In the previous chapter, we had a discussion on motion.
• We saw objects moving with a uniform velocity.
♦ But we did not discuss about how those objects were able to move with uniform velocity.
• Also we saw that objects can move with uniform acceleration.
♦ But we did not discuss about how those objects were able to move with uniform acceleration.
• In this chapter we will discuss those topics.
1. Consider a ball resting on a level ground. If we give it a mild hit, it will begin to roll.
2. But we can see that, after a short while, it's velocity begins to decrease, and will become zero.
3. That is., the ball will come to rest again.
4. It is as if the ball does not want to move. It is as if the ball wants to be at rest always.
5. Such examples suggest that rest is the 'natural state' of objects. But is it true? We will soon see.
Let us see some more examples from our day to day life:
■ In the super market we use a trolley to move the items that we want to buy from the shelves to the billing counter.
• But to move the trolley, we must push it. If we do not apply the pushing force, the trolley will not reach up to the billing counter.
■ To close the door while leaving a room, we must pull the door by the handle.
• If we do not apply the pulling force, the door will not shut and the room will remain open.
■ For removing dust and dirt from a carpet, we hit it with a stick. Here it is not pushing or pulling. But it is 'hitting'.
• If we do not apply the hitting force, the dust and dirt will not be removed.
In the above examples, the objects: trolley, door and dust were at rest. We wanted them to move. We achieved it by applying a pushing force, pulling force or a hitting force.
■ Now consider a rolling ball. We want to stop it before it reaches the boundary. The only way is to apply a force. It can be viewed in this way:
• You are standing guard at the boundary. The ball is rolling towards you.
• You do not want to get dirt on your hands. So you decide to stop the ball with a wooden board. You hold the board towards the incoming ball and wait.
• When the ball come and hit the board, you must apply a force. Then only it will stop. If you do not apply a force, both the ball and the board will reach the boundary.
■ Some one opened the window shutter and forgot to fasten it. A wind blew and swung the shutter towards the window.
• You must apply a force to stop it. If you apply a force, the window will shut with a bang and cause damage.
In the above examples, the ball and window shutter were in motion. We wanted them to stop. We achieved it by applying a force.
■ So we can write:
• A force is required to put an object at rest into motion
• A force is required to put an object in motion to rest
Let us get to know more about force:
We cannot actually see a force. We can only see the effect. For example:
■ Consider a man in a supermarket. His hands are on the handles of the trolley. He is standing still and looking at the items in the shelf. Do we see any force? No
• After some time, the man decides to move on. He pushes the trolley forward. Now do we see any force?
Actually no. We see only the effect produced by the pushing force. We see the effect which is: 'the forward motion of the trolley'.
• So any applied force can be described only by 'describing it's effect'.
■ By studying the 'effect produced by a force', we will be able to write the following:
• Whether the force is a moving force or stopping force
• Whether the force is large or small. That is., the magnitude of the force.
1. Consider a wooden block resting on a smooth surface. It is shown in fig.2.1 below:
2. A force F1 pushes it from the left towards the right. Then the block will move towards the right.
3. Now consider Fig.2.2(a) below. The same force F1 is now applied from the right side also.
4. This time the block is being pushed from opposite sides by the same amounts of forces. In this case, the block will not move. So it is a case of balanced forces.
5. Now consider fig.2.2(b). The opposing force F2 which acts from right to left is smaller. Now the block will move towards the right. This is a case of unbalanced forces.
• There is friction between the 'bottom surface of the block' and the 'surface of the floor'
• It caused an opposing 'frictional force' Ff
4. Like any force, we cannot see Ff. We can only see it's effects. Here the effect is:
'Not allowing the block to move even when F1 is applied'.
• In later chapters, when we learn about friction, we will see the method to measure the magnitude of Ff.
5. But at present, we can sure realise that, Ff is greater than F1. That is why the block is not moving.
6. Let us apply a larger force F3. This is shown in the fig.2.4 below:
7. Now the forces are unbalanced. The right ward push can overcome the left ward frictional force. So the block begins to move towards the right. What will happen after it begins to move?
• If we want to move it from one end of the play ground to the other end, We have to keep on pushing it until it reaches the other end. That is., we have to keep on applying the force F3
• If at any time during this journey, we stop applying the force or reduce the force, then the block will come to a stop.
• So, the question is this:
If there is no opposing force, do we have to keep pushing it until it reaches the other end?
• The answer is no. Let us see the explanation:
1. Suppose that there is no friction between the block and the floor of the play ground.
2. The block is initially at rest. We give the block a single push.
3. Since there is no force acting in the opposite direction, the push is an unbalanced force.
4. So the block will begin to move. We do not have to keep pushing it. The block will reach the other end of the play ground.
5. But it will not stop there. Unless there is some thing to stop it, the block will continue it's journey and move out of the play ground.
6. In fact, it will continue it's journey for ever. This is because, there is no friction or any other force to stop it.
7. If we do not want to lose the block, we will have to apply a force and stop it. Either get behind it and apply a pulling force or get ahead of it and apply a pushing force.
8. Once the block has attained a velocity due to the push, it will continue to move with that velocity.
■ Consider a person standing in a moving bus. When the bus stops suddenly, he tends to fall forward. Why does that happen?
1. When the bus is in motion, the person standing is also in the state of motion.
2. His body has the tendency to keep moving for ever.
3. But when the bus stops, the lower part of his body which is in contact with the bus, comes to a stop.
4. The upper part of his body tends to continue moving. That is why he falls forward.
■ This happens while travelling in a car too. In the moving car, we are sitting. But if the car stops suddenly, we will be thrown forward causing injuries. That is why wearing seat belts are made compulsory.
■ What we saw above are examples for:
• The tendency of an object in motion to continue in the state of motion.
■ We can show the ‘opposite of the above situations’ as examples for:
• The tendency of an object at rest to continue in the state of rest.
Let us write them:
1. When the bus is at rest, the person standing is also in the state of rest.
2. His body has the tendency to continue in the state of rest.
3. But when the bus starts to move, the lower part of his body which is in contact with the bus, starts to move.
4. The upper part of his body tends to continue at rest. That is why he falls backward.
■ This happens while travelling in a car too. In the car which is stationary, we are sitting. But if the car starts to move suddenly, we will be thrown backwards.
In other words, all objects resist a change in their state of rest or motion. The tendency of undisturbed objects to stay at rest or to keep moving with the same velocity is called inertia. This is why, the first law of motion is also known as the law of inertia.
A video demonstrating the law can be seen here.
1. We see two objects in motion.
2. They are moving with the same velocity. But they have different masses. That is., one is heavier than the other.
3. We want to stop them both. We know that those two objects will be wanting to continue in motion.
4. In other words, those two moving objects have acquired a special tendency. The tendency to continue in motion.
5. We saw that this tendency is called 'inertia'.
6. More specifically, as those objects are in motion, we can call the tendency: ‘inertia of motion’.
7. Since they have acquired this inertia of motion, they wont stop unless we put an effort.
8. How much effort do we have to put for each?
• Is the effort required, same for both the objects?
• Does one of them require a greater effort? If so which one need the greater effort?
♦ If we know that before hand, we can send a stronger person to stop it.
9. So we see that measuring inertia is essential.
10. By experience we find that, if two objects of different masses travel with the same velocity, the heavier object will require a greater effort.
11. For example, if a tennis ball and a cricket ball travel with the same velocity, a greater effort is required to stop the cricket ball. Because it has greater mass. [Note that, for making a comparison, velocities have to be the same. It will be more difficult to stop a tennis ball if it is travelling at a very high speed]
12. Another example: Huge and sophisticated brakes are required to stop a train. While smaller brakes are sufficient for a car. Even smaller brakes are sufficient for a bicycle.
■ So we can write:
Heavier objects have greater inertia of motion.
■ Let us consider inertia of rest. It also depends on mass.
• Take the example of a cricket ball and a tennis ball at rest.
• It requires greater effort to set the cricket ball in motion.
■ So we can write this:
Mass is a measure of inertia.
In the next section, we will see Newton's Second Law of motion.
• We saw objects moving with a uniform velocity.
♦ But we did not discuss about how those objects were able to move with uniform velocity.
• Also we saw that objects can move with uniform acceleration.
♦ But we did not discuss about how those objects were able to move with uniform acceleration.
• In this chapter we will discuss those topics.
1. Consider a ball resting on a level ground. If we give it a mild hit, it will begin to roll.
2. But we can see that, after a short while, it's velocity begins to decrease, and will become zero.
3. That is., the ball will come to rest again.
4. It is as if the ball does not want to move. It is as if the ball wants to be at rest always.
5. Such examples suggest that rest is the 'natural state' of objects. But is it true? We will soon see.
Let us see some more examples from our day to day life:
■ In the super market we use a trolley to move the items that we want to buy from the shelves to the billing counter.
