Forces and Newton's Laws of Motion [Overview]

In this video we introduce forces and Newton's laws of motion.

0:00 - Force Definition
1:17 - Contact Forces
6:08 - Non-contact Forces
8:11 - Newton's 1st Law
9:00 - Newton's 2nd Law
10:02 - Newton's 3rd Law
10:57 - Mechanical Equilibrium
12:48 - Free-Body Diagrams
14:47 - Forces with and without Formulas
16:04 - Weight, Mass, and Gravity
18:50 - Normal Force
21:57 - Tension Force
23:52 - Spring Force
27:11 - Kinetic and Static Friction

A force is a push or pull and the role of force is to cause acceleration. We measure force in units of newtons.

We can break forces into two categories; contact forces and non-contact forces. Contact forces are the normal force or support force, static friction, kinetic friction, tension, spring force, air drag and thrust. Non-contact forces are gravity or the force of gravitation, weight, the electric force and the magnetic forces.

Newton's 1st law states that an object at rest will remain at rest, and an object in motion will remain in motion at a constant speed in a straight line unless acted upon by an external force. The property of objects to resist changes to their motion, or changes to their velocity, is called inertia. Inertia is measured in mass. Since change in velocity is called acceleration, inertia is also the resistance to acceleration.

Newton's 2nd law states that an object feeling an nonzero net force will feel an acceleration. This acceleration is proportional to the magnitude of the net force and inversely proportional to the mass of the object. The direction of this acceleration is the same as the direction of net force. This is often stated as F = ma.

Newton's 3rd law states that for every action there is a reaction that is equal in magnitude and opposite in direction. Both action and reaction are just words for force. This also means that forces always occur in pairs, and these are called action-reaction pairs.

When there is no net force on an object, it is said to be in a state of mechanical equilibrium. This applies to objects that are at rest and remain at rest, but also objects that are moving at constant velocity. The net force can be found by summing of all of the forces, and in equilibrium, this sum will be zero. This also means that the sum of force components must individually equal zero.

To help organize problems, we use free-body diagrams. A free body diagram is where we represent our objects as a particle or dot, and draw all of the forces acting on that object. Often these forces are the only thing we need to solve a particular problem, so a free-body diagram will free us from all of the other unnecessary information.

Some forces have formulas and some forces do not. In particular, the normal force and tension do not have formulas, along with any generic applied or external force that may just be given in a problem or scenario.

Weight, mass and gravity are often confused. Mass is a measure of inertia and is often thought of as the amount of matter in an object. Mass is measured in kilograms. Gravity is a force between any two masses. Weight is a special case of gravity near the surface of earth. Both are forces and therefore they are measured in newtons.

The normal force is the force that prevents an object from moving through a surface. It is always perpendicular to the surface and will be determine by the physical scenario. Tension is also a force that depends on the scenario. Tension is pulling force exerted via a rope, cable, string, etc. The direction of tension is always away from the object along the rope or other medium.

The spring force is an elastic force that has a formula referred to as Hooke's law. Hooke's law describes restoring forces. A restoring force acts in such as way to return an object to its original configuration. A spring that is stretched or compressed will exert a force that opposes its displacement and act to restore it to its original length. The spring force is also proportional to the displacement. The proportionality constant is called the spring constant and it represents the stiffness of the spring.

Kinetic friction is a force that oppose motion across a surface. Kinetic in general refers to motion. Static friction is a force that opposes impending motion when a force is acting to accelerate an object from rest. Both are proportional to the normal force and have a proportionality constant called the coefficient of friction.

In general static friction is stronger than kinetic friction, so it is more difficult to get an object moving that it is to keep an object moving. Static friction can also vary from zero up to some maximum amount. If there is no force trying to move the object, there is no static friction. Static friction will increase until some maximum value, after which it will start to move, and kinetic friction will become kinetic friction.