Force and Pressure: Motion | ICSE Class 7 Physics Notes

This note covers rest and motion, reference points, translatory and circular motion, oscillatory and repetitive motion, random motion, uniform and non-uniform motion, distance, time, average speed, simple calculations, weight, and the differences between mass and weight.

How do we decide whether an object is at rest or in motion?

An object's position tells us where it is. To describe that position, we choose a reference point: a fixed object or point against which the position is compared. The reference point must be clear before we decide whether the object is moving.

Definition: An object is in motion when its position changes with time relative to a reference point. It is at rest when its position does not change with time relative to that reference point.

Why does the reference point matter?

Passengers sitting in a moving bus remain in the same seats. Their positions relative to the bus do not change, so they are at rest relative to it. Relative to a building outside, their positions change as the bus travels, so they are in motion.

These descriptions do not contradict each other. They use different reference points. A complete statement about rest or motion identifies what the object's position is being compared with, rather than simply saying that the object is moving or stationary.

How can we check for motion?

1. Choose the object whose motion is being studied.
2. Select a reference point and identify the object's position relative to it.
3. Observe the object's position again after some time.
4. Compare the positions using the same reference point and decide whether they changed.

Walking, running, cycling and a bird flying involve motion. However, identifying the activity is only the beginning of a physical description. We can also describe the path, meaning the route followed, and the distance, meaning its length. A time interval is the duration between the beginning and end of an observation.

What are translatory and circular motion?

Translatory motion is movement in which an object shifts from one position to another without turning as a whole. Its parts move through equal distances in the same direction during the same interval. The path of translation may be straight or curved.

Linear motion, also called rectilinear motion, follows a straight path. A heavy box sliding straight without turning illustrates straight-line translation. A dropped eraser also provides a simple observation of a straight downward path.

What makes motion circular?

In circular (circulatory) motion, the moving object follows a circular path. An eraser tied to a thread and whirled around the hand illustrates this path. Its position changes as it moves around the circle, even though the circle itself stays in the same place.

The path is the route followed during motion. To distinguish linear from circular motion, examine this route rather than whether the object moves quickly or slowly. A description of the path does not, by itself, tell us the distance travelled in a given time.

When describing a demonstration, name both the moving object and the feature used to classify it. For the eraser on a thread, the relevant feature is the circular path of the eraser. For the sliding box, it is the shift of the box along a straight path without turning.

How does a pendulum show oscillatory and periodic motion?

Oscillatory motion is repeated movement to and fro about a fixed position. A swing and an eraser suspended from a thread can show this motion. A simple pendulum consists of a small metallic ball suspended from a rigid support by a long thread.

The ball is called the bob. The position where the pendulum rests is its mean position. The farthest positions reached on either side during a swing are its extreme positions. Moving the bob slightly to one side and releasing it starts oscillatory motion.

The first drawing labels a rigid support, a long thread and a bob. The second shows the mean position O below the support, with extreme position B to the left and extreme position A to the right.

See Fig. 8.7 in your NCERT textbook

What counts as one complete oscillation?

Here O, A and B are labels for the bob's positions. One oscillation is a complete to-and-fro movement. Starting at O, the bob moves to A, then to B, and returns to O. Starting at A, it moves to B and returns to A.

Periodic motion repeats its path after a fixed interval of time. A pendulum demonstrates periodic motion. Its time period is the time taken for one complete oscillation. Going from one extreme position to the other is only part of a complete oscillation.

How is the time period measured?

1. Suspend the bob by a thread from a rigid support and wait until it rests.
2. Move it slightly to one side, then release it without pushing; keep the thread taut.
3. Use a watch or stopwatch to measure the time for 10 complete oscillations.
4. Divide that measured time by 10, then repeat the measurement to compare the results.

In the following relationship, = means “equals” and ÷ means “divided by”.

Time period = Time for the counted oscillations ÷ Number of oscillations. A stopwatch is a watch used to time an interval. In repeated observations, the time period is almost the same each time. Count complete oscillations consistently before comparing the readings.

How do periodic, non-periodic and random motion differ?

Different classifications answer different questions. Linear and circular motion describe the shape of a path. Oscillatory motion describes to-and-fro movement. Periodic motion describes repetition after a fixed time interval. A pendulum can therefore be both oscillatory and periodic.

Repetitive motion is movement that occurs again and again. To decide whether repetition is periodic, check the time intervals between matching stages of the movement. Repetition alone does not establish that those intervals are equal.

What is non-periodic motion?

Non-periodic motion does not repeat at fixed intervals of time. Motion can occur repeatedly without keeping a regular timing pattern. The distinction concerns the timing of repetition, so simply seeing an object move again is insufficient evidence for calling its motion periodic.

What is random motion?

Random motion has an irregular, unpredictable path, with no fixed pattern of direction. Its classification concerns the lack of a regular path pattern. Non-periodic describes the lack of fixed-time repetition. These descriptions concern different features and should not be treated as identical definitions.

When classifying motion, support the chosen term with the observation that defines it. For a pendulum, describe both the to-and-fro movement and the repeated cycle. This gives a clearer explanation than attaching a single label without stating what was observed.

How can we distinguish uniform and non-uniform motion?

Uniform motion covers equal distances in equal intervals of time. Non-uniform motion does not maintain this equality. For motion along a straight line, uniform motion has constant speed, meaning unchanging speed; non-uniform motion has a speed that changes.

Speed describes the distance covered per unit time, such as during one second. Distance is the length of the path travelled. A time interval is the duration between the beginning and end of an observation. Equal intervals must be used when comparing successive distances.

How does a train illustrate the difference?

A train leaving a station starts slowly and then speeds up. It may travel at constant speed over part of the route before slowing to stop at the next station. Its straight-line motion is non-uniform during speeding up and slowing down.

The drawing marks A at the left station, B and C along the track, and D at the right station. These letters label positions: the described motion is uniform from B to C and non-uniform from A to B and C to D.

See Fig. 8.11 in your NCERT textbook

What do equal-time observations show?

The table records two trains, X and Y, at intervals of 10 minutes. X and Y identify the trains. A kilometre, written km, is a unit of distance; AM identifies morning clock times. Each distance column gives the distance for the preceding interval, with zero at the starting entry.

Train X covers 20 km in each interval and is uniform. Train Y covers unequal distances in equal intervals and is non-uniform. Uniform linear motion is an idealisation: a simplified description. In everyday life, we seldom find constant speed over long distances or long time intervals.

Which measurements and units are needed to describe speed?

To calculate speed, measure the distance travelled and the time taken to travel that distance. The measurements must belong to the same journey. A distance without a time, or a time without a distance, does not give the speed of that journey.

SI means International System of Units. The SI unit of distance is the metre, written m. The SI unit of time is the second, written s. The SI unit of speed is metre per second, written m/s.

How are larger units written?

The symbol min means minute, and h means hour. Kilometres per hour is written km/h. The slash in m/s or km/h means “per”, so these units combine a distance unit with a time unit.

Use metres and seconds to obtain m/s. Use kilometres and hours to obtain km/h. Convert before dividing if the given units differ from those requested. Leave a space between a number and its unit, and write unit symbols without plural endings.

Which instruments measure distance and speed?

A speedometer displays a vehicle's speed, commonly in km/h. An odometer measures the distance travelled by the vehicle, in kilometres. These instruments report different quantities. A watch or stopwatch provides the time interval needed alongside distance to calculate average speed.

What does average speed tell us about a journey?

Average speed is the total distance covered divided by the total time taken. The word “total” matters: the distance and time must cover the whole journey being considered. Average speed gives one value for that journey even when its speed changes along the way.

Definition: $\text{Average speed} = \frac{\text{Total distance covered}}{\text{Total time taken}}$. With distance in metres and time in seconds, the resulting average speed is in metres per second.

How can speeds be compared?

For the same time interval, the object covering more distance has the greater average speed. For the same distance, the object taking less time has the greater average speed. If both distance and time differ, calculate the speeds and compare them in the same unit.

A journey's average does not mean that the object maintained that speed throughout. It might have travelled more slowly during one part and faster during another. To judge uniformity, examine the distances covered in successive equal time intervals, rather than relying on the final average.

