Newton’s Laws of Motion

Dynamics and Forces Dynamics: Connection between force and motion. Explains why things move. Dynamics: Connection between force and motion. 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 (change its velocity)  Force is a vector quantity  Force can be measured using a spring scale.

4th Century BC: Aristotle ( BC) A force (push or pull) is needed to keep an object moving along a horizontal plane. The natural state of objects is at rest. The greater the force, the greater the velocity.

1630’s: Galileo Galilei ( ) Any object resists changes to its state of motion (inertia). Once an object starts moving, it continues to move in a straight line at constant speed An object slows down because a force acts on it.

1687: Isaac Newton ( ) Published the book Philosophae Naturalis Principia Mathematica in which he explained and extended Galileo’s ideas about motion and completