What is the Weight and Normal Reaction of a 10 kg Box on a Table at Rest?

[Nahsor1]

Homework Statement

weight of the box 10 kg on table at rest.

Homework Equations

deternmine weight of box and normal reaction acting on it.
also determine w and Fn if force 40N pushes down the box.

The Attempt at a Solution

s
i did like
w=mg =NORMAL REACTION(R)
and
40=10 * acceleration due to gravity.
i feeel like this is wrong.[/B]

 

[BvU]

Hello Nahsor, welcome to PF !

$W = mg$ seems excellent to me. And if the box doesn't move, the normal force has to be $F_N = - mg$, because then the net force on the box (in a vertical direction) is $F_{\rm net} = F_{\rm gravity} + F_N = mg - mg = 0$.

And your feeling of doubt in the next part is very justified.

Where you find the 10 in your next expression is a mystery to me.
Simply continue the reasoning that if the box doesn't move, the net force has to be 0. Only now htere is an extra force acting. W is related to the box only, so it is and it stays $W = mg$.

 

[Nahsor1]

is this the answer for the next part
Fn=40N+10*9.8

 

[BvU]

That's correct. You found it from $F_{\rm net} = 0 = -40 \;{\rm N} + mg + F_N$

As you can see, I use the convention up = positive.
And mg is pointing downwards, so if I write $F_{\rm gravity} = mg$ I have to take g = -9.8 m/s².
Result: $F_N = 40 \;{\rm N} - 10 \;{\rm kg} * (-9.8 \; {\rm m/s^2})$ .

Important tip: always use dimensions -- and check them !

 

[Nahsor1]

THANKS FOR URS HELP BVU

 

[Nahsor1]

I UNDERSTOOD NOW FORCE UPWARD=FORCE DOWNWARD

 

[BvU]

Yes, that's all there's to it.

 

[haruspex]

One small correction to the question statement. 10kg is the mass of the box, not the weight. The weight is 10g N.

 

[Nahsor1]

oh yus yus.