# Normal force

1. Aug 20, 2010

### barthayn

1. The problem statement, all variables and given/known data
How to know when one is suppose to add or subtract to get the normal force of an object.

2. Relevant equations
FN = Fg, FN = F sin(-)

3. The attempt at a solution

I believe to get the normal force of a 2 kg object when it is being pulled upwards by a force of 45 Newtons at an angle of 40 degrees is:
FN = 45 N sin(40) - 19.6 N
FN = 9.32 N

or is it:

FN = 45 N sin(40)
FN = 28.9 N

Which one is correct? How do I know when to add or subject the normal force and gravitational force?

2. Aug 20, 2010

### mizzy

I usually draw the FBD and look at my positive and negative values.

Fn is perpendicualr to the slope of the incline, therefore it's equal to the y component of mg.

3. Aug 20, 2010

### barthayn

How is it the normal force equal to the force of gravity? That is impossible with an incline. It is only possible if the mass is on a flat surface. However, I see your point of labeling the FBD. Therefore, my example that I used, the mass was being pulled upwards because the normal force was higher than the force of gravity?

4. Aug 20, 2010

### rl.bhat

If you push a cart with a force making an angle θ with the horizontal on the horizontal surface, then the normal force = mg + Fsinθ.
If you pull the cart in the above case, the normal force = mg - Fsinθ.
In your problem two forces are acting on the body.Net normal force is the sum of the components of two forces perpendicular to the surface of the inclined plane. 45 N is acting parallel to the inclined plane. So it has no vertical component with respect to the inclined plane. Hence the normal reaction is mg*cos(θ)

Last edited: Aug 20, 2010