Unit 5: Force and Motion - I

5.1 - Newton's First and Second Laws

First Law

If no net force acts on a body (F⃗net=0\vec F_{net} = 0), the body’s velocity cannot change; that is, the body cannot accelerate. External forces are required to cause acceleration.

Inertial Reference Frames

Newton’s first law is not true in all reference frames, but we can always find reference frames in which it (as well as the rest of Newtonian mechanics) is true. Such special frames are referred to as inertial reference frames, or simply inertial frames.

Second Law

The net force on a body is equal to the product of the body’s mass and its acceleration.

F⃗net=ma⃗\vec F_{net} = m \vec a

Where F⃗net\vec F_{net} is the sum of all forces acting on the body (the net force).

This does not necessarily mean that there are no forces on the objects, but rather that all of the forces on the body are in equilibrium, meaning that the net force is zero.

5.2 - Some Particular Forces

The Gravitational Force

Fg=mgF_g = mg

The gravitational force is a force directly downward and only in the y direction, which equals the mass of an object times the acceleration due to gravity (9.8m/s).

Weight

The weight W of a body is the magnitude of the net force required to prevent the body from falling freely, as measured by someone on the ground.

The Normal Force

The force of a solid object pushing back on an object pushing into it. Results in a balanced force, when, for example, an object affected by gravity is sitting on a flat surface.

FN−Fg=mayF_N - F_g = ma_y

Friction

Friction is any force which opposes motion. There are two forms of friction, kinetic and static, which we will learn more about later.

Tension

When a cord (or a rope, cable, or other such object) is attached to a body and pulled taut, the cord pulls on the body with a force T⃗\vec T directed away from the body and along the cord. The force is often called a tension force because the cord is said to be in a state of tension (or to be under tension), which means that it is being pulled taut. The tension in the cord is the magnitude T of the force on the body. For example, if the force on the body from the cord has magnitude T=50NT = 50 N, the tension in the cord is 50 N.