Friction between 2 objects stacked on top of each other

Question

Let's say I have a particle on top of a block that is resting on top of a rough plane. Let's also assume that there is friction between block & particle as well as block & surface. In which direction would friction act and if we were given some information such as the frictional force between the particle & block, how would I be able to calculate things like frictional force acting between the block and surface, provided the block is in equilibrium?

I appreciate that these questions are hypothetical but the main purpose of these questions is for me to understand that when 2 objects are stacked on top of one another, how do the forces act on them etc?

Thanks

Answer 1

Generally speaking, you should always draw each internal force between objects according to the 3rd law of Newton, and then apply the 2nd law of Newton on each object.

Usually one thinks that friction always is in the opposite of movement, but this is not true. For example, put a book on the palm of your hand and slowly move your hand; the book moves too! This is because there is a friction between your skin & book, and technically speaking, the friction causes the book to move in the direction which you move your hand. Duo to 3rd law of Newton, there exists an equal and opposite force (friction) which you feel it as some kind of "resistance" from the book.