Force And Motion
Class 7 | Science — Physics | Unit 2
Class 7 Unit 2 Science
Unit 2
Force and Motion
Picture, picture description begins, basketball court showing the ball is in motion. picture description ends.
Learning Objectives
After studying this unit, students will be able to:
- define distance and displacement.
- differentiate distance and displacement.
- define speed, velocity and acceleration.
- differentiate speed and velocity.
- draw and explain distance - time and velocity - time graphs.
- measure and calculate the speed of moving objects.
- know the day to day uses of centre of gravity and stability
QR code, QR code description begins QR CODE IS GIVEN, NUMBER, Z, F, A, L,3. QR code description ends.
Introduction
Look at the picture given below. Kavitha can reach her school in two ways, as shown in the picture. Can you tell, by choosing which path she could reach the school early?
Picture, picture description begins two roads marked A, 1.5 kilometer and B, 2 kilometer from kavitha's home to school. picture description ends.
In the picture given below, you can see leaf falling from a tree. In which path the leaf will reach the ground first?
Picture, picture description begins, shows path of the leaf, when there is no wind and another path of the leaf when there is wind. picture description ends.
Uma and Priya are friends studying in the same school. After school hours, they go to the nearby playground, play games and return back home. One day Uma told that she would reach the playground after visiting her grandmother's house. The paths which they took to reach the playground is shown here. Take a twine and measure the length of the two paths (A and B). Which is the longest path among the two?
Picture, picture description begins, shows that the path A is longer than the path B. picture description ends.
From the above examples, we could conclude that when an object travels from one place to another, it will reach faster if it travels along the straight line path. The straight line path is the shortest distance between two points. In this lesson we are going to study about distance and displacement, speed and velocity, acceleration, distance - time graph, velocity - time graph, centre of gravity and stability
2.1 Distance and Displacement
The total length of a path taken by an object to reach one place from another place is called distance. The shortest distance from the initial position to the final position of an object is called displacement. Both distance and displacement possess the same unit. The SI unit distance and displacement is metre (m).
The figure given below shows the motion of a person between two places A and B
Picture, picture description begins, shows the motion of a person between two places A and B, path 1, 10 kilo meter, path 2, 7 kilometer, path 3, 5 kilometer. picture description ends.
He travels 10 kilometres along the first path. Along the second path, he travels 7 kilometres. The distance between A and B in the case of first path is 10 kilometres. In the case of second path, the distance is 7 kilometres. The shortest distance between the two places is 5 kilometre which is represented by the third path. So, the displacement is 5 kilometres (In east direction).
The path of an object moving from point A to point B is shown in the figure. Total distance travelled by the object is 120 metres. The displacement of the object is 40 metres (south - east direction).
Picture, picture description begins, shows the distance travelled by the object 120 meter and the displacement by the object is 40 meter south east direction. picture description ends.
The path in which a rabbit ran is shown in the figure below. Let us consider that each square is in an unit of one square meter. The rabbit starts from point A and reaches the point B. Find the distance and displacement of it in the two figures. When will the distance and displacement be equal? (The starting point and the finishing point should be different).
Picture, picture description begins, the distance and displacement of two points given in two different pictures with the example of rabbit motion. picture description ends.
When we represent the displacement, we use a positive or negative sign depending on the direction in which it travels
Picture, picture description begins, shows the displacement of the object from a to b. picture description ends.
Let us consider the point A as the starting point. While the object moves from A to B the displacement is considered to be positive and it is negative, when it travels from B to A.
Subha goes to the nearby playground from her home. Look at the picture and answer the following questions.
1. What is the distance she travelled?
2. What is her displacement?
Picture, picture description begins, shows the distance travelled and displacement by subha from home to playground, distance travelled 400 meter, displacement 100 meter. picture description ends.
Can you answer the following questions?
The distance travelled by an object is 15 kilometre and its displacement is 15 kilometres. What do you infer from this?
