- #1

aaaa202

- 1,170

- 3

You are using an out of date browser. It may not display this or other websites correctly.

You should upgrade or use an alternative browser.

You should upgrade or use an alternative browser.

- Thread starter aaaa202
- Start date

- #1

aaaa202

- 1,170

- 3

- #2

tiny-tim

Science Advisor

Homework Helper

- 25,838

- 255

Why is it that the normal force always point perpendicular to the plane?

it's a mathematical

alternative answer: it

the

it can be at any angle, but we choose to simplify it by calling the normal component the normal force, and the parallel component the friction force

- #3

aaaa202

- 1,170

- 3

- #4

tiny-tim

Science Advisor

Homework Helper

- 25,838

- 255

… Surely the box slides, because there isn't any component balancing the projection of the gravity along the slope. So the total reaction force must be the normal force...

no, the box slides (ie stays on the surface) because the normal force (ie the normal component of the reaction force) is

- #5

aaaa202

- 1,170

- 3

When gravity acts on the box, it will push directly downwards on the layers of atoms on the slope. Agreed so far?

Good, now by Newtons 3rd law these atoms will push back on the box. I want to know this:

Why doesn't the reaction force from the atoms point in the same way as the downwards gravity? This would be quite bizarre of course since the box then wouldn't slide. But I want to know what limits them from pressing directly upwards.

Please try not give an answer in terms of the ideal physical world presented in clasiccal mechanics, but rather an answer in terms of what actually happens between the molecules. I have studied several lectures in both ordinary Newtonian mechanics and lagrangian mechanics, and I'm not looking for trivial answers :P

- #6

tiny-tim

Science Advisor

Homework Helper

- 25,838

- 255

When gravity acts on the box, it will push directly downwards on the layers of atoms on the slope. Agreed so far?

nope!

gravity acts on the molecules in the box

the molecules in the box also act on each other via the electromagnetic force

they also act on the molecules in the slope via the electromagnetic force

the resultant can be in any direction

… rather an answer in terms of what actually happens between the molecules.

hmm … you do know that, on that level of mangification,

- #7

aaaa202

- 1,170

- 3

Edit: hmm.. maybe it doesn't really make sense theoretically to talk about the box pressing directly down on the molecules if the surface is smooth.

- #8

tiny-tim

Science Advisor

Homework Helper

- 25,838

- 255

and what makes you think they're going to be normally in-line, with a normal reaction force, anyway?

- #9

aaaa202

- 1,170

- 3

Here's how I understand it:

When the box molecules meet the slope molecules they interact. The nature of these interactions is very hard to describe because it essentially involves all kinds of forces. What we however CAN see is:

The box doesn't break through the slope, i.e. the total force from the molecules must sum to a normal force that balances the component of gravity perpendicular to the slope. Similarly we observe that there is a force of friction which must be the rest of the resulting force...

- #10

tiny-tim

Science Advisor

Homework Helper

- 25,838

- 255

yes

unfortunately, with solids, the*molecular level* really isn't very practical!

unfortunately, with solids, the

Share:

- Replies
- 5

- Views
- 647

- Replies
- 3

- Views
- 236

- Replies
- 15

- Views
- 303

- Replies
- 8

- Views
- 428

- Replies
- 3

- Views
- 510

- Last Post

- Replies
- 13

- Views
- 450

- Replies
- 2

- Views
- 219

- Replies
- 5

- Views
- 567

- Replies
- 22

- Views
- 503

- Replies
- 6

- Views
- 174