### Fwd: NEWTON'S THIRD LAW OF MOTION

---------- Forwarded message ----------
From: ROHITH M
Date: Sun, Oct 4, 2009 at 11:46 PM
Subject: NEWTON'S THIRD LAW OF MOTION
To: mathewnjappallil@gmail.com

Newton's 3rd Law .

 Newton's 3rd law may be formally stated: "Forces always occur in pairs. If object A exerts a force F on object B, then object B exerts an equal and opposite force –F on object A" or in slogan style: "Every action has an equal and opposite reaction" Note the important provision: two objects must be involved! There exists a whole set of situations where two equal and opposite forces act on the same object, cancelling each other so that no acceleration (or even no motion) occurs. This is not an example of the third law, but of equilibrium between forces. Some examples:     A heavy object stands on the floor, pulled down by the Earth with a force mg (drawing). However, it does not move in that direction, because the floor stops it. Obviously, the floor is exerting on it an equal and opposite force -mg (velocity v=0, acceleration a=0).     An elevator is pulled up from the street level to the 5th floor. It senses two forces: downwards, its weight and that of the people in it, and upwards the pull of the cable which holds it up. Between the floors, as long as the elevator does not accelerate, the net force must be zero, hence the two forces must be equal and opposite (v>0, a=0).

To every action there is always opposed an equal reaction; or, the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.
- Newton's Third Law of Motion, translated from the Principe's Latin

We represent the Third Law by looking at two bodies A and B that are interacting. We define FA as the force applied to body A by body B and FA as the force applied to body B by body A. These forces will be equal in magnitude and opposite in direction. In mathematical terms, it is expressed as:

FB = - FA

or

FA + FB = 0

This is not the same thing as having a net force of zero, however. If you apply a force to an empty shoebox sitting on a table, the shoebox applies an equal force back on you. This doesn't sound right at first - you're obviously pushing on the box, and it is obviously not pushing on you. But remember that, according to the Second Law, force and acceleration are related - but they aren't identical!

Because your mass is much larger than the mass of the shoebox, the force you exert causes it to accelerate away from you and the force it exerts on you wouldn't cause much acceleration at all.

Not only that, but while it's pushing on the tip of your finger, your finger in turn pushes back into your body, and the rest of your body pushes back against the finger, and your body in turn pushes on the chair or floor (or both), all of which keeps your body from moving and allows you to keep your finger moving to continue the force. There's nothing pushing back on the shoebox to stop it from moving.

If, however, the shoebox is sitting next to a wall and you push it toward the wall, the shoebox will push on the wall - and the wall will push back. The shoebox will, at this point, stop moving. You can try to push it harder, but the box will break before it goes through the wall because it isn't strong enough to handle that much force.

Tug of War: Newton's Laws in Action

Most people have played tug of war at some point. A person or group of people grab the ends of a rope and try to pull the person or group at the other end, usually past some marker (sometimes into a mud pit in really fun versions), thus proving that one of the groups is stronger. All three of Newton's Laws can be seen very obviously in tug of war.

There frequently comes a point in tug of war - sometimes right at the beginning but sometimes later - where neither side is moving. Both sides are pulling with the same force and therefore the rope does not accelerate in either direction. This is a classic example of Newton's First Law.

Once a net force is applied, such as when one group begins pulling a bit harder than the other, an acceleration begins, and this follows the Second Law. The group losing ground must then try to exert more force. When the net force begins going in their direction, the acceleration is in their direction. The movement of the rope slows down until it stops and, if they maintain a higher net force, it begins moving back in their direction.

The Third Law is a lot less visible, but it's still there. When you pull on that rope, you can feel that the rope is also pulling on you, trying to move you toward the other end. You plant your feet firmly in the ground, and the ground actually pushes back on you, helping you to resist the pull of the rope.

Next time you play or watch a game of tug of war - or any sport, for that matter - think about all the forces and accelerations at work. It's truly impressive to realize that you could, if you worked at it, understand the physical laws that are operating in your favorite sport.

### Newton's Third Law

A force is a push or a pull upon an object which results from its interaction with another object. Forces result from interactions! As discussed in Lesson 2, some forces result from contact interactions (normal, frictional, tensional, and applied forces are examples of contact forces) and other forces are the result of action-at-a-distance interactions (gravitational, electrical, and magnetic forces). According to Newton, whenever objects A and B interact with each other, they exert forces upon each other. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body. There are two forces resulting from this interaction - a force on the chair and a force on your body. These two forces are called action and reaction forces and are the subject of Newton's third law of motion.

The statement means that in every interaction, there is a pair of forces acting on the two interacting objects. The size of the forces on the first object equals the size of the force on the second object. The direction of the force on the first object is opposite to the direction of the force on the second object. Forces always come in pairs - equal and opposite action-reaction force pairs.

A variety of action-reaction force pairs are evident in nature. Consider the propulsion of a fish through the water. A fish uses its fins to push water backwards. But a push on the water will only serve to accelerate the water. Since forces result from mutual interactions, the water must also be pushing the fish forwards, propelling the fish through the water. The size of the force on the water equals the size of the force on the fish; the direction of the force on the water (backwards) is opposite the direction of the force on the fish (forwards). For every action, there is an equal (in size) and opposite (in direction) reaction force. Action-reaction force pairs make it possible for fish to swim.

Consider the flying motion of birds. A bird flies by use of its wings. The wings of a bird push air downwards. Since forces result from mutual interactions, the air must also be pushing the bird upwards. The size of the force on the air equals the size of the force on the bird; the direction of the force on the air (downwards) is opposite the direction of the force on the bird (upwards). For every action, there is an equal (in size) and opposite (in direction) reaction. Action-reaction force pairs make it possible for birds to fly.

Consider the motion of a car on the way to school. A car is equipped with wheels which spin backwards. As the wheels spin backwards, they grip the road and push the road backwards. Since forces result from mutual interactions, the road must also be pushing the wheels forward. The size of the force on the road equals the size of the force on the wheels (or car); the direction of the force on the road (backwards) is opposite the direction of the force on the wheels (forwards). For every action, there is an equal (in size) and opposite (in direction) reaction. Action-reaction force pairs make it possible for cars to move along a roadway surface.

Rohith.M, Std XI E, K.V Pattom