The distance travelled by a person is 30 kilometre and his displacement is 0 kilometre. What do you infer from this?
Do You Know?
Nautical mile
Nautical mile is the unit for measuring the distance in the field of aviation and sea transportation. One nautical mile is 1.852 kilometre. The unit for measuring the speed of aeroplanes and ships is knot. It means that they travel one nautical mile in one hour.
2.2 Speed – Velocity
2.2.1 Speed
In sixth standard you have already studied about speed in detail. Speed is the rate of change of distance.
Speed = Distance ÷ Time
The unit of speed is metre per second (m/s).
We can classify speed into two types
1. Uniform speed
If a body in motion covers equal distances in equal intervals of time, then the body is said to be in uniform speed.
2. Non-uniform speed
If a body covers unequal distances in equal intervals of time, the body is said to be in non-uniform speed.
Average Speed = Total distance travelled ÷ Time taken to travel the distance
Do You Know?
1 kilometre per hour = 5/18 metre per second
How we got this?
1 kilometre = 1000 metre
1 hour = 3600 seconds
1 kilometre per hour = 1000 metre ÷ 3600 second = 5/18 metre per second
Do You Know?
Know the speed
- Tortoise: 0.1 metre per second
- Person walking: 1.4 metre per second
- Falling raindrop: 9 to 10 metre per second
- Cat running: 14 metre per second
- Cycling: 20 - 25 kilometre per hour
- Cheetah running: 31 metre per second
- Bowling speed of fast bowlers: 90 to 100 miles per hour
- Badminton smash: 80 to 90 metre per second
- Passenger jet: 180 metre per second
- Rocket: 5200 metre per second
2.2.2 Velocity
Velocity is the rate of change in displacement.
Velocity (v) = Displacement ÷ Time
SI unit of velocity is metre/second (m/s).
Look at the figure. An athlete takes 25 second to complete a 200-metre sprint event. Find her speed and velocity
Speed = Distance ÷ Time = 200 ÷ 25 = 8 metre per second
Velocity = Displacement ÷ Time = 50 ÷ 25 = 2 metre per second
Picture, picture description begins, shows the athlete sprinting on the ground with a displacement 50 meter. picture description ends.
Uniform velocity
A body is said to have uniform velocity, if it covers equal displacement at equal intervals of time in the same direction.
Example: Light travels through vacuum.
Non-uniform velocity
If either speed or direction changes, the velocity is non-uniform.
Example: A train starting and moving out of the station
Average velocity
If the total displacement of an object is divided by the total time taken by the object we get the average velocity.
Average velocity = Total displacement ÷ Total time taken
In the figure given below, a car travels 5-kilometre due east and makes a U – turn to travel another 7 kilometres. If the time taken for the whole journey is 0.2 hour, calculate the average velocity of the car.
Picture, picture description begins, shows the situation of the motion of the car explained above. picture description ends.
Average velocity = Total displacement ÷ Time taken. (Taking the direction due east of point O as positive)
Average velocity = (5 - 7) ÷ 0.2 = -2 ÷ 0.2 = -10 kilometre per hour or -10 × 5/18 = -25/9 = -0.28 metre per second
The triangle method can help you to recall the relationship between velocity (v), displacement (d), and time(t)
Picture, picture description begins, triangle is given to know the relationship between velocity displacement and time. picture description ends.
- Velocity = displacement ÷ time
- time = displacement ÷ velocity
- displacement = velocity × time
Answer the following questions.
1. Calculate the velocity of a car travelling with a uniform velocity covering 100 metre in 4 seconds.
2. Usain Bolt covers 100 metre in 9.58 seconds. Calculate his speed. If Usain Bolt competes with a Cheetah which is running at a speed of 30 metre per second, who will be the winner?
3. You are walking along east direction covering a distance of 4 metre, then 2 metre towards south, then 4 metre towards west and at last 2 metre towards north. You cover the total distance in 21 seconds. What is your average speed and average velocity?
