Newton’s Laws of MotionBeginning with:

The Law of Inertia

Force

A force is a push or a pull on an object.

Forces are measured in Newtons (N).

1 N = 1 kg m/s2

Net Force External force – acts from outside the system

or object Internal force – acts from inside the system

A net external force is the sum of the external forces acting of the object. Simply, the net force is the overall force on an object.

Balanced vs. Unbalanced Forces If the net force on an object

is zero, the forces are balanced. (This condition is called equilibrium.)

If the net force is not zero, then the forces are unbalanced.

Free-Body Diagrams Free-body diagrams show the external forces

acting upon an object.

FBDs Since forces are vectors, free-body diagrams

are helpful for analyzing a situation. The diagrams can be used to determine the

net force or individual forces. The object is typically represented as a dot. Roughly scaled arrows represent the forces. The arrows should be labeled.

Galileo (1564-1642)

Galileo asked how things move, not why they moved.

Inertia is a property of matter. It is that property of matter which opposes changes in velocity. Simply stated, a common object will not change its velocity spontaneously.

Friction is the name given to the force that acts between materials that touch as they move past one another.

Isaac Newton (1642-1727)Issac Newton’s monumental work, Philosophiae Naturalis Principia Mathematica, was published in 1687. It proposed scientific laws that are still used today to describe the motion of objects. (credit: Service commun de la documentation de l'Université de Strasbourg)

The Law of Inertia First Law of Motion: Every object continues

in a state of rest, or continues moving in a straight line at constant velocity, unless it is compelled to change that state by forces exerted upon it.

Main Idea: There must be a cause (a net force) for there to be a change in velocity.

Equilibrium The Law of Inertia describes objects at

equilibrium. For an object to be at equilibrium it must:

AND

Have balanced forces (net force = 0)

Be at rest Move at constant velocityOR

Mass: A Measure of Inertia

It is more difficult to change the motion of an object with more inertia (mass).

Mass is a measurement of the amount of stuff in something.

The mass of an object is NOT dependent on its location.

Mass is typically measured in kilograms.

Weight is a Force Weight depends on the acceleration due to

gravity, which can vary with location. In a gravity free environment, there is no

weight. Mass and weight are proportional. An object

with a mass of 1 kg will have a weight of 9.8 N.

W = m g

Newton’s Second Law of Motion Formal Version: The acceleration produced by a net

force on an object is directly proportional to the magnitude of the net force, is in the same direction as the net force, and is inversely proportional to the mass of the object.

Plain Version: A net force causes a net acceleration. When mass is constant, a larger force causes a larger acceleration. When force is constant, a smaller mass gets a larger acceleration (and vice versa).

2nd Law – Mathematical Version

F = ma OR

a = F/mThe units are:

Force (F) is in Newtons (N) Mass (m) is in kilograms (kg) Acceleration is in meters/second2 (m/s2)

Newton’s Third Law Whenever one object exerts a force on a

second object, the second object exerts an equal and opposite force on the first.

Action force - Reaction force Forces are on different objects