My book states that contact force between two surface is equal to $\sqrt{N^2+F^2}$. where $N$ is the normal force and $F$ is the friction

Question

Applied Force is our label for a contact force that a person exerts. When an applied force acts at an angle, it is actually a combination of two forces: normal and friction. The component of the applied force that is perpendicular to the surface is a normal force, and the component parallel to the surface is a friction force. I have two questions;

1. When the applied force is at an angle then forces of friction and normal are between the surface of the object and the hand, not between the object and the ground so can we say that the parallel component of applied force is responsible for the acceleration of the object? (Is it okay to say that it is the friction force between our hand and the object that accelerates the object)?

2. What will happen if an applied force acts on a thread also can we say that the applied force is the combination of normal and friction force between my hands and thread if so then tension force will be also called a combination of normal force and friction force or if I am wrong then please correct me.

Answer 1

It depends, what you call the friction force, the friction force is in the opposite direction of the movement, your hand moves in the direction. the magnitude of the friction force diminishes the horizontal acceleration force. Only if you move with constant speed, the two forces are equal. it is not important if the force is applied through a thread in pulling or direct by pushing its always the horizontal force which accelerates or does work.