• But to move the trolley, we must push it. If we do not apply the pushing force, the trolley will not reach up to the billing counter.
■ To close the door while leaving a room, we must pull the door by the handle.
• If we do not apply the pulling force, the door will not shut and the room will remain open.
■ For removing dust and dirt from a carpet, we hit it with a stick. Here it is not pushing or pulling. But it is 'hitting'.
• If we do not apply the hitting force, the dust and dirt will not be removed.
In the above examples, the objects: trolley, door and dust were at rest. We wanted them to move. We achieved it by applying a pushing force, pulling force or a hitting force.
■ Now consider a rolling ball. We want to stop it before it reaches the boundary. The only way is to apply a force. It can be viewed in this way:
• You are standing guard at the boundary. The ball is rolling towards you.
• You do not want to get dirt on your hands. So you decide to stop the ball with a wooden board. You hold the board towards the incoming ball and wait.
• When the ball come and hit the board, you must apply a force. Then only it will stop. If you do not apply a force, both the ball and the board will reach the boundary.
■ Some one opened the window shutter and forgot to fasten it. A wind blew and swung the shutter towards the window.
• You must apply a force to stop it. If you apply a force, the window will shut with a bang and cause damage.
In the above examples, the ball and window shutter were in motion. We wanted them to stop. We achieved it by applying a force.
■ So we can write:
• A force is required to put an object at rest into motion
• A force is required to put an object in motion to rest
In our day to day life we experience many such situations where we must put an effort to achieve some thing. Mostly they are muscular efforts. We experience them while pushing, pulling, hitting, stopping etc.,
Let us get to know more about force:
We cannot actually see a force. We can only see the effect. For example:
■ Consider a man in a supermarket. His hands are on the handles of the trolley. He is standing still and looking at the items in the shelf. Do we see any force? No
• After some time, the man decides to move on. He pushes the trolley forward. Now do we see any force?
Actually no. We see only the effect produced by the pushing force. We see the effect which is: 'the forward motion of the trolley'.
• So any applied force can be described only by 'describing it's effect'.
■ By studying the 'effect produced by a force', we will be able to write the following:
• Whether the force is a moving force or stopping force
• Whether the force is large or small. That is., the magnitude of the force.
Balanced and Unbalanced Forces
1. Consider a wooden block resting on a smooth surface. It is shown in fig.2.1 below:
 |
| Fig.2.1 |
3. Now consider Fig.2.2(a) below. The same force F1 is now applied from the right side also.
 |
| Fig.2.2 |
5. Now consider fig.2.2(b). The opposing force F2 which acts from right to left is smaller. Now the block will move towards the right. This is a case of unbalanced forces.
In the above example, the wooden block was resting on a smooth surface. But in day to day life, we rarely come across surfaces that are perfectly smooth. All the surfaces will have a certain amount of friction. Consider fig.2.3 below:
 |
| Fig.2.3 |
1. The wooden block is now resting on an ordinary surface. Like the floor of the school play ground.
2. A force F1 is applied from the left towards the right. The block does not move.
• Earlier we saw that, the block readily moved when F1 was applied.
• To keep the block stationary, we had to apply the same F1 in the opposite direction. But here we are applying no opposing forces.
3. So why is the block stationary?
Ans: We did not apply any opposing force. But friction did.• There is friction between the 'bottom surface of the block' and the 'surface of the floor'
• It caused an opposing 'frictional force' Ff
4. Like any force, we cannot see Ff. We can only see it's effects. Here the effect is:
'Not allowing the block to move even when F1 is applied'.
• In later chapters, when we learn about friction, we will see the method to measure the magnitude of Ff.
5. But at present, we can sure realise that, Ff is greater than F1. That is why the block is not moving.
6. Let us apply a larger force F3. This is shown in the fig.2.4 below:
 |
| Fig.2.4 |
• If we want to move it from one end of the play ground to the other end, We have to keep on pushing it until it reaches the other end. That is., we have to keep on applying the force F3
• If at any time during this journey, we stop applying the force or reduce the force, then the block will come to a stop.
Based on the above discussion, we encounter a very interesting question. This question can be formulated as follows:
• To move the block from one end of the play ground to the other, we have to keep applying the force. This is because we have to keep overcoming the Ff which is opposing the motion.• So, the question is this:
If there is no opposing force, do we have to keep pushing it until it reaches the other end?