Note: Average speed and uniform motion answer different questions. Average speed summarises distance and time for a journey; uniformity concerns whether equal distances are covered in equal intervals throughout the motion.

How can we find distance or time instead?

The same relationship can be rearranged when a different quantity is unknown. Use the average speed for the particular journey, or a constant speed maintained throughout it. The stated time and distance must refer to that same journey.

The multiplication sign × means “multiplied by”.

Derivation: How is the distance formula obtained?

Rearrange the average speed relationship to find the total distance covered.

1. Start with $\text{Average speed} = \frac{\text{Total distance covered}}{\text{Total time taken}}$.
2. Multiply both sides by the total time taken: $\text{Average speed}\times\text{Total time taken} = \frac{\text{Total distance covered}}{\text{Total time taken}}\times\text{Total time taken}$.
3. Cancel the total time taken in the denominator against the multiplying time. The right side is now the total distance covered.

Result: $\text{Total distance covered} = \text{Average speed}\times\text{Total time taken}$.

Derivation: How is the time formula obtained?

Rearrange the distance relationship to find the time for a journey with a non-zero average speed.

1. Start with $\text{Total distance covered} = \text{Average speed}\times\text{Total time taken}$.
2. Divide both sides by the average speed: $\frac{\text{Total distance covered}}{\text{Average speed}} = \frac{\text{Average speed}\times\text{Total time taken}}{\text{Average speed}}$.
3. Cancel the average speed in the numerator and denominator on the right. This leaves the total time taken.

Result: $\text{Total time taken} = \frac{\text{Total distance covered}}{\text{Average speed}}$.

These equations use quantity names rather than letter symbols. Keeping the names visible helps distinguish the distance being sought from the time interval used in the calculation.

How are simple motion calculations worked out?

Begin each calculation by identifying what is known and what is required. Choose the speed relationship, check the units, and substitute the given values. End with a number and its unit. A correct calculation without the unit does not fully describe a physical quantity.

How do we calculate speed and convert units?

Worked example 1. Swati cycles 3.6 km to school in 15 min. Calculate her average speed in m/s. Use 1 km = 1000 m and 1 min = 60 s.

Given: distance 3.6 km; time 15 min. Formula: Distance in metres = Distance in kilometres × 1000; Time in seconds = Time in minutes × 60; Average speed = Total distance ÷ Total time.

Substitute: $\text{Distance} = 3.6\times1000\,\mathrm{m} = 3600\,\mathrm{m}$; $\text{Time} = 15\times60\,\mathrm{s} = 900\,\mathrm{s}$. Answer: $\text{Average speed} = \frac{3600\,\mathrm{m}}{900\,\mathrm{s}} = 4\,\mathrm{m/s}$, or 4 m/s.

Worked example 2. A car travels 150 m in 10 s. Find its average speed in km/h. Use 1 km = 1000 m and 1 h = 3600 s.

Formula: Average speed = Distance ÷ Time. Substitute: 150 ÷ 10 = 15 m/s. Converting the units gives 15 × 3600 ÷ 1000. Answer: the average speed is 54 km/h.

Both calculations divide distance by time. The difference is the required unit. In the bicycle calculation, convert the input measurements before dividing. In the car calculation, calculate metres per second first, then convert that result to kilometres per hour.

How can speed give a distance or a time?

Worked example 3. Raghav travels by bus at 50 km/h for 2 h. Find the distance covered.

Formula: Distance = Speed × Time. Substitute: 50 × 2. Answer: the bus covers 100 km. Kilometres per hour multiplied by hours gives a distance in kilometres.

Worked example 4. A train travels at 90 km/h. Find the time taken to cover 360 km, in hours and seconds. Use 1 h = 3600 s.

Formula: Time = Distance ÷ Speed. Substitute: 360 ÷ 90. Answer: the time taken is 4 h, or 4 × 3600 = 14400 s. Division gives hours because the speed was given in kilometres per hour.

How do we handle more than one calculation?

Worked example 5. A train covers 180 km in 3 h. Find its average speed and the distance it would cover in 4 h if it maintained the same speed throughout.

Formula: Average speed = Distance ÷ Time; New distance = Speed × New time. Substitute: average speed = 180 ÷ 3 = 60 km/h; new distance = 60 × 4. Answer: 60 km/h and 240 km, respectively.

