Newton's Laws of Motion

GPB Education - 8:30

Free Body Diagram

GPB Education - 6:40

Newton's Law of Motion

Newton's 1st Law of Motion (Law of Inertia):

An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
Another way of stating this is that if the resultant force on a free body is zero then the object will move at a constant speed in a straight line. This speed may be zero.
Another way of stating this is that if the forces on a free body are balanced, then it will be stationary or moving with a constant velocity.

Newton's 2nd Law of Motion:

If an unbalanced force acts on a free object, it will be accelerated in the direction of the force with an acceleration which is proportional to the size of the unbalanced force.
Another way of stating this is that if the resultant force on a free body is greater than zero than the object will change its speed or direction or both.

Newton's 3rd Law of Motion:

For every action there is an equal and opposite reaction force. The reaction force results from the action force. The action force acts on one object, and the the reaction force acts on a different object.

Important Skill: Free Body Diagram

Force Diagrams

In a force diagram, each force is shown as a force arrow. An arrow shows:

the size of the force (the longer the arrow, the bigger the force)
the direction in which the force acts.

The arrow is usually labelled with the name of the force and its size in newtons.

Free Body Diagram (FBD).

A free body diagram is a graphic, dematerialized, symbolic representation of the body (structure, element or segment of an element) in which all connecting "pieces" have been removed.

In order to effectively use free body diagram to analyze a body's motion you must be accomplish four skills.

Isolate the object (called system) from the environment
Identify the force acting on the system (the free body)
- Non-contact forces: gravity (weight), state electric force, magnetic force
- Contact forces: tension (rope, pulling), normal force (support from ground, table, or inclined plane), thrust (push), friction
Identify the direction of each acting force and draw vectors representing the forces
Create a pair of equations from a free body
Do the math. This usually involves systems of equations.

Torque and Equilibrium

Torque refers to the turning effect about a pivot.
The size of a torque depends on the size of the force and the perpendicular distance from the pivot to the point where the force is applied.
Torque, moment or moment of force is also called moment.
The symbol for torque is typically τ, the Greek letter tau. ( When it is called moment, it is commonly denoted M. )
The SI units for torque is the newton metre (N. m).

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Inertia

Inertia is a concept in Physics that is defined as the tendency of an object to resist a change in its motion.

In other words, Inertia acts as a resistance to any change in the velocity of any physical object.

According to Newton's First Law of Motion, also known as the Law of Inertia, an object will continue to be in a state of rest or in a state of motion unless an external force acts on it.

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