# Force: action and reaction

In a recent class , I was learning how to draw a free body diagram of an object. In the first illustration my teacher taught how to draw a free body diagram of block resting on a horizontal surface. He said that earth is pulling block towards itself with a force equal to it's weight, therefore block has tendency to go vertically downwards and block pushes surface due to that tendency and in return surface pushes block by normal force.

I was confused by the point that surface pushes block by a normal reaction force because block pushes it with a force equal to it's weight or because block pushes it by contact forces electromagnetically. I think surface exert normal force on block because also exert normal force on surface ( contact forces ) and not because block pushes it with it's weight.

that surface pushes block by a normal reaction force because block pushes it with a force equal to it's weight or because block pushes it by contact forces electromagnetically.

These are both true. The block pushes on the surface. This pushing force is done via contact forces electromagnetically (well, it's slightly more complicated than just "electromagnetically" but you've got the right idea. Here's way more than you'll ever want to know about that if you're interested).

If it weren't for the block's weight due to gravity, i.e. if there was no gravity and these were objects floating in space, the block wouldn't remain in contact with the surface. It would simply float away, so no normal force.

• so you want to convey that gravity makes block remain in contact with surface and that's why they are able to exert contact forces on each other May 16 at 16:37
• even if we had no gravity and if block remains in contact with surface then will they not exert contact forces on each other? May 16 at 16:37
• If there's no gravity then the block and the surface will not remain in contact with each other, they'd gradually drift apart. May 16 at 16:51
• @lalittolani If the block does not exert a force on the surface then the surface does not exert a force on the block. If the block exerts a force on the surface (because of gravity, or just because it is being pushed against the surface) then the surface exerts an equal and opposite force on the block. May 16 at 17:03
• @SeñorO If no other forces were involved they would remain in contact as mass attracts mass. They would have their own microgravity attracting each other, but then their normal forces would be quite small. May 16 at 17:04

...because block pushes (the surface) with a force equal to it's weight or because block pushes (the surface) by contact forces ...

Both are true.

The block exerts a downward force on the surface that is equal to its weight (assuming there are no other vertical forces acting on the block). This force is a contact force because the block is in contact with the surface. It is also called a "normal force" because its direction is normal or perpendicular to the surface. And contact/normal forces ultimately result from electromagnetic interactions between the molecules of the block and the molecule of the surface.