a book rests on a table, gravity pulls down on it at 20 newtons, what prevents the book accel. down at 9.8 m/s

Answer 1

Imagine the table is not made of wood but by a thin rubber or of paper sheet stretched across the table top.

As the weight of the 20 Newton force pushes the paper down, the paper is deformed in shape; the surface becomes concave; there is a small depression on the point where the object is.

Therefore, by elastic property of the paper the paper tends to regain its original shape; it tends to move up pushing the 20 N up. A stage is reached when the down ward 20 N force is equal to the up ward push by paper. If these two forces are equal and opposite then the object will be at rest.

If that paper sheet is not able to withstand the strain produced by the force, the paper will be torn and because of gravity the object will fall down.

The wooden table provides the necessary upward force and is called normal reaction force.

If the object's weight is heavy enough, and the table top is not providing equal and opposite force, even that wooden top will break and the object will fall down.

When you drop an object in air, the air is not able to give the necessary upward force and hence the object falls down.

If a balloon filled with light gas, it goes up; this is because the surrounding air pushes the balloon up with a force greater than the weight of the balloon.

Answer 2

The table pushes up on the book with a force of 20 Newtons making the net force on the book 0 Newtons. So, the acceleration is 0 m/s^2

Answer 3

The book is pushing the table downward as it is being pulled downward by its weight force (force of gravity from the Earth), the table also exerts an upward force, which is equal to 20 newtons. Thus, the book has two equal and opposite forces forces acting on it. The net force on the book is thus zero and it has no acceleration. Note that the the two forces acting on the book are not the "action-reaction" pair discussed in the Newton's third law.

Answer 4

The acceleration of the book on the Table is 9.81 meters/seconds ^2.
The weight of the book on the tabel is 20 newton gravity force.
The mass of the book is M=Gravity force pushing
down on the Book divided by the value of Gravity field.
M =20/9.81=2.038 Kilograms
Applying Newtons 3rd law the Earth is pushing up against the gravity force ;hence the book relative to the table is going nowhere.
The total acceleration between the Earth and the book equals the Sum of the acceleration of the Book on the table plus the acceleration of the Earth at that point , due to the mass of the book on the table.

Its very vely simple.

Answer 5

the force that prevents the books acceleration is the reaction force which is againes the down pull

Answer 6

Normal force.