In the previous chapter we completed a discussion on motion in two dimensions. In this chapter, we will see Laws of motion.
• In the preceding chapters we saw the details about velocity and acceleration of objects.
• Based on the knowledge about velocity and acceleration, we were able to write the details about position and displacement of objects.
• We saw that:
♦ If the velocity is constant, the body will not be having any acceleration
♦ If the velocity changes, the body will be having an acceleration/retardation
• If we say that 'a body is in motion', it implies that it has a velocity
■ But how did the body attain that velocity?
■ Further, 'if that velocity is not uniform', how did the body attain the acceleration/retardation?
In this chapter we try to answer those questions. Let us consider some simple cases:
1. We see a football in motion. We can certainly guess how is attained its velocity
• Someone must have clicked it.
♦ That is., someone applied a force on it
2. We see a stone rising in the air
• Clearly, someone must have thrown it
♦ That is., someone applied a force on it.
3. We see the branches of a tree swinging
• The breeze must be causing them to swing
♦ That is., the breeze applied a force on it.
■ We can easily conclude that a force is required to create motion
• In the preceding chapters we saw the details about velocity and acceleration of objects.
• Based on the knowledge about velocity and acceleration, we were able to write the details about position and displacement of objects.
• We saw that:
♦ If the velocity is constant, the body will not be having any acceleration
♦ If the velocity changes, the body will be having an acceleration/retardation
• If we say that 'a body is in motion', it implies that it has a velocity
■ But how did the body attain that velocity?
■ Further, 'if that velocity is not uniform', how did the body attain the acceleration/retardation?
In this chapter we try to answer those questions. Let us consider some simple cases:
1. We see a football in motion. We can certainly guess how is attained its velocity
• Someone must have clicked it.
♦ That is., someone applied a force on it
2. We see a stone rising in the air
• Clearly, someone must have thrown it
♦ That is., someone applied a force on it.
3. We see the branches of a tree swinging
• The breeze must be causing them to swing
♦ That is., the breeze applied a force on it.
■ We can easily conclude that a force is required to create motion
■ It may be noted that in the examples above, the forces which caused the motion were in physical contact with the objects
1. To kick a ball, the player has to make physical contact with the ball
2. To throw a stone, the thrower has to make physical contact with the stone
3. To swing the branches, the air has to make physical contact with the branches
■ But there are cases where such physical contact is not required. Let us see some examples:
1. Consider a stone dropped from the top of a building
• The person who was holding the stone just dropped it.
♦ He did not apply any force
• Yet we see that stone is moving downwards
• We are inclined to think that no force is required for producing motion in this case
• But the truth is that, force is be applied in this case also.
♦ It is the force applied by Earth. It is called the gravitational force. We will learn about this force in the next chapter
• If gravitational force was absent, the stone would not fall.
• This will be clear if we consider dropping the stone in outer space. Let us see how:
• Consider a person holding the stone in outer space
• He just lets it go without applying any force
♦ He does not apply any force in the horizontal direction
♦ He does not apply any force in the vertical direction (upwards or downwards)
♦ He does not apply any force in any inclined directions
• He just let it go. Then the stone will not fall. In fact, the stone will not move in any direction.
• This is clear proof that, a force is required to cause motion.
2. Consider an iron nail lying on the table.
• If we bring a bar magnet near the nail, then the nail will move towards the magnet
• Here no physical contact is made. Yet the magnetic force is able to give motion to the nail
■ So we can write: Gravitational and magnetic forces are capable of setting objects in motion, with out physical contact
■ The overall conclusion is: A force is required to start a motion
• Thus we know how an object begins to move.
■ Our next aim is to get the 'details about an object which is already in motion'.
• We see a body in motion.
♦ When will it come to rest?
♦ How will it come to rest?
Let us see an example. We will write it in steps:
1. Consider a ball rolling on the floor.
• The ball will come to a stop when the 'energy available for the motion' is completely used up
2. What if there is no agent to use that energy?
• In other words, What if the ball experience no resistance against it's motion?
• Then it would mean that, the ball has no work to do.
■ All it's available energy will remain as such and it will continue to move for ever
• In such a situation is encountered, we will have to apply an external force to stop the ball
3. This will be clear if we consider a scenario in outer space.
• Let a ball be kicked in outer space.
• It will continue it's motion for eternity.
• It will never come to a stop unless acted upon by an external stopping force.
4. But on earth, we see that the ball comes to a stop after some time, even if we see no external stopping force.
• It is not true that there is no stopping force acting on the ball.
• There is indeed a stopping force. We just do not notice it.
5. It is the frictional force. It is a force acting at the interface between the ball and the ground.
• We will learn more about frictional force later in this section.
