Law of InteractionThe 3rd Law

of Motionby Isaac Newton

OVERVIEW

Definition of term

The general statement for the Law of Interaction

Basic concepts of the Law of Interaction

Examples of situations exhibiting Law of Interaction

Summary

References

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ISAAC NEWTON was the one who formulated the Three Laws of Motion - Law of Inertia, Law of Acceleration, and the Law of Interaction.

DEFINITION OF TERM

Force is a “push” or a “pull” of an object which results from its interaction with another object.

Forces therefore, results from interactions!

 

Consider the example of interaction between a hammer and a nail.

When you strike the nail with a hammer, the hammer exerts a downward force on the nail.  The nail in turn, also exerts a force on the hammer.

Force exerted by the hammer

Force exerted by the nail

Force exerted by the nail

Force exerted by the hammer

The downward force or the force exerted by the hammer on the nail is the action force.

The upward force or the force exerted by the nail on the hammer is the reaction reaction force.force.

Action and reaction forces always come in pairs.

They are of equal magnitude, but in opposite direction.

They act on different bodies

Remember:

Law 3: For every action, there is an equal (in strength) and opposite (in direction) reaction.

In simplified form, Law of interaction can be stated as:

Fundamentally, the reason why we know that forces behave like this is that experiment tells us so.

Never forget that physics isn't merely a collection of rules and equations; rather, it’s a systematic description of the natural world based on experiment and observation.

Identifying Action and Reaction Force Pairs

Action & Reaction Forces

The action force is the force exerted by the wings of the bird on the air.

The reaction force is the force exerted by the air on the bird’s wings.

EXAMPLES OF SITUATIONS EXHIBITING LAW OF INTERACTION

The forward movement of the rocket is the reaction force.

The expulsion of exhaust gases from the rear is the action force.

The action force is the downward gravitational force exerted by earth on the book.

The reaction force is the upward force exerted by the table on the book.

Check Your Understanding

While driving down the road, an unfortunate bug strikes the windshield of a bus.

Quite obviously, this is a case of Newton's third law of motion.

The bug hit the bus and the windshield hit the bus.

Check Your Understanding A gun recoils when it is fired. The

recoil is the result of action-reaction force pairs.

As the gases from the gunpowder explosion expand, the gun pushes the bullet forwards and the bullet pushes the gun backwards.

It’s true that the opposite reaction force by the cart on the horse restrains the horse. This force tends to hold the horse back.

Without this force the horse could freely gallop to the market. So how does the horse move forward?

By interacting with the ground. At the same time the horse pushes backward against the ground, the ground pushes forward on the horse. If the horse pushes the ground with a greater force than its pull on the cart, then there will be a net force on the horse. Acceleration occurs.

When the cart is up to speed, the horse needs only to push against the ground with enough force to offset the friction between the cart’s wheels and the ground.

SUMMARY

SUMMARY …

• We emphasize once more a fundamental truth: the two forces in an action-reaction pair never act on the same body.

• Remember this simple fact to help you avoid confusion about action-reaction pairs and Newton’s third law.

Are there any questions?

1. Find the net force produced by a 30-N and 20-N force in each of the following cases:   a. Both forces act in the same direction.  b. Both forces act in opposite directions. 

2. A horizontal force of 100 N pushes a box across a floor at a constant speed.   

a.What is the net force acting on the box?   b.What is the force of friction on the box? 

3. Phil Physicer weighs 600 N and stands on two bathroom scales. He stands so one scale reads twice as much as the other. What are the scale readings?

4. If you stand next to a wall on a frictionless skateboard and push the wall with a force of 30 N, how hard does the wall push on you? If your mass is 60 kg, what’s your acceleration?

REFERENCES

University Physics by Young & Freedman pp. 98-103

Conceptual Physics by Paul Hewitt

pp. 87-92

http://www.images.com

Thank you very much

for listening!!!