• The answer is no. Let us see the explanation:
1. Suppose that there is no friction between the block and the floor of the play ground.
2. The block is initially at rest. We give the block a single push.
3. Since there is no force acting in the opposite direction, the push is an unbalanced force.
4. So the block will begin to move. We do not have to keep pushing it. The block will reach the other end of the play ground.
5. But it will not stop there. Unless there is some thing to stop it, the block will continue it's journey and move out of the play ground.
6. In fact, it will continue it's journey for ever. This is because, there is no friction or any other force to stop it.
7. If we do not want to lose the block, we will have to apply a force and stop it. Either get behind it and apply a pulling force or get ahead of it and apply a pushing force.
8. Once the block has attained a velocity due to the push, it will continue to move with that velocity.
It is not possible to actually see a block 'moving for ever' in our play ground. Because we cannot eliminate the friction between the play ground and the block. But we can experience some side effects of such 'never ending motion' in our day to day life. Let us see some examples:
■ Consider a person standing in a moving bus. When the bus stops suddenly, he tends to fall forward. Why does that happen?
1. When the bus is in motion, the person standing is also in the state of motion.
2. His body has the tendency to keep moving for ever.
3. But when the bus stops, the lower part of his body which is in contact with the bus, comes to a stop.
4. The upper part of his body tends to continue moving. That is why he falls forward.
■ This happens while travelling in a car too. In the moving car, we are sitting. But if the car stops suddenly, we will be thrown forward causing injuries. That is why wearing seat belts are made compulsory.
■ What we saw above are examples for:
• The tendency of an object in motion to continue in the state of motion.
■ We can show the ‘opposite of the above situations’ as examples for:
• The tendency of an object at rest to continue in the state of rest.
Let us write them:
1. When the bus is at rest, the person standing is also in the state of rest.
2. His body has the tendency to continue in the state of rest.
3. But when the bus starts to move, the lower part of his body which is in contact with the bus, starts to move.
4. The upper part of his body tends to continue at rest. That is why he falls backward.
■ This happens while travelling in a car too. In the car which is stationary, we are sitting. But if the car starts to move suddenly, we will be thrown backwards.
Sir Isaac Newton studied the above phenomenon and published his findings in the form of three laws. They came to be known as ‘Newton’s laws of motion’.
■ The first law of motion states that:
An object remains in a 'state of rest' or 'state of uniform motion' in a straight line unless compelled to change that state by an applied force.In other words, all objects resist a change in their state of rest or motion. The tendency of undisturbed objects to stay at rest or to keep moving with the same velocity is called inertia. This is why, the first law of motion is also known as the law of inertia.
A video demonstrating the law can be seen here.
Now a question arises: Can we measure inertia?
The situations that make us to ask this question can be explained using examples as follows:1. We see two objects in motion.
2. They are moving with the same velocity. But they have different masses. That is., one is heavier than the other.
3. We want to stop them both. We know that those two objects will be wanting to continue in motion.
4. In other words, those two moving objects have acquired a special tendency. The tendency to continue in motion.
5. We saw that this tendency is called 'inertia'.
6. More specifically, as those objects are in motion, we can call the tendency: ‘inertia of motion’.
7. Since they have acquired this inertia of motion, they wont stop unless we put an effort.
8. How much effort do we have to put for each?
• Is the effort required, same for both the objects?
• Does one of them require a greater effort? If so which one need the greater effort?
♦ If we know that before hand, we can send a stronger person to stop it.
9. So we see that measuring inertia is essential.
10. By experience we find that, if two objects of different masses travel with the same velocity, the heavier object will require a greater effort.
11. For example, if a tennis ball and a cricket ball travel with the same velocity, a greater effort is required to stop the cricket ball. Because it has greater mass. [Note that, for making a comparison, velocities have to be the same. It will be more difficult to stop a tennis ball if it is travelling at a very high speed]
12. Another example: Huge and sophisticated brakes are required to stop a train. While smaller brakes are sufficient for a car. Even smaller brakes are sufficient for a bicycle.
■ So we can write:
Heavier objects have greater inertia of motion.
■ Let us consider inertia of rest. It also depends on mass.
• Take the example of a cricket ball and a tennis ball at rest.
• It requires greater effort to set the cricket ball in motion.
■ So we can write this:
Mass is a measure of inertia.
In the next section, we will see Newton's Second Law of motion.
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