• At present it is sufficient to know that, the ball comes to a stop because of frictional force. The energy of the moving ball is gradually used up to do work against friction.
• If this frictional force was completely absent, the ball would roll on forever just like it would, in outer space
6. Let us see a real life example of the scenario in (3):
• The voyager 1 space probe launched by NASA in 1977 is still travelling at a very high speed of 15 km/s. (Details here)
• It is not using any propulsion for the travel.
■ It is still moving because, there is no external force available to stop it.
• Also note that, it is not moving in any random direction. It was set to move in a definite direction by giving it an initial velocity in that direction.
• The following two points are worth noting:
(i) Voyager has not deviated from it's intended path
(ii) It is continuing to travel at a uniform velocity away from the earth
• The 'continuity of motion' and 'consistency in direction' are accomplished without any help from it's rocket engines
• If a change in direction becomes necessary, then those engines will have to be turned on
■ Based on the above 6 steps, we can write: An external force is required to stop a body in motion
■ Earlier, we saw that, an external force is required to set a body in motion
Now let us consider each of the above two points again. We must add one more information.
1. The first point:
• An external force is required to set a body in motion.
This is illustrated in fig.5.1(a) below:
• We see a wooden block at rest. It will continue to be at rest because no forces are acting on it.
♦ That is., no force is available to set the block in motion
• Now, we see another wooden block at rest. Based on what we have learned, we are inclined to think that no force is acting on the block. But it need not be true.
• Consider the wooden block in fig.5.1 (b) above. It is acted upon by several forces. But the block is at rest. How is that possible?
• The answer is that, the forces on the left cancel the effects of the forces on the right.
• In other words, the net force on the block is zero.
• That is., in effect, no force is acting on the block. That is why the block is at rest.
• So we must analyze each situation carefully. An object being at rest does not necessarily mean that, no force is acting on it. But we can say no net force is acting on it.
• We must clearly distinguish between no force and no net force
2. The second point:
• An external force is required to stop a body in motion
This is illustrated in fig.5.2(a) below:
• We see a wooden block in motion in space. It will continue to be in motion because no forces are acting on it.
♦ That is., no force is available to stop the block
• Now, we see another wooden block in motion in space. Based on what we have learned above, we are inclined to think that, no force is acting on the block. But it need not be true.
• Consider the wooden block fig.5.2(b) above. It is acted upon by several forces. But the block is in motion. How is that possible?
• The answer is that, the forces on the left cancel the effects of the forces on the right.
• In other words, the net force on the block is zero.
• That is., in effect, no force is acting on the block. That is why the block is in motion.
• So we must analyze each situation carefully. An object being in uniform motion does not necessarily mean that, no force is acting on it. But we can say no net force is acting on it.
• We must clearly distinguish between no force and no net force.
■ If the net external force is zero, then:
(i) A body at rest continues to remain at rest
(ii) A body in motion continues to move with uniform velocity
■ This property of the body is called inertia.
• We can write: Inertia is a property of a body by which it resists the following two items:
(i) Any change in it's state of rest
(ii) Any change in it's state of uniform motion
• Because of this property, we encounter the following two situations:
(i) We have to apply a net external force to make a body (at rest) to move
(ii) We have to apply a net external force to make a body (in motion) to stop
• Later, the studies made by the English scientist Sir Isaac Newton gave more insight about them
1. To kick a ball, the player has to make physical contact with the ball
2. To throw a stone, the thrower has to make physical contact with the stone
3. To swing the branches, the air has to make physical contact with the branches
■ But there are cases where such physical contact is not required. Let us see some examples:
1. Consider a stone dropped from the top of a building
• The person who was holding the stone just dropped it.
♦ He did not apply any force
• Yet we see that stone is moving downwards
• We are inclined to think that no force is required for producing motion in this case
• But the truth is that, force is be applied in this case also.
♦ It is the force applied by Earth. It is called the gravitational force. We will learn about this force in the next chapter
• If gravitational force was absent, the stone would not fall.
• This will be clear if we consider dropping the stone in outer space. Let us see how:
• Consider a person holding the stone in outer space
• He just lets it go without applying any force
♦ He does not apply any force in the horizontal direction
♦ He does not apply any force in the vertical direction (upwards or downwards)
♦ He does not apply any force in any inclined directions
• He just let it go. Then the stone will not fall. In fact, the stone will not move in any direction.
• This is clear proof that, a force is required to cause motion.