Worked example 6. A car covers 60 km in the first hour, 70 km in the second and 50 km in the third. Determine whether its motion is uniform and find its average speed.

Formula: Total distance = Sum of the distances; Average speed = Total distance ÷ Total time. Substitute: total distance = 60 + 70 + 50 = 180 km; total time = 3 h.

Answer: average speed = 180 ÷ 3 = 60 km/h. The motion is non-uniform because the car covers unequal distances in equal one-hour intervals.

In the final example, the average speed is calculated for the complete three-hour journey. It does not erase the differences between the hourly distances. The calculation and the classification are separate parts of the answer, each supported by the given information.

What is weight and how can it be measured?

A force is a push or pull arising from an interaction between objects. The Earth's pull on an object is a gravitational force, also called gravity. The weight of an object is the force with which the Earth pulls it towards itself.

The SI unit of weight is the newton, written N, because weight is a force. A statement of weight tells us how strongly an object is pulled, rather than directly stating how much matter the object contains.

How does a spring balance work?

Mass is the amount of matter in an object. The gram, written g, is a unit of mass. Mass and weight describe different quantities, even when both appear on the same instrument.

A spring balance measures force using the stretching of a spring. One end of the spring is fixed, and an object hangs from a hook at the other end. The amount of stretching is read against a scale marked in newtons.

Photograph: A spring balance and close-up of its scale. The balance has a top suspension ring and a lower hook. The enlarged scale has columns headed GRAMS and NEWTONS, with a red indicator at zero. Photograph: Object suspended from a spring balance (Figure 5.14). Below Figure 5.13, it shows an object hanging from the hook.

See Fig. 5.13 in your NCERT textbook

The range is the interval of values an instrument can measure. The illustrated balance has a range of 0 to 10 N. Between markings differing by 1 N, there are five smaller divisions. Each division therefore represents 1 ÷ 5 = 0.2 N.

What should be checked before measuring?

1. Look at the scale and identify the maximum weight it can measure.
2. Determine the value of a small division from the labelled marks and the number of divisions between them.
3. Choose an object whose weight does not exceed the balance's maximum reading.
4. Suspend it from the hook, read the weight scale carefully, and record the value with its unit.

Different spring balances may have different ranges and division sizes. Read the particular instrument being used. Hanging an object heavier than the maximum permitted weight may damage the balance, so the range check is part of making the measurement correctly.

How are mass and weight related, and how do they differ?

Mass is the amount of matter in an object. The SI unit of mass is the kilogram, written kg. A smaller unit is the gram, written g. Weight is a gravitational force, so its unit is the newton rather than the kilogram.

At the same place, weight increases in proportion to mass: an object with greater mass has greater weight under the same gravitational conditions. Changing location can change the gravitational pull without changing the amount of matter in the object.

What changes when an object is taken elsewhere?

An object's mass remains the same from place to place. Gravitational force can vary very slightly from place to place on Earth and can be very different on different planets. Consequently, weight can change while mass remains unchanged.

A beam balance compares an object's weight with that of an object of known mass to determine its mass. A spring balance usually also has a mass scale. That scale assumes the instrument is used on Earth, under Earth's gravitational pull.

What do mass and weight values show?

The following values illustrate the distinction for the same object. Its mass is unchanged in every column, while its weight depends on location.

Weight remains almost the same everywhere on Earth, so weighing is acceptable for finding mass for practical purposes. Nevertheless, scientific writing distinguishes the quantities: a wheat bag described in everyday speech as “weighing 10 kg” has a mass of 10 kg.

Glossary

• Reference point — A fixed object or point used to describe another object's position.
• Motion — Change in an object's position with time relative to a reference point.
• Translatory motion — Movement in which an object changes position without turning as a whole.
• Circular motion — Motion in which the moving object follows a circular path.
• Oscillatory motion — To-and-fro movement of an object about a fixed position.
• Periodic motion — Motion that repeats its path after a fixed interval of time.
• Non-periodic motion — Motion that does not repeat at fixed intervals of time.
• Random motion — Irregular motion with an unpredictable path and no fixed directional pattern.
• Time period — The time taken by a pendulum to complete one oscillation.
• Uniform motion — Motion in which equal distances are covered in equal intervals of time.
• Average speed — Total distance covered during a journey divided by the total time taken.
• Mass — The amount of matter in an object, measured in kilograms or grams.
• Weight — The gravitational force pulling an object towards the Earth or another attracting body.
• Spring balance — An instrument that measures force through the stretching of a spring.

