Newton’s Laws of Motion

Dynamics and Forces

Dynamics: Connection between force and motion. Dynamics: Connection between force and motion. Explains why things move.Explains why things move.

Force: Any kind of push or pull on an object.Force: Any kind of push or pull on an object. Force is required to accelerate an object Force is required to accelerate an object

(change its velocity)(change its velocity) Force is a vector quantityForce is a vector quantity Force can be measured using a Force can be measured using a spring scalespring scale..

4th Century BC: Aristotle (384-322 BC)

A force (push or pull) A force (push or pull) is needed to keep an is needed to keep an object moving along a object moving along a horizontal plane. horizontal plane.

The natural state of The natural state of objects is at rest. objects is at rest.

The greater the force, The greater the force, the greater the velocity.the greater the velocity.

1630’s: Galileo Galilei (1564-1642)

Any object resists Any object resists changes to its state of changes to its state of motion (inertia).motion (inertia).

Once an object starts Once an object starts moving, it continues to moving, it continues to move in a straight line move in a straight line at constant speed at constant speed

An object slows down An object slows down because a force acts on because a force acts on it. it.

1687: Isaac Newton (1642-1727)

Published the book Published the book Philosophae Philosophae Naturalis Principia Naturalis Principia MathematicaMathematica in which in which he explained and he explained and extended Galileo’s extended Galileo’s ideas about motion ideas about motion and completely and completely changed the way the changed the way the universe is viewed.universe is viewed.

Newton’s First Law of Motion (The Law of Inertia)

Every object continues in a state of rest, or Every object continues in a state of rest, or uniform velocity in a straight line, as long uniform velocity in a straight line, as long as no net force acts on it.as no net force acts on it.

InertiaInertia: Tendency of an object to maintain : Tendency of an object to maintain its state of rest or of uniform velocity in a its state of rest or of uniform velocity in a straight line (resists changes in motion)straight line (resists changes in motion)

Inertial Reference Frame

One in which Newton’s 1One in which Newton’s 1stst Law is valid. Law is valid. Reference frames fixed on the Earth are Reference frames fixed on the Earth are

inertial frames.inertial frames. Any reference frame that moves with a Any reference frame that moves with a

constant velocity relative to an inertial constant velocity relative to an inertial frame is also an inertial frame.frame is also an inertial frame.

Noninertial framesNoninertial frames are ones in which the are ones in which the 11stst law does not hold (accelerated frames) law does not hold (accelerated frames)

MASS (m)

Measure of the inertia of an object. Measure of the inertia of an object. The greater the mass, the greater the The greater the mass, the greater the

inertia.inertia.Inertia is a property of an object Inertia is a property of an object

(quantity of matter)(quantity of matter)Standard is the Standard is the kilogram.kilogram.

Net force (F)

Sum of all of the forces that act on an Sum of all of the forces that act on an object. It is a vector sum. object. It is a vector sum. WhenWhen F = 0F = 0, There is no change in , There is no change in

the motion of the object. This is also the motion of the object. This is also called called equilibrium.equilibrium.

When When F F 0, 0, the object accelerates.the object accelerates.

Newton’s 2nd Law Of Motion

The acceleration of an object is directly The acceleration of an object is directly proportional to the net force acting on it, proportional to the net force acting on it, and is inversely proportional to its mass. and is inversely proportional to its mass. The direction of the acceleration is in the The direction of the acceleration is in the direction of the net forcedirection of the net force

a = a = F/F/mm

F = F = mmaa

Newton’s 2nd Law Of Motion

Units:Units: F F = kg= kg∙∙m/sm/s22 = = Newtons (N)Newtons (N)

gg∙∙cm/scm/s22 = = dynedyne (1 dyne = 10 (1 dyne = 10-5-5 N)N)

slugslug∙∙ft/sft/s22 = = poundpound

1 N = 0.225 lbs1 N = 0.225 lbs

1 lb = 4.45 N1 lb = 4.45 N

Newton’s 3rd Law Of Motion

Whenever an object exerts a force on a Whenever an object exerts a force on a second object, the second object exerts an second object, the second object exerts an equal force in the opposite direction on the equal force in the opposite direction on the first.first. Forces are exerted on objects by other Forces are exerted on objects by other

objectsobjects Forces act on different objectsForces act on different objects Notation: FNotation: Fon, byon, by

Examples of Newton’s 3rd Law