Reaction Force (1 Viewer)

[Dumsum]

Ok, example. Ball of mass m sitting on a table. It experiences a force F = mg, yes? There is also a reaction force N = mg. What? I don't understand why it's not N = -mg, considering it's acting in the opposite direction. Shouldn't the force vectors add to give 0 (no net force)?

 

[gman03]

Ok, example. Ball of mass m sitting on a table. It experiences a force F = mg, yes? There is also a reaction force N = mg. What? I don't understand why it's not N = -mg, considering it's acting in the opposite direction. Shouldn't the force vectors add to give 0 (no net force)?

Shouldn't F and N have directions as well since they are vectors? Why not make F and N points in different direction!

 

[acmilan]

Yes you do add them.

Normal force is upwards, weight is downwards, hence you add them, ie N + (-mg) = 0, so N = mg. If N = -mg, then N + mg = 0, which implies that N and mg act in the same direction (since their sign is the same) which is not true.

 

[Dumsum]

But when doing calculations with things like velocity, negative velocity implied it was the other direction? Wouldn't negative force imply that it's acting in an opposite direction?

 

[acmilan]

Yes.

If you take up as + and down as - :

Normal force acts up, so its +N
Weight force acts down, so its -mg

The sum of the vectors is zero, so + N - mg = 0, hence N = mg.

 

[Dumsum]

Yes.

If you take up as + and down as - :

Normal force acts up, so its +N
Weight force acts down, so its -mg

The sum of the vectors is zero, so + N - mg = 0, hence N = mg.

Text book is saying weight force is +mg, and normal force is +mg, I'm confoozled

 

[acmilan]

yeah well in that situation they took positive as downwards, hence weight is positive, ie +mg.

Using the same logic as my previous post, normal force is now -N.

Adding up again gives -N + mg = 0, ie N = +mg

Thus both weight and normal = +mg. However normal force doesnt always equal mg, only in this case. When stuff like friction and pulling forces are added to the equation, the value of the normal force changes.

 

[Dumsum]

Ah I see it now. Thanks dude