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

In summary, the conversation discusses determining the weight of a box with a mass of 10 kg on a table at rest. The formula W=mg is used, and the normal reaction (Fn) is also determined. The next part involves a force of 40N pushing down on the box, resulting in a net force of 0. The correct formula for Fn is found to be Fn=40N+10*9.8, and it is important to always use and check dimensions when solving problems. It is also mentioned that the box has a mass of 10 kg, not a weight of 10 kg.
  • #1
Nahsor1
5
0

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]
 

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  • #2
Hello Nahsor, welcome to PF :smile: !

## 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 ##.
 
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  • #3
is this the answer for the next part
Fn=40N+10*9.8
 
  • #4
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 !
 
  • #5
THANKS FOR URS HELP BVU
 
  • #6
I UNDERSTOOD NOW FORCE UPWARD=FORCE DOWNWARD
 
  • #7
Yes, that's all there's to it.
 
  • Like
Likes Nahsor1
  • #8
One small correction to the question statement. 10kg is the mass of the box, not the weight. The weight is 10g N.
 
  • #9
oh yus yus.
 

1. What is the weight of the box?

The weight of the box is 10 kg.

2. What is the force of gravity on the box?

The force of gravity on the box is equal to its weight, which is 98.1 Newtons (9.8 m/s^2 x 10 kg).

3. How is the box able to stay on the table?

The box stays on the table because of the normal force exerted by the table on the box, which is equal to the weight of the box. This is due to Newton's third law of motion, which states that for every action, there is an equal and opposite reaction.

4. What is the net force acting on the box?

The net force acting on the box is zero, as the box is at rest and all the forces acting on it (gravity and the normal force) are equal and opposite.

5. What would happen if the box was on a different planet?

If the box was on a different planet with a different gravitational pull, its weight and the normal force exerted on it by the table would also be different. This would affect the net force acting on the box and could potentially cause it to move or accelerate.

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