牛顿第三定律

Newton's Third Law has been, over the multitude of years, been stated in many ways. The shortest way we know it today is:

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

Some websites state it as follows:

When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.

One from of the original formulation, translated to English, states:

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.

All of these are trying to communicate that there is always an action-reaction pair of forces. Continue below for examples.

The standard example is walking. When we use our feet to push the floor, if there was no reaction force, our feet would just plunge through the floor. In fact, this is why we can stand on the Earth! A person's gravitational force (i.e. weight) applies a force on the ground and a normal force is applied by the ground to the person.

He can clearly pull the box with a rope like we always do. The trouble is the following. The donkey forgot that the two forces that he took into consideration are acting on different objects. The forward force is exerted on the sled, whereas the backward force is exerted on the donkey. Thus, the box could accelerate and we would have to use Newton's Second Law on the box to compute the acceleration of the box.

Best Film on Newton's Third Law. Ever.

Newton's third law can be depicted by using few simple words- Every action there is equal to opposite reaction But these few words have the power to dominate our whole universe. Every interaction in our universe must obey newton's third law.

If we want to get more insight about Newton's third law, we can detach it into six particular statements. We have assumed that you have basic conception about the definition of force.

1. Action and reaction have same magnitude.
2. Action and reaction are opposite in direction.
3. Action and reaction work on two different bodies.
4. Action and reaction work simultaneously.
5. Action and reaction are same kind of forces.

6.Action and reaction work along a common straight line.

Example of 1st statement: If someone punches you in the face and you can't take revenge back, there is no cause to be disappointed. Because physics says, if he applies 100 N force on your face by punching, your face will give the same amount of force on his hand.

Example of 2nd statement: Force is a vector. If the gravitational force on apple due to the earth works along the negative 'Y' axis, the gravitational force on earth due to apple will work along positive 'Y' axis.

Example of 3rd statement: In our 2nd example of earth and apple system, action works on earth and reaction works on apple. There is no obligation to choose which one should be action and which one should be reaction between these two forces. You are free to choose it. We can also say that action works on apple and reaction works on earth. But the main theme is that action and reaction work on two different objects.

Example of 4th statement : If you turn away from pushing your car forward, your car will also finish to push you backward simultaneously. Action and reaction appear simultaneously and disappear simultaneously too.

Example of 5th statement: There are four fundamental forces in our universe. All interactions in our universe are nothing but these four fundamental forces. In our first example of punching or our third example of pushing car are the effect of electromagnetic force. Action and reaction of those two cases are electromagnetic force.

In our second example of apple and earth system, the action and reaction both are gravitational force. In universe, you will never find any event where action and reaction are two different form of forces. For instance, action is gravitational force and reaction is electromagnetic force. This will never happen.

Example of 6th statement : Action and reaction work along a common straight line. We know that action and reaction are opposite in direction. They are opposite vectors. It may happen that two vectors are opposite in direction on a same plane but don't lie on the same straight line. They can be just parallel with each other. But whenever we are talking about action and reaction case, these two forces work along a common straight line but opposite in direction.