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Newton's Third Law of Motion

Definition

 

For each and every action, there is equal and opposite reaction.

Newton`s Third Law of Motion | Science Class 9

lf a body A exerts force Newton`s Third Law of Motion | Science Class 9Newton`s Third Law of Motion | Science Class 9 on another body B, then B exerts a force -Newton`s Third Law of Motion | Science Class 9 on A, the two forces acting along the same line. The two forces Newton`s Third Law of Motion | Science Class 9 and -Newton`s Third Law of Motion | Science Class 9 connected by Newton's third law are called action-reaction pair. Any one may be called 'action' and the other 'reaction'.

Action and reaction acts on different bodies hence they never cancel each other. 

Note: Action and reactions forces occur simultaneously. It is wrong to think that first action occurs and it is followed by reaction.

An important conclusion: When action-reaction forces are acting on two bodies of masses mA and mB then
 From eqn. FAB = -Newton`s Third Law of Motion | Science Class 9BA
 mAaA = -mBaB
Newton`s Third Law of Motion | Science Class 9

Misconception: The action and reaction forces acting on a body are equal and opposite and therefore cancel out the effect of each other.
Actual Concept: Action and reaction do not cancel out the effect of each other because they act on different bodies.
Misconception: Any pair of equal and opposite forces are an action-reaction pair.
Actual Concept: Only those forces from an action-reaction pair that involve the two bodies responsible for action-reaction.

Some Other Examples of Newton's Third Law 


1. When a bullet is fired from a gun, we observe recoiling of the gun. The gun acts on bullet i.e., exerts a force on the bullet; simultaneously, the bullet reacts on the gun.

 

Newton`s Third Law of Motion | Science Class 9

 2. While walking or running, you push the ground in the backward direction with your feet. The ground simultaneously exerts a force of equal magnified in the forward direction on feet. This force enables us to walk.

 

Newton`s Third Law of Motion | Science Class 9

3. When a man jumps from a boat, the boat also experiences a backward jerk. This is due to the action-reaction pair as shown in figure.

 

Newton`s Third Law of Motion | Science Class 9

4. Inflate a balloon and leave it. You will observe that the balloon moves in opposite direction to the opening in balloon through which the air is coming out.

 

Newton`s Third Law of Motion | Science Class 9 
5. A ball rebounds after striking against a floor.
 When a ball strikes against a floor, it exerts a force on the floor. According to Newton's third law of motion, the floor exerts an equal and opposite force on the ball. Due to this reaction, the ball rebounds.

6. How do we swim ?
 While swimming, a swimmer pushes the water backward with his hands. The reaction offered by the water to the swimmer pushes him forward.

7. A boatman pushes the river bank with a bamboo pole to take his boat into the river.
 When the boatman pushes the river bank with a bamboo pole, the river bank offers an equal and opposite reaction. This reaction helps the boat to move into the river.
 

8. Why a fireman struggles to hold a hose-pipe ?
 A fireman has to make a great effort to hold a hose-pipe to throw a stream of water on fire to extinguish it. This is because the stream of water rushing through the hose-pipe in the forward direction with a large speed exerts a large force on the hose-pipe in the backward direction.
 

Important Points to Remember 

  • Unit of force is dyn or newton whereas that of mass is gram or kilogram.
  • It is wrong to say "Weight of Ankit is 40 kg." We should say "Mass of Ankit is 40 kg" or "Weight of Ankit is 40 × 9.8 = 392N."
  • 1 metric ton = 1,000 kg , 1 quintal = 100 kg
  • 1 metric ton = 10 quintals , 1 kg = 1,000 g

 

Competitive Window 

 1. In solving the numericals, always take the force in absolute units i.e., in dynes or newtons.

 2. 1 N = 105 dyn.

 3. The kg f (kilogram force) and kg wt (kilogram weight) are two names of same quantity i.e., of practical units of force.

 4. Momentum is a vector quantity.

 5. Weight of the body or force applied should be never states in kg. Kilogram is a unit of mass and not of weight or force. Never say, "My weight is 50 kg". Say "My weight is 50 kg for 50 kg wt or 50 × 9.8 N" or say, "My mass is 50 kg".

 6. Hold 0.1 kg on your hand placed parallel to earth. Your  hand is applying a force of nearly 1 N (0.98 N to be exact). 
Newton`s Third Law of Motion | Science Class 9

7. In all numericals, use only absolute units (N or dyn). If the force is given in kg f or g f, convert these to N or dyn. 

 

 

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FAQs on Newton's Third Law of Motion - Science Class 9

1. What is Newton's Third Law of Motion?
Ans. Newton's Third Law of Motion states that for every action, there is an equal and opposite reaction. This means that when an object exerts a force on another object, the second object exerts an equal and opposite force on the first object.
2. How does Newton's Third Law of Motion relate to the Law of Conservation of Momentum?
Ans. Newton's Third Law of Motion is closely related to the Law of Conservation of Momentum. According to the Law of Conservation of Momentum, the total momentum of a closed system remains constant if no external forces act on it. When two objects interact, their momentum may change, but the total momentum of the system remains unchanged. This is because the forces exerted on each other by the objects are equal in magnitude and opposite in direction, as stated by Newton's Third Law.
3. Can you provide an example of Newton's Third Law of Motion?
Ans. Sure! A classic example of Newton's Third Law of Motion is when you push against a wall. When you apply a force to the wall, the wall exerts an equal and opposite force on you. This is why you feel resistance when pushing against a wall.
4. How does Newton's Third Law of Motion apply to everyday life?
Ans. Newton's Third Law of Motion can be observed in various everyday situations. For example, when you swim, you push the water backward with your arms and legs, and in return, the water pushes you forward, propelling you through the water. Similarly, when you walk, your foot exerts a force backward on the ground, and the ground pushes your foot forward, allowing you to move forward.
5. Can you explain the concept of a competitive window in the context of Newton's Third Law of Motion?
Ans. In the context of Newton's Third Law of Motion, a competitive window refers to a specific time period during which an athlete or a team can maximize their performance by utilizing the principles of equal and opposite forces. By understanding and applying Newton's Third Law, athletes can optimize their movements, such as running, jumping, or throwing, to generate more force and achieve better results. The competitive window represents the period when an athlete's training, technique, and physical condition align to produce the best possible performance.
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