# B Actual value of normal reaction

1. Mar 26, 2017

### Wong Tak

I am a high school student in Hong Kong studying Physics. I am having some problems about atmospheric pressure. Imagine there is a cube block with volume 1 m3 and top surface area 1 m2 . The cube block is being placed on the floor and it has mass of 1 kg. What is the normal reaction force F acting on the block by the floor?
Considering the free body diagram of the block, there are effectively three forces acting on it . They're the weight , F , and the force based on the atmospheric pressure on the top surface. ( Since the side surface's force canceled out each other , they're neglected) Since net force= 0 , F = mg + Area of top surface x atmospheric pressure= 1x9.81 + 1x101x1000 =101009.81 N. Is it really that large? Or my calculation is wrong? My teacher said that it includes a concept called buoyancy in this situation and it is out of our syllabus. Can anyone explain it to me ? Another alternative question I have is that what is the principle of the pressure balance ? Doesn't it measure the normal reaction force acting on it? I mean even if I didn't step on it , it should still have a large reading , right ? Or is it adjusted ? Thanks

2. Mar 26, 2017

### Staff: Mentor

Yes, atmospheric pressure really does exert that great a force on the block. But it is not easy to make an 'air-tight' seal between block and surface, so usually that force is balanced (except for the small buoyant force).