2.3 Acceleration
Acceleration is the rate of change of velocity. In other words, if a body changes its speed or direction then it is said to be accelerated.
Picture, picture description begins, explaining the acceleration of the body, a) change in speed, b) change in direction, c) change in both speed and direction. picture description ends.
Acceleration = Change in velocity ÷ Time = Final velocity (v) – Initial velocity (u) ÷ Time
a = (v – u) ÷ t
SI unit of acceleration is metre per second square
A car at rest starts to travel in a straight-line path. It reaches a velocity of 12 metre per second in 4 second. What is its acceleration, assuming that it accelerates uniformly?
Initial velocity, u = 0 metre per second (Since the car starts from rest)
Final velocity (v) = 12 metre per second
Time taken (t) = 4 second
Acceleration (a) = (v - u) ÷ t = (12 - 0) ÷ 4 = 3 metre per second square
Picture, picture description begins, explains the situation of the car mentioned above. picture description ends.
QR code, QR code description starts QR code number is S18DJ is given. QR code description ends.
Do You Know?
Picture, picture description begins picture of a cheetah is given. picture description ends.
See how brisk I am! My name is cheetah. I can run at great speed. Do you know what my speed is? It is 25 metre per second to 30 metre per second. My speed changes from 0 to 20 metre per second in 2 second. See how good my acceleration is! Can you calculate it?
2.3.1 Positive acceleration
If the velocity of an object increases with respect to time, then the object is said to be in positive acceleration
The velocity of a train at different times is given in the figure. Analyse this and complete the table.
Picture, picture description begins to analyse the velocity of train at different time is given. picture description ends.
| The distance travelled by train | Initial velocity (u) meter per second | Final velocity (v) meter per second | Change in velocity (v - u) meter per second | Time taken (t) second | Acceleration = Change in velocity ÷ Time a = (v - u) ÷ t meter per second square |
|---|
| A to B | 0 | 6 | 6 | 10 | 0.6 |
| B to C | | | | | |
| C to D | | | | | |
| D to E | | | | | |
| E to F | | | | | |
2.3.2 Negative acceleration or Deceleration or Retardation
If the velocity of an object decreases with respect to time, then the object is said to be in negative acceleration or deceleration or retardation
Picture, picture description begins shows the deceleration of the golf ball mentioned below. picture description ends.
The velocity of a golf ball rolling in a straight-line change from 8 metre per second, to 2 metre per second, in 10 seconds. What is its deceleration, assuming that it is decelerating uniformly?
Initial velocity (u) = 8 meter per second
Final velocity (v) = 2 meter per second
Time taken (t) = 10 second
Acceleration (a) = (v - u) ÷ t = (2 - 8) ÷ 10 = -0.6 meter per second square
The deceleration is -0.6 meter per second square.
2.3.3 Uniform acceleration
An object undergoes uniform acceleration when the change (increase or decrease) in its velocity for every unit of time is the same.
The table given below table description begins shows the uniform acceleration of a bus. picture description ends.
| Time (s) | 1 | 2 | 3 | 4 | 5 |
|---|
| Velocity (metre per second) | 20+20 | 40+20 | 60+20 | 80+20 | 100+20 |
|---|
| acceleration | |
|---|
| Velocity (metre per second) | 100-20 | 80-20 | 60-20 | 40-20 | 20-20 |
|---|
| Deceleration | |
|---|
When the velocity of the object is increasing by 20 meter per second the acceleration is 20 meter per second square. When the velocity of the object is decreasing by 20 meter per second the deceleration is 20 meter per second square.
2.3.4 Non – uniform acceleration
An object undergoes non–uniform acceleration if the change in its velocity for every unit of time is not the same.
| Time (second) | 0 | 1 | 2 | 3 | 4 | 5 |
|---|
| Velocity [meter per second] | 0 | 10 | 40 | 60 | 70 | 50 |
|---|
| Change in Velocity [meter per second] | 0 | 10 | 30 | 20 | 10 | 20 |
|---|
Note here that the change in velocity is not the same for every second. Thus, the moving object is undergoing non-uniform acceleration.