2. Consider an iron nail lying on the table.
• If we bring a bar magnet near the nail, then the nail will move towards the magnet
• Here no physical contact is made. Yet the magnetic force is able to give motion to the nail
■ So we can write: Gravitational and magnetic forces are capable of setting objects in motion, with out physical contact
■ The overall conclusion is: A force is required to start a motion
• Thus we know how an object begins to move.
■ Our next aim is to get the 'details about an object which is already in motion'.
• We see a body in motion.
♦ When will it come to rest?
♦ How will it come to rest?
Let us see an example. We will write it in steps:
1. Consider a ball rolling on the floor.
• The ball will come to a stop when the 'energy available for the motion' is completely used up
2. What if there is no agent to use that energy?
• In other words, What if the ball experience no resistance against it's motion?
• Then it would mean that, the ball has no work to do.
■ All it's available energy will remain as such and it will continue to move for ever
• In such a situation is encountered, we will have to apply an external force to stop the ball
3. This will be clear if we consider a scenario in outer space.
• Let a ball be kicked in outer space.
• It will continue it's motion for eternity.
• It will never come to a stop unless acted upon by an external stopping force.
4. But on earth, we see that the ball comes to a stop after some time, even if we see no external stopping force.
• It is not true that there is no stopping force acting on the ball.
• There is indeed a stopping force. We just do not notice it.
5. It is the frictional force. It is a force acting at the interface between the ball and the ground.
• We will learn more about frictional force later in this section.
• At present it is sufficient to know that, the ball comes to a stop because of frictional force. The energy of the moving ball is gradually used up to do work against friction.
• If this frictional force was completely absent, the ball would roll on forever just like it would, in outer space
6. Let us see a real life example of the scenario in (3):
• The voyager 1 space probe launched by NASA in 1977 is still travelling at a very high speed of 15 km/s. (Details here)
• It is not using any propulsion for the travel.
■ It is still moving because, there is no external force available to stop it.
• Also note that, it is not moving in any random direction. It was set to move in a definite direction by giving it an initial velocity in that direction.
• The following two points are worth noting:
(i) Voyager has not deviated from it's intended path
(ii) It is continuing to travel at a uniform velocity away from the earth
• The 'continuity of motion' and 'consistency in direction' are accomplished without any help from it's rocket engines
• If a change in direction becomes necessary, then those engines will have to be turned on
■ Based on the above 6 steps, we can write: An external force is required to stop a body in motion
■ Earlier, we saw that, an external force is required to set a body in motion
Now let us consider each of the above two points again. We must add one more information.
1. The first point:
• An external force is required to set a body in motion.
This is illustrated in fig.5.1(a) below:
 |
| Fig.5.1 |
♦ That is., no force is available to set the block in motion
• Now, we see another wooden block at rest. Based on what we have learned, we are inclined to think that no force is acting on the block. But it need not be true.
• Consider the wooden block in fig.5.1 (b) above. It is acted upon by several forces. But the block is at rest. How is that possible?
• The answer is that, the forces on the left cancel the effects of the forces on the right.
• In other words, the net force on the block is zero.
• That is., in effect, no force is acting on the block. That is why the block is at rest.
• So we must analyze each situation carefully. An object being at rest does not necessarily mean that, no force is acting on it. But we can say no net force is acting on it.
• We must clearly distinguish between no force and no net force
2. The second point:
• An external force is required to stop a body in motion
This is illustrated in fig.5.2(a) below:
 |
| Fig.5.2 |
♦ That is., no force is available to stop the block
• Now, we see another wooden block in motion in space. Based on what we have learned above, we are inclined to think that, no force is acting on the block. But it need not be true.
• Consider the wooden block fig.5.2(b) above. It is acted upon by several forces. But the block is in motion. How is that possible?
• The answer is that, the forces on the left cancel the effects of the forces on the right.
• In other words, the net force on the block is zero.
• That is., in effect, no force is acting on the block. That is why the block is in motion.
• So we must analyze each situation carefully. An object being in uniform motion does not necessarily mean that, no force is acting on it. But we can say no net force is acting on it.
• We must clearly distinguish between no force and no net force.
So we can write a summary:
(i) A body at rest continues to remain at rest
(ii) A body in motion continues to move with uniform velocity
■ This property of the body is called inertia.
• We can write: Inertia is a property of a body by which it resists the following two items:
(i) Any change in it's state of rest
(ii) Any change in it's state of uniform motion
• Because of this property, we encounter the following two situations:
(i) We have to apply a net external force to make a body (at rest) to move
(ii) We have to apply a net external force to make a body (in motion) to stop
• It was the studies made by the Italian scientist Galileo Galilei that brought these phenomena to the notice of the world for the first time
In the next section, we will see how Sir Isaac Newton developed those phenomena.
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