Why reaction force must exist

Calculate the acceleration produced when the professor exerts a backward force of N on the floor. System 1 is appropriate for this example, because it asks for the acceleration of the entire group of objects. All other forces either cancel or act on the outside world.

Since they accelerate as a unit, we define the system to be the professor, cart, and equipment. This is System 1 in Figure. Because all motion is horizontal, we can assume there is no net force in the vertical direction. Therefore, the problem is one-dimensional along the horizontal direction. There are no other significant forces acting on System 1.

See the free-body diagram in the figure. The net external force on System 1 is deduced from Figure and the preceding discussion to be. Another way to look at this is that forces between components of a system cancel because they are equal in magnitude and opposite in direction.

For example, the force exerted by the professor on the cart results in an equal and opposite force back on the professor. In this case, both forces act on the same system and therefore cancel. Thus, internal forces between components of a system cancel. Choosing System 1 was crucial to solving this problem. Calculate the force the professor exerts on the cart in Figure , using data from the previous example if needed.

If we define the system of interest as the cart plus the equipment System 2 in Figure , then the net external force on System 2 is the force the professor exerts on the cart minus friction. This force is significantly less than the N force the professor exerted backward on the floor. Not all of that N force is transmitted to the cart; some of it accelerates the professor.

The choice of a system is an important analytical step both in solving problems and in thoroughly understanding the physics of the situation which are not necessarily the same things. What force will give the second block, with the mass of 6. View this video to watch examples of action and reaction.

Identify the action and reaction forces in the following situations: a Earth attracts the Moon, b a boy kicks a football, c a rocket accelerates upward, d a car accelerates forward, e a high jumper leaps, and f a bullet is shot from a gun. Newton's third law is about pairs of objects interacting. The force that acts on one object is equal and opposite to the force acting on the other object. So you can never have a third law pair acting on the same object.

The equality of the reaction force and the weight force is nothing to do with the third law, and is just as a result of the first law applied to the forces acting on the book. A lot of questions here talk about "normal force", but I get the feeling that you're still confused about what that is. First consider the book - Whether it is resting on the table or not, it has a weight.

Here weight is different from mass. The same goes for the table. Now this is the important part - The weight isn't gravitational force. In the case of the table and the book, the gravitational attraction is absolutely negligible, since they are both so tiny. The force that the table experiences because of the book is what is being called normal force. The table then exerts an equal and opposite force.

This is also clearly seen, because if the table didn't exert an equal and opposite force, the book would be accelerating downward. But the whole system is at rest, therefore the total force on the book-table system must be zero. Basically normal force is only indirectly due to gravity.

Khan Academy has a brilliant explanation of these concepts. So book-table has force pairs due to interaction forces, balanced and oppsite, call them normal due to book, normal due to table. Both same kind. Book-earth has force pair due to gravity of each acting on other.

Both same kind of forces, equal and opposite, and on different bodies. Table-earth, there is contact, which is electric interaction at electronic charge level. Equal, opposite yet same kind of force. Finally, each mass has gravity and the mass exerts force on other mass - NOTE: "on other mass!!!! Sign up to join this community. The best answers are voted up and rise to the top.

Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. Why is a book on a table not an example of Newton's third law? Ask Question. Asked 9 years, 5 months ago. Active 5 years, 7 months ago. Viewed k times. Improve this question. The answer to this question is wrapped up in the same issues as the answer to your question about the ball. The Newtonian pair are the force of the book on the table and the force of the table on the book.

They are both equal in magnitude to the weight of the book, but that is because the problem is static nothing undergoing acceleration. I recommend that you try to understand the other question first, and then come back to this one. 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.

Action-reaction force pairs make it possible for cars to move along a roadway surface. While driving down the road, a firefly strikes the windshield of a bus and makes a quite obvious mess in front of the face of the driver.

This is a clear case of Newton's third law of motion. The firefly hit the bus and the bus hits the firefly. Which of the two forces is greater: the force on the firefly or the force on the bus? Trick Question! Each force is the same size. For every action, there is an equal The fact that the firefly splatters only means that with its smaller mass, it is less able to withstand the larger acceleration resulting from the interaction. Besides, fireflies have guts and bug guts have a tendency to be splatterable.

Windshields don't have guts. There you have it. For years, space travel was believed to be impossible because there was nothing that rockets could push off of in space in order to provide the propulsion necessary to accelerate. This inability of a rocket to provide propulsion is because I t is a common misconception that rockets are unable to accelerate in space. The fact is that rockets do accelerate. There is indeed nothing for rockets to push off of in space - at least nothing which is external to the rocket.

But that's no problem for rockets. Rockets are able to accelerate due to the fact that they burn fuel and push the exhaust gases in a direction opposite the direction which they wish to accelerate. Many people are familiar with the fact that a rifle recoils when fired. This recoil is the result of action-reaction force pairs. A gunpowder explosion creates hot gases that expand outward allowing the rifle to push forward on the bullet.

Consistent with Newton's third law of motion, the bullet pushes backwards upon the rifle.