2.4 Distance – Time Graphs
A car travelling along a straight line away from the starting point O is shown in the figure. The distance of the car is measured for every second. The distance and time are recorded and a graph is plotted using the data. The results for four possible journeys are shown below.
Picture, picture description begins shows the motion of a car with displacement s. picture description ends.
a. Car at rest
| Time [second] | 0 | 1 | 2 | 3 | 4 | 5 |
|---|
| Distance | 20 | 20 | 20 | 20 | 20 | 20 |
|---|
Picture, picture description begins the graph shows car at rest. picture description ends.
The graph has zero gradient. That is the distance is constant for every second. Thus, the car is at rest.
b. Car travelling at uniform speed of 10 meters per second
| Time [second] | 0 | 1 | 2 | 3 | 4 | 5 |
|---|
| Distance | 0 | 10 | 20 | 30 | 40 | 50 |
|---|
Picture, picture description begins the graph shows car travelling at uniform speed. picture description ends.
The graph has constant gradient. The distance increases 10 metre in every second. Thus, the car moves with uniform speed.
c. Car travelling at increasing speed
| Time [second] | 0 | 1 | 2 | 3 | 4 | 5 |
|---|
| Distance | 0 | 5 | 20 | 45 | 80 | 125 |
|---|
Picture, picture description begins the graph shows the car travelling at increasing speed. picture description ends.
The graph has an increasing gradient, that is, the speed increases
d. Car travelling at decreasing speed
| Time [second] | 0 | 1 | 2 | 3 | 4 | 5 |
|---|
| Distance | 0 | 45 | 80 | 105 | 120 | 125 |
|---|
Picture, picture description begins the graph shows car travelling at decreasing speed. picture description ends.
The graph has a decreasing gradient. That is, the speed decreases
2.5 Speed – Time Graphs
Let us consider a bus travelling from Thanjavur to Trichy. The speed of the bus is measured for every second. The speed and time are recorded and a graph is plotted using the data. It is known as speed-time graph. The results for four possible journeys are shown.
a. Bus at rest
| Time (second) | 0 | 1 | 2 | 3 | 4 | 5 |
|---|
| Speed (metre per second⁻¹) | 0 | 0 | 0 | 0 | 0 | 0 |
|---|
Picture, picture description begins the graph shows bus at rest. picture description ends.
The speed of the bus remains at 0 metre per second. So, the bus has zero acceleration
b. Bus travelling at uniform speed of metre per second
| Time (In Seconds) | 0 | 1 | 2 | 3 | 4 | 5 |
|---|
| Speed (metre per second⁻¹) | 10 | 10 | 10 | 10 | 10 | 10 |
|---|
Picture, picture description begins the graph shows bus travelling at uniform speed. picture description ends.
The speed of the bus remains at 10 metre per second⁻¹. Here, slope of the line is zero. So, the bus has zero acceleration.
c. Bus travelling uniform acceleration
| Time (In Seconds) | 0 | 1 | 2 | 3 | 4 | 5 |
|---|
| Speed (metre per second⁻¹) | 10 | 10 | 20 | 30 | 40 | 50 |
|---|
Picture, picture description begins the graph shows that the bus travelling in uniform acceleration. picture description ends.
The speed of the bus increases by 10 metre per second⁻¹ every second. Hence, the graph has a positive and constant gradient, and the acceleration is constant.
d. Bus travelling uniform deceleration
| Time (In Seconds) | 0 | 1 | 2 | 3 | 4 | 5 |
|---|
| Speed (metre per second⁻¹) | 50 | 40 | 30 | 20 | 10 | 0 |
|---|
Picture, picture description begins the graph shows that the bus is travelling in uniform deceleration. picture description ends
The speed of the bus decreases by 10 metre per second⁻¹ very second. Hence, the graph has a negative and constant gradient and the acceleration is negative and constant.
e. Bus travelling with increasing acceleration (Non-uniform acceleration)
| Time (In Seconds) | 0 | 1 | 2 | 3 | 4 | 5 |
|---|
| Speed (metre per second⁻¹) | 10 | 2 | 8 | 18 | 32 | 50 |
|---|
Picture, picture description begins The graph shows that the bus travelling with increasing acceleration (non- uniform acceleration). picture description ends.
