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Note on Newton’s Laws of Motion

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After Galileo, Sir Isaac Newton (1642 – 1727 AD) of England made a detailed and systematic study of the motion of bodies and formulated the three laws of motion.

Newton’s first law of motion
Newton’s first law of motion

Newton’s first law of motion

Newton’s first law states that ‘everybody continues in its state of rest or of uniform motion in a straight line unless an unbalanced force acts on it.’ Examples:

  1. When we shake mango tree, the mangoes fall down.
  2. When a blanket is given a sudden jerk, the dust particles in it fall off due to inertia at rest.
  3. A rider on a running horse is thrown forward when the horse stops suddenly due to the inertia of motion.
  4. When a person jumps out of a moving vehicle, he falls forward due to the inertia of motion.

Interpretation of first law: According to the first law, a body continues to remain at rest if no unbalanced forces act on it. Similarly, the body will remain moving with uniform velocity in a straight line unless unbalanced forces act on it.

Newton’s second law of motion

Newton’s second law of motion
Newton’s second law of motion

Newton’s second law of motion states that ‘acceleration produced in a body is directly proportional to the force applied to it in the direction of motion and inversely proportional to its mass.’ Example:

  1. A cricket player while catching a ball moves his hands backwards.
  2. A person falling on the cemented floor is injured more than a person falling on a sandy floor or mattress.
  3. It is difficult to catch a cricket ball as compared to a tennis ball moving at the same velocity.
  4. It is easier to drag a stone than to kick.

Interpretation of second law: If a small body is pushed gently, a small acceleration is produced. If it is pushed harder, a larger acceleration is produced. If two bodies, one lighter and another heavier, are pulled by the same amount of force in the same direction separately, it is found that the heavier body has less acceleration than the lighter body.

Newton’s third law of motion
Newton’s third law of motion

Newton’s third law of motion

Newton’s third law of motion states that ‘to every action there is equal and opposite reaction.’ Example:

  1. While rowing a boat, a person pushes water backwards with the help of the oars. Due to the reaction offered by water, the boat moves forwards.
  2. Birds, while flying, push air with their wings (action). The air exerts an equal and opposite force on the birds (reaction) and that force causes birds to move forward.
  3. In order to swim, a man pushes water backwards with his hands. Due to the reaction offered by water to the man, the man is pushed forwards.
  4. When the mouth of an inflated balloon is left open, the balloon flies away. This is because when the air inside escapes out (action), the balloon gets pushed (reaction) in the opposite direction by the air escaping out.

Interpretation of third law: Third law implies that forces always occur in a pair and a single force is thus impossible.

  • Newton’s first law states that ‘everybody continues in its state of rest or of uniform motion in a straight line unless an unbalanced force acts on it.'
  • Newton’s second law of motion states that, ‘acceleration produced in a body is directly proportional to the force applied to it in the direction of motion and inversely proportional to its mass.’
  • Newton’s third law of motion states that ‘to every action there is equal and opposite reaction.’
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Very Short Questions

Sir Isaac Newton developed three laws of motion which is known as Newton's laws of motion.

It states that "Every body continues in its state of rest or uniform motion in a straight line unless it is acted by external unbalanced force." Example: A pen kept on the table continues to be at rest unless it is given pushing or pulling force.

According to Newton's 1st law of motion, when a person is in motion, it tries to be in motion towards the same direction due to which person running with high speed cannot stop suddenly.

Newton's 2nd law of motion states that acceleration produced in a body is directly proportional to the force applied and inversely proportional to the mass of the body.

Newton's 2nd law of motion states that acceleration produced in a body is directly proportional to the force applied and inversely proportional to the mass of the body. i.e. a ∝ F and a ∝ By combining these two, we get,
a ∝
or, F ∝ ma
or, F = k. ma where k is proportionality constant, In unit system, F = 1N
m = 1kg
a = 1m/s2
Then, k = 1
By putting value of k, we have
F = 1.ma
F = ma
Hence, proved.

From the relation, F = ma = m (), force is inversely proportional to the time taken. The time of impact is less for the concrete floor than the sand pile which gets depressed down. Hence, force is more on the concrete floor, and hurts more and less force of the sand pile causes less hurt.

As we know that force is inversely proportional to the time of impact, while moving hands backward time of flight of ball becomes more and less force acts on hand which hurts less since F ∝. Hence, a cricketer moves his hands backward while catching a ball.

It states that "To every action, there is an equal and opposite reaction".

Action and reaction are equal and opposite but they do not cancel each other because action and reaction occur in different bodies. For example, when you are sitting on a bench, you are pressing down (action) on the bench and at the same time, the bench pushes you up (reaction).

When a bullet is fired from the gun, the gun exerts force on the bullet (action). At the same time, reaction force is given by the bullet to the gun which forces the gun backward. Hence, gun recoils when a bullet is fired and to stand at a place, the soldier puts the gun on his shoulder while firing.

When boatman pushes the water in backward direction, it gives equal and opposite reaction in the forward direction according to Newton's 3rd law of motion. Hence, boatman pushes water backward which produces sufficient forces to move forward.

When rocket throws large smoke downward (action), then it provides necessary reaction force to the rocket to move upward according to Newton's 3rd law of motion.

While walking, a man presses the ground with his feet in the backward direction (action) and the earth gives an equal and opposite force on the person in the forward direction (reaction) which helps the body to move forward.

While flying, wings push the air downward (action), the air exerts an equal and opposite force on the bird (reaction) which makes the bird fly upward easily.

Solution:
Given,
Force (F) = 4N
Mass (m) = 1kg
Change in velocity = 0.3 – 0.1 = 0.2 m/s
Distance travelled (s) = ?
Now,
Force (F) = m x a
or, a = = 4m/s2

Again,a =
or, 4 =
or, t = 0.05 s Now, distance travelled = x t = x t = 0.01m
Hence, the distance travelled by the body is 0.01m.

Solution:
Given,
Mass (m) = 200g = 0.2kg
Force (F) = 10N
According to newton's 2nd law of motion,
F = ma
or, 10 = 0.2 x a
or, a = 50m/s2
Hence, the acceleration produced is 50m/s2.

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  • Which is not one of Newton's laws of motion?

    body will continue to be in motion or rest unless affected by external force
    Motion is directly proportional to velocity
    acceleration is directly proportional to force
    to every action there is equal and opposite reaction
  • Who made a detailed and systematic study of the motion of bodies and formulated the three laws of motion?

    Michael Faraday
    Sir Isaac Newton
    Benjamin Franklin
    Einstein
  • Everybody continues in its state of rest or of uniform motion in a straight line unless an unbalanced force acts on it. Which law states this?

    Newton's third law of motion
    Newton's first law of motion
    Newton's second law of motion
    Galileo's first law of motion
  • A cricket player while catching a ball moves his hands backwards.' Which one of the following explains this example?

    Newton's second law of motion
    Galileo's first law of motion
    Newton's third law of motion
    Newton's first law of motion
  • Which one of the following lawimplies that forces always occur in pair and a single force is impossible?

    none of the option are correct
    Newton's first law of motion
    Newton's second law of motion
    Newton's third law of motion
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DISCUSSIONS ABOUT THIS NOTE

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surya

if there is equal and opposite force between two pairs why the magnitude of force not equal to zero


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a moving ball on the ground is stopped even if we don't touch it.Does it violet the newton's first law of motion? why?


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Varun

Why first law of motion is called law of inertia?


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abhi

Action and reaction are equal in magnitude and opposite in direction.Then,why do they not balance each other?


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