover the same distance of 7 m but have different displacements.

5. Direction is important because displacement is a vector quantity, which has both magnitude and direction. The direction tells us which way the object has moved from its starting point. Without direction, we cannot correctly describe the displacement of a body.

1. An object is said to be in motion if its position changes with respect to its surroundings and time.
2. The shortest possible distance between the initial and final points is called displacement.
3. The motion of a pendulum is an example of oscillatory (periodic) motion.
4. When a body covers equal distances in equal intervals of time, it is said to be in uniform motion.
5. The laws of motion were discovered by Isaac Newton.

1. True
2. True
3. False
   Corrected Statement — Circular motion is a special type of curvilinear motion.
4. False
   Corrected Statement — In vibratory motion, only a part of the object moves to and fro.
5. True

The weight of a body is the force with which the earth attracts the body towards its centre. Its SI unit is newton (N). Its value changes from place to place as it depends on gravity.

Two examples of rectilinear motion are an automobile moving on a straight road and an apple falling from a tree.

In uniform motion, a body covers equal distances in equal intervals of time and moves with a constant speed. In non-uniform motion, a body covers unequal distances in equal intervals of time and its speed keeps on changing.

Periodic motion is the motion of a body which gets repeated after a regular interval of time. For example, the motion of the earth around the sun and the movement of the hands of a clock are periodic motions.

$\text{Speed} = \frac{\text{Distance covered}}{\text{Time taken}}$

The SI unit of speed is metre per second (m/s).

Rest and motion are relative terms because whether a body is at rest or in motion depends on the point of observation and the surroundings. The same object can appear to be at rest with respect to one observer and in motion with respect to another.

For example, when you are sitting in a moving car, the person sitting next to you appears to be at rest with respect to you. But to a person standing on the roadside, both of you appear to be in motion. Similarly, the trees outside appear to move backwards to you, but they are at rest to the person standing on the road.

This shows that an object may be at rest with respect to some objects and in motion with respect to others. Hence, rest and motion are relative terms.

The SI unit of both distance and displacement is metre (m).

When a body exhibits more than one type of motion at the same time, it is said to have multiple motion. Many objects show two types of motion together.

Some examples are:

1. Motion of the earth: The earth rotates on its axis and at the same time revolves around the sun in a curved path.
2. Vehicle moving on a straight road: The wheels of a vehicle rotate in a circular motion while the vehicle moves forward in a linear motion.
3. Drawing water from a well: The bucket moves up and down in linear motion while the pulley rotates in circular motion.
4. Motion of a drill: While making a hole, the drill rotates (rotatory motion) and at the same time moves forward into the wood (translatory motion).

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

When an object is in non-uniform motion, its speed keeps on changing. In such cases, we calculate the average speed of the object. Average speed is the ratio of the total distance travelled by the object to the total time taken during the journey.

It is calculated using the formula:

$\text{Average speed} = \frac{\text{Total distance travelled}}{\text{Total time taken}}$

For example, if a car travels a total distance of 75 km in 2 hours, then its average speed = $\frac{75\ \text{km}}{2\ \text{h}}$ = 37.5 km/h. The SI unit of average speed is metre per second (m/s).