The speed of the bus is increasing with time. Hence, the graph has a positive and increasing gradient and the acceleration increases.
f. Bus travelling with decreasing acceleration (non-uniform acceleration)
| Time (In Seconds) | 0 | 1 | 2 | 3 | 4 | 5 |
|---|
| Speed (metre per second⁻¹) | 10 | 18 | 32 | 42 | 48 | 50 |
|---|
Picture, picture description begins The graph shows bus travelling with decreasing acceleration (non-uniform acceleration) picture description ends.
The speed is decreasing with time. Hence, the graph has a positive and decreasing gradient, and the acceleration decreases.
2.5.1 Comparison between Distance – Time and Speed – Time Graphs
The Speed – Time graphs and Distance – Time graphs may look very similar. But, they give different information. We can differentiate them by looking at the labels.
Graph and Story
Raju began walking to his school. Suddenly he remembered that he forgot his pen and walked back home. But he stopped suddenly when he heard a noise.
Picture, picture description begins the graph is given according to the given story. picture description ends.
Draw a graph for the given story
Rani was waiting for her mother for some time. When she saw her mother, she ran out of her home hugged her and stood there for a while.
Picture, picture description begins a graph to be drawn according to the given story. picture description ends.
Imagine and write a story on your own for the given graph
Picture, picture description begins a graph is given to write a story. picture description ends.
| | From A to B | From B to C | From C to D |
|---|
| | Car accelerates uniformly from rest. | From C to D Car moves at constant speed | Car decelerates uniformly to a stop. |
Distance and Time Graph
Picture, picture description begins a distance time graph is given with the curves explained. picture description ends.
| Distance in metres | Description |
|---|
| Distance moved increases at an increasing rate. Hence, gradient increases (Represented by a concave curve). | Distance moved increases uniformly over time. Hence, gradient is a constant (Represented by a straight line) |
| Distance moved increases at a decreasing rate. Hence, gradient decreases (Represented by a convex curve). |
Speed and Time Graph
Picture, picture description begins a speed- time graph is given with the curves explained. picture description ends.
| Speed Metre per second⁻¹ | Description |
|---|
| Speed increases uniformly over time. Hence, gradient is a positive constant (represented by a straight line). | Speed is constant. Hence, the graph here is a horizontal line. |
| Speed decreases uniformly over time. Hence, gradient is a negative constant (Represented by a straight line) |
2.6 Centre of Gravity
Try to balance a cardboard on your finger tip. What do you observe? You can notice that there is only one point at which the cardboard is balanced. The point at which the cardboard is balanced is called the centre of gravity of the cardboard. The centre of gravity of an object is the point through which the entire weight of the object appears to act.
How do we find the centre of gravity of an object?
QR code, QR code description begins QR Code is given, Number, B, R, X, S, O. QR code description ends.
ACTIVITY 1
What about irregular shaped objects?
Apparatus: Irregularly shaped card, string, pendulum bob, stand
- Make three holes in the lamina.
- Suspend the lamina from the optical pin through one of the holes as shown in figure.
- Suspend the plumbline from the pin and mark the position of the plumbline on the lamina.
- Draw lines on the lamina representing the positions of the plumbline.
- Repeat the above steps for the other holes.
- Label the intersection of the three lines as X, the position of the centre of gravity of the lamina.
Picture, picture description begins explaining the set-up of the activity. picture description ends.
Centre of Gravity
Total pull of the earth (weight) appears to act through the centre of gravity
Picture, picture description begins showing the weight of an object appears to act through the centre of gravity. picture description ends.
