2
$\begingroup$

If a body is on the earth’s surface at equilibrium, the forces acting on it are gravitational (by the earth’s centre) and normal (by the surface). These balance each other and allow the body to maintain a state of equilibrium.
What about the reaction forces to these? I understand that the gravitational force exerted by the body on the earth is insignificant with respect to the earth’s mass. Shouldn’t the body apply an equal and opposite normal to the earth’s surface? If it does, what balances that normal? Can you give a description of the total forces acting on the body and by the body in this case and how they are balanced/unbalanced?

$\endgroup$
  • 1
    $\begingroup$ Action and reaction pair: object A exerts force $\mathbf{F}$ on object B. Then object B exerts a force $-\mathbf{F}$ on object A. $\endgroup$ – K_inverse Mar 19 at 4:47
  • $\begingroup$ This question is too broad. You should narrow it down to a single question $\endgroup$ – Aaron Stevens Mar 19 at 4:51
1
$\begingroup$

Newton's third law always causes conceptual difficulties.

When considering the law it is worth remembering the following three points:

  1. Newton's third law is always true.
  2. The forces must be acting on different bodies.
  3. Both forces must be of the same type. The two forces must either both be contact forces where the forces are due to the bodies touching each other or they must be the same type of non-contact forces where the bodies do not need to touch for the forces to act eg gravitational attraction.

When a book rests on a bench it might appear that

  • the force on the book due to the bench and
  • the force on the book due to the gravitational attraction of the Earth

are a Newton third law pair but the diagram below shows why this is not so.

enter image description here

Indeed, if the bench is removed only the force on the book due to the Earth’s gravity would be acting which is not a pair of forces.
Remember Newton's third law is always true.

The N3L pairs in book on bench situation are shown below:

enter image description here

You will note that when you drop a book there are no contact forces and the book accelerates downwards due to the Earth’s gravitational attraction.
However, at the same time the Earth will accelerate upwards due to the book’s gravitational attraction.

$\endgroup$
0
$\begingroup$

As you are studying basics right now so Iwould like to tell concepts easily.

When a body is kept on earth's surface only two forces are being exerted on the body by earth namely reaction force and the gravitational attraction.

See,we will only consider the vertical reaction force(contact force) by the earth on the body because the parallel component of reaction force by earth on the body is zero until and unless another force by some external body is applied on that body or the body is slipping with respect to the ground.

Contact reaction forces arise due to the electromagnetic interaction between the surface of object and the surface of earth itself.By observations the vertical component of the reaction forces is repelling in nature(almost every time leaving exceptions behind) and the parallel component of reaction force always oppose relative motion(while slipping) and impending motion when it is at rest and acted by an external force(I have taken earth and the body as the system).

Now coming to forces altogether in the system.See earth exerts a force of gravitational attraction on the body so by 3rd law of motion object also exerts an equal and opposite gravitational force on the earth.The earth exerts a vertical reaction force on the object so by Newton's 3rd law the object also exerts an equal and opposite vertical reaction force on the earth.Considering equilibrium of the object the weight of the object(downwards) and vertical reaction(upwards) forces balance each other.Considering equilibrium of earth the vertical reaction force(downwards) and gravitational attraction(upwards) balance each other.

Hope this helps.

$\endgroup$
0
$\begingroup$

enter image description here

Action Equals Reaction, so the sum is zero

$\endgroup$
0
$\begingroup$

When you perform a force balance on a body, you only include the forces that other bodies exert on it. You don't include the forces that it exerts on other bodies. The net resultant of the forces that other bodies exert on body A is equal to the mass of body A times its acceleration.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.