Thread: What is the difference between force due to gravity and normal force?

In class, my physics teacher gave us the equation F = ma.

He also gave us this equation: Fg+Fn=m(0)
-mg + Fn = 0
Therefore, Fn=mg
Why is mass multiplied by acceleration in the first equation, while being multiplied by gravity in the second equation? Are they two different forces, and if so, what forces are they? When do I use ma, and when do I use mg? Thanks for any help.

Gravity and acceleration are the same thing. They both move something towards a point at 9.81 m/s^2. Kilograms (mass) times acceleration (essentially anything with the units 9.81 m/s^2) is a force.

The normal force and force due to gravity are two different forces but equal forces. Remember Newton's law? Every action (in this case a force) has an equal and opposite reaction.

So think of it this way. Gravity is acting on you correct? You are exerting a force on the Earth. The reason why you don't sink into the Earth is because of a normal force. The Earth is pushing back on you.

My physics teacher joked that when you punched somebody, they punched you back. If you punched someone in the face. Your hand would exert a force on the person's face, and his face would push back on your hand (which is why your hand can hurt when you punch someone and why boxers wear gloves).


mg is used more when gravity is involved like if you shoot something into the air and then the object acted upon by gravity. ma is generally used for objects like cars. Ex: a car accelerates when person presses the peddle at 2 m/s^2, what is the velocity after two seconds?

But like I said, I like to think of the two as not really independent. Pushing a gas peddle to accelerate something and using gravity to accelerate something is basically the same thing.

Hope that helped!

forget about everything ur teacher said just for the sake of this ...delema, just know that..

weight is the product you get when you multiply mass and gravity.

w=mg

but what is weight measured in in physics? Newtons right? if so, then, isn't weight really telling you how much "force" (F) your pushing into the ground?

F=ma, in that equation you of course noe that "a" is acceleration. well "g" is also an acceleration. "g" is the acceleration of the earth, or, in other words, it defines how fast it will accelerate you if you were to freefall from the sky. which is a constant, it's (-9.81m/s).

kk so now you know that F=ma and w=mg are really two sides of the same coin.

force in physics is measured in units called "newtons", kina like, i weigh 92 pounds... i weigh 50 newtons.

the force due to gravity IS the normal force lol. "normal force" is the reactionary force, it defines how much force an object is pushing back. okay so for example if you weigh 400 newtons and you stand on the floor then the ground is also pushing back up 400 newtons. if you push a wall with 32 newtons the wall will be pushing back 32 newtons. i hope that answers ur questions, if so can i get best answer =D

F=m*a is just Newton's second law of motion.
Newton's second law of motion explains how an object will change velocity if it is pushed or pulled upon by a force.

Fg=m*g means that an object having a mass (m) is pulled towards the gravitational center of the earth
If you stand on a scale it will measure the gravitational force of your mass and because "g" is almost constant (9.81m/s^2) your mass can be calculated.
Fn, the normal force is there for the balance (Fg+Fn=0). If you stand on the floor, the floor will push against your feet (-g*m) with the same force the gravity pulls your mass down to the earth (+g*m). So two opposite forces of the same magnitude cancel each other. thus you don't move.

Force is measured in Newton, the symbol is "N" 1N=1kg*m/s^2