2.6.1 Centre of gravity of regular – shaped objects
Generally, the centre of gravity of the geometrical shaped objects lie on the geometric centre of the object.
Picture, picture description begins shows the weight of a square shaped card acting through the centre of gravity. picture description ends.
Picture, picture description begins shows the weight of a triangle acting through the centre of gravity. picture description ends.
Picture, picture description begins shows the weight of a triangle disc through the centre of gravity picture description ends.
Picture, picture description begins shows the weight of a ring acting through the centre of gravity. picture description ends.
The ruler is in equilibrium when supported at its centre of gravity. For a regular object such as a uniform meter ruler, the centre of gravity is at the centre of the object. When the object is supported at that point, it will be balanced. If it is supported at any other point, it will topple.
2.7 Stability
Stability is a measure of the body's ability to maintain its original position. Three types of stability are:
- A. Stable equilibrium
- B. Unstable equilibrium
- C. Neutral equilibrium
Let us demonstrate them by taking a frustum.
Stable Equilibrium
In stable equilibrium, the frustum can be tilted through quite a big angle without toppling.
Picture, picture description begins shows the stable equilibrium of the frustum of a cone. picture description ends.
Its centre of gravity is raised when it is displaced. The vertical line through its centre of gravity still falls within its base. So, it can return to its original position.
Unstable Equilibrium
In this equilibrium, the frustum will topple with the slightest tilting. Its centre of gravity is lowered when it is displaced.
Picture, picture description begins shows the unstable equilibrium of the frustum of a cone. picture description ends.
Here, the vertical line through its centre of gravity falls outside its base. So, it will not come back to its position
Neutral Equilibrium
It causes frustum to topple. The frustum will roll about but does not topple. Its centre of gravity remains at the same height when it is displaced. The body will stay at any position to which it has been displaced.
Picture, picture description begins shows the neutral equilibrium of the frustum of a cone. picture description ends.
2.7.1 Condition for Stability
Stability can be increased by the following ways.
- Lowering its centre of gravity
- Increasing the area of its base
- A heavy base lowers the centre of gravity
So, the object will be stable.
A broad base makes the object more stable.
Do You Know?
The Thanjavur Doll
It is s type of traditional toy made in Thanjavur from terracotta material. The centre of gravity and the total weight of the doll is concentrated at its bottom most point, generating a dance-like continuous movement with slow oscillations
Picture, picture description begins picture of a Tanjavur doll is given. picture description ends.
2.7.2 Real Life Applications of Centre of Gravity
- In order to have stability, the luggage compartment of a tour bus is located at the bottom and not on the roof.
- Extra passengers are not allowed on the upper deck of a crowded double decker bus.
- Racing cars are built low and broad for stability.
- Table lamps and fans are designed with large heavy bases to make them stable.
Points to Remember
- The total length of a path taken by an object to reach one place from another place is called distance.
- The shortest distance from the initial to the final position of an object.
- Acceleration is the rate of change in velocity.
- SI unit of acceleration is metre per second square
- Velocity is the rate of change in displacement.
- SI unit of velocity is metre per second (m/s).
- The centre of gravity of an object is the point through which the entire weight of the object appears to act.
- Generally, the centre of gravity of the geometrical shaped object lie on the geometric centre of the object.
- Stability is a measure of the body's ability to maintain its original position.
- The three types of stability are: stable equilibrium, unstable equilibrium, neutral equilibrium.
Evaluation
QR Code, QR code description begins QR Code is given, Number, 1, L, T, E, D. QR Code description ends.
Exercise I. Choose the best answer
1. A particle is moving in a circular path of radius r. The displacement after half a circle would be
a. Zero b. R c. 2r d. r/2
2. Which of the following figures represent uniform motion of a moving object correctly?
Picture, picture description begins distance time graph of an object is in uniform motion is given. picture description ends.
Picture, picture description begins distance time graph of an object is at rest is given. picture description ends.