Common errors and misconceptions

• Misconception: A seated passenger in a moving bus must be described simply as at rest. Correct: The passenger is at rest relative to the bus but in motion relative to a building outside.
• Misconception: Every repeated movement is periodic. Correct: Periodic motion repeats after fixed time intervals. Repetition without fixed-time regularity is non-periodic.
• Misconception: Moving from one extreme of a pendulum's swing to the other completes an oscillation. Correct: Starting at one extreme, the bob must reach the other and return to its starting extreme.
• Misconception: Finding an average speed proves that motion was uniform. Correct: An average summarises total distance and total time; successive equal-time observations are needed to examine uniformity.
• Misconception: Any two distances are enough to compare which object was faster. Correct: Their times must also be known. For equal times, a greater distance indicates a greater average speed.
• Misconception: Dividing kilometres by minutes produces a speed in m/s. Correct: Convert kilometres to metres and minutes to seconds before calculating a result in m/s.
• Misconception: Mass and weight are the same because both can be found by weighing. Correct: Mass is the amount of matter; weight is gravitational force. Their SI units are kg and N, respectively.
• Misconception: All spring balances have the same smallest division. Correct: Ranges and divisions may differ, so inspect the particular scale before measuring.

Exam-style questions with model answers

Q1. Passengers remain seated in a moving bus. Explain their state of rest or motion relative to the bus and to a building outside. [2 marks]

1. Relative to the bus, the passengers are at rest because their positions in their seats do not change with time.
2. Relative to the building, they are in motion because their positions change as the bus travels.

Q2. Define average speed. A car travels 150 m in 10 s. Calculate its average speed in m/s and state whether this result alone proves uniform motion. [3 marks]

1. Average speed is the total distance travelled divided by the total time taken for that distance. Both measurements must refer to the same journey.
2. Using the given distance and time, average speed = 150 m ÷ 10 s = 15 m/s. The units already match the requested unit.
3. This result alone does not prove uniform motion. We would need information about distances covered in successive equal time intervals to establish uniformity.

Q3. Swati cycles 3.6 km to school in 15 min. Calculate her average speed in m/s in four steps: distance conversion, time conversion, formula and substitution. Use 1 km = 1000 m and 1 min = 60 s. [4 marks]

1. Convert the complete journey's distance to metres: 3.6 km = 3.6 × 1000 m = 3600 m.
2. Convert the corresponding journey time to seconds: 15 min = 15 × 60 s = 900 s.
3. Use the relationship average speed = total distance covered ÷ total time taken, keeping both measurements for the same journey.
4. Substituting gives 3600 m ÷ 900 s = 4 m/s. This is the average speed for the journey.

Q4. A pendulum has mean position O and extreme positions A and B. Explain its construction, how to start it, a complete oscillation, its time period, and why it is periodic. [5 marks]

1. A simple pendulum consists of a small metallic ball, called a bob, suspended by a long thread from a rigid support. Its resting position is O.
2. Move the bob slightly to one side and release it without pushing, keeping the thread taut. It begins moving to and fro about its mean position.
3. Starting at extreme position A, one complete oscillation takes the bob to extreme position B and then back to A, its starting position.
4. The time period is the time taken for one complete oscillation. It can be found by timing several complete oscillations and dividing by their number.
5. The motion is periodic because it repeats its path after a fixed interval. It is also oscillatory because it moves to and fro about O.

Q5. A car covers 60 km in the first hour, 70 km in the second hour and 50 km in the third hour. Find its total distance, total time and average speed, classify the motion, and explain the classification. [5 marks]

1. Add the distances for the three parts of the journey: total distance = 60 + 70 + 50 = 180 km. All distances use the same unit.
2. The journey includes the first, second and third hours, so its total duration is 3 h. This time corresponds to the entire distance just calculated.
3. Average speed = total distance ÷ total time = 180 km ÷ 3 h = 60 km/h. This is one summary value for the complete journey.
4. The motion is non-uniform. This classification refers to the pattern of movement over the journey, rather than merely the value obtained by dividing the totals.
5. The car covers unequal distances, 60 km, 70 km and 50 km, in equal one-hour intervals. Uniform motion would require equal distances in equal intervals.

