# Physics principle(s) to explain how to stack spherical items

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1. Apr 22, 2015

### SelenaT

I need to think of all the physics principles to explain how one can stack spherical items (ex. baseballs) on top of each other. So far I've thought of one.
1. Newton's third law
In this case, the reaction is the normal force in each baseball that is stacked and the action is the force of gravity acting on each ball.

Could Newton's first or second law also explain this? Are there other physics principles that can explain this?

2. Apr 23, 2015

### CWatters

Yes.

When you stack balls the normal reaction between balls isn't vertical. What might stop the ones on the bottom being pushed or rolling away?

3. Apr 23, 2015

### SelenaT

The fact that every object will remain at rest unless acted upon by an unbalanced force (Newton's first law)? But I don't understand. How is the normal reaction between balls not vertical? Isn't it perpendicular to the horizontal surface?

4. Apr 23, 2015

### haruspex

"Normal" in "normal force" means perpendicular to the contact plane. It can be hard to define contact plane in general, but assuming the surfaces are differentiable then it means the plane tangent to the two surfaces at the point of contact.