Picture, picture description begins distance time graph of an object is at increasing motion is given. picture description ends.
Picture, picture description begins distance time graph of an object is at decreasing motion is given. picture description ends.
a. First graph b. Second graph c. Third graph d. Fourth graph
3. Suppose a boy is enjoying a ride on a merry go round which is moving with a constant speed of 10 metre per second. It implies that the boy is
a. at rest b. moving with no acceleration c. in accelerated motion d. moving with uniform velocity
4. From the given v to t graph it can be inferred that an object is
Picture, picture description begins distance time graph of an object is at uniform motion. picture description ends.
a. in uniform motion b. at rest c. in non - uniform motion d. moving with uniform accelerations
5. How can we increase the stability of an object?
a. Lowering the centre of gravity b. Raising the centre of gravity c. Increasing the height of the object d. Shortening the base of the object Exercise II. Fill in the blanks
1. The shortest distance between two places is
2. The rate of change of velocity is
3. If the velocity of an object increases with respect to time, then the object is said to be in acceleration.
4. The slope of the speed–time graph gives
5. In equilibrium, the centre of gravity remains at the same height when it is displaced.
Exercise III. Match the following
| Displacement | |
| Light travelling through vacuum | |
| Speed of ship | |
| Centre of gravity of geometrical shaped objects | |
| Stability | |
Options: Knot, Geometric centre, Metre, Larger base area, Uniform velocity
Exercise IV. Analogy
1. Velocity : metre per second :: Acceleration :
2. Length of scale : metre :: Speed of aeroplane :
3. Displacement ÷ Time : Velocity :: Speed ÷ Time :
Exercise V. Answer very briefly
1. Asher says all objects having uniform speed need not have uniform velocity. Give reason.
2. Saphira moves at a constant speed in the same direction. Rephrase the same sentence in fewer words using concepts related to motion.
3. Correct your friend who says that acceleration gives the idea of how fast the position changes.
Exercise VI. Answer briefly
1. Show the shape of the distance and time graph for the motion in the following cases.
a. A bus moving with a constant speed.
b. A car parked on a road side.
2. Distinguish between speed and velocity.
3. What do you mean by constant acceleration?
4. What is centre of gravity?
Exercise VII. Answer in detail
1. Explain the types of stability with suitable examples.
2. Write about the experiment to find the centre of gravity of the irregularly shaped plate.
Exercise VIII. Numerical problems
1. Geetha takes 15 minutes from her house to reach her school on a bicycle. If the bicycle has a speed of 2 metre per second, calculate the distance between her house and the school.
2. A car starts from rest and it is travelling with a velocity of 20 metre per second in 10 seconds. What is its acceleration?
3. A bus can accelerate with an acceleration of 1 metre per second square. Find the minimum time for the bus to attain the speed of 100 km/s from 50 km/s.
Exercise IX. Fill in the boxes
| Serial Number | First Move | Second Move | Distance (in metre) | Displacement |
|---|
| 1 | Move 4 metres east | Move 2 metres west | 6 metre | 2metres East |
| 2 | Move 4 metres north | Move 2 metres south | | |
| 3 | Move 2 metres east | Move 4 metres west | | |
| 4 | Move 5 metres east | Move 5 metres west | | |
| 5 | Move 5 metres south | Move 2 metres north | | |
| 6 | Move 10 metres west | Move 3 metres east | | |
📝 Summary
- Distance is the total length of the path taken by an object to travel from one place to another, while displacement is the shortest distance from the initial position to the final position of an object.
- Both distance and displacement have the same SI unit which is metre (m), but displacement includes direction while distance does not.
- The straight line path between two points is always the shortest distance, allowing objects to reach their destination faster compared to longer curved paths.
- Distance and displacement are equal only when an object moves in a straight line path from starting point to finishing point without changing direction.
- This unit also covers the study of speed, velocity, acceleration, distance-time graphs, velocity-time graphs, centre of gravity and stability in motion.