Q6. A spring balance has a range of 0 to 10 N. Adjacent large marks differ by 1 N, with five equal small divisions between them. State what it measures, find one small division's value, and give the load precaution. [3 marks]

1. A spring balance measures force, including an object's weight, by the stretching of its spring. The weight reading is expressed in newtons, whose symbol is N.
2. The value of one small division is the difference between adjacent large marks divided by the number of small divisions: 1 N ÷ 5 = 0.2 N.
3. Do not suspend an object with weight above 10 N from this balance. Its stated maximum is 10 N, and exceeding that value may damage it.

Q7. An object has mass 1 kg on Earth and on the Moon. Its listed weights are 10 N on Earth and 1.6 N on the Moon. Explain the meanings and units of mass and weight, then describe separately what happens to each quantity when the object moves from Earth to the Moon. [4 marks]

1. Mass describes the amount of matter in an object; weight describes the gravitational force pulling that object towards the attracting body.
2. Mass is expressed here in kilograms, symbol kg. Weight is expressed in newtons, symbol N, because weight is a force.
3. The object's mass stays at 1 kg at both locations. Taking it to the Moon does not change its amount of matter.
4. The listed weight changes from 10 N to 1.6 N. Weight depends on gravitational pull, which differs between Earth and the Moon.

Q8. Over six successive 10-minute intervals, train X covers 20, 20, 20, 20, 20 and 20 km; train Y covers 20, 15, 15, 25, 20 and 25 km. Classify each train's motion and compare their average speeds over the full hour. [3 marks]

1. Train X has uniform motion because each of the equal 10-minute intervals contains the same distance, 20 km. Its total distance is 120 km.
2. Train Y has non-uniform motion because its distances in equal intervals differ. Adding 20 + 15 + 15 + 25 + 20 + 25 gives 120 km.
3. Each travels 120 km in the given one hour, so each average speed is 120 km/h. Equal average speeds can therefore accompany different patterns of motion.

Key takeaways

• Decide rest or motion by comparing an object's position over time with the same stated reference point.
• Classify motion by the feature being considered: path shape, to-and-fro movement, regular repetition, or an irregular path.
• A pendulum's time period is the time for one complete oscillation, including the return to its starting stage.
• Uniform motion covers equal distances in equal time intervals; non-uniform motion does not maintain that equality.
• Average speed equals total distance divided by total time, and does not by itself establish uniform motion.
• Convert distance and time into compatible units before calculating, and include the appropriate unit in the final answer.
• Weight is gravitational force measured in newtons; a spring balance measures it through the stretching of a spring.
• Mass remains unchanged with location, while weight can change because gravitational pull can differ from place to place.

Test yourself

Why must a reference point be stated when describing rest or motion?

An object's position may remain unchanged relative to one reference point while changing relative to another, as for passengers in a moving bus.

What feature distinguishes circular motion from linear motion?

Circular motion follows a circular path, whereas linear motion follows a straight path.

A pendulum starts at extreme position A. What journey completes one oscillation if the other extreme is B?

The bob moves from A to B and returns to A, completing a full to-and-fro movement.

Does every repetitive movement qualify as periodic motion?

No. Periodic motion requires repetition at fixed time intervals, rather than repetition without a regular timing pattern.

What distinguishes a speedometer from an odometer?

A speedometer displays a vehicle's speed, while an odometer measures the distance travelled by the vehicle.

A train travels at 90 km/h for a distance of 360 km. How long does it take?

Time equals distance divided by speed: 360 km ÷ 90 km/h = 4 h.

Which changes with gravitational pull: mass or weight?

Weight can change with gravitational pull; mass remains the amount of matter in the object.

Why should the divisions on a spring balance be checked before using it?

Spring balances may have different ranges and division sizes, so each instrument's scale must be read correctly.