Another physics question?

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Jearred

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In an equation that I was given in order to find normal force, it tells me that normal force is equal to negative(mass times gravity).

F = -mg

I was just wondering why is it negative? Is it because it is acting against something?

Also, in another question I was given this great help:

F = ma = -μmg
μ = -a / g
= -(-1.5m/s²) / 9.8m/s²
= 0.15

I was just wondering why is the negative included?(the first negative, not the negative on the acceleration) What if the acceleration would have been positive? I don't think friction can be negative, can it? Please explain.
 
F = -mg? Generally, the normal force is positive, because it acts upward, opposite of gravity (on a horizontal surface).

For F = ma = -μmg, this is because the friction force (μmg) acts in the opposite direction of motion. That is from my answer to another of your questions, I guess I could have explained that to you then.

Hope this clears it up.
 
This equations are about earth's gravitation forces

that is why if you want to work on a thing against earth's gravity you
will have to face earths gravitational force.
 
This equations are about earth's gravitation forces

that is why if you want to work on a thing against earth's gravity you
will have to face earths gravitational force.
 
The negative is included to indicate that the normal force is the 'opposite' of the gravitational force. For example:

If gravity was -9.8m/s^2 and the mass of the object was 20kg, then you would find the gravitational force like so:

F = ma
F = (20)(-9.8)
F = -196 Newtons

The negative sign signifies that the force is acting downwards. The normal force and the gravitational force are equal, but since the normal force is the force of the ground on the person, it has to be the exact opposite of the gravitational force, thus:

F = -(gravitational force)
F = -(-196)
F = 196

Also, I don't think friction can be negative, at least the coefficients can't be.

I hope this helped :-]
 
Remember, friction is a force and a force is a vector. The negative refers to the direction of the force.

If your frame of reference considers weight pushing down as being the positive direction, then normal force pushing up would be the negative direction.

For friction, the motion is usually considered in the positive direction, so friction would be acting in the negative direction (opposing motion).
 
Simple answer, Force is a vector, meaning it needs direction. Weight points toward the Earth, while a normal force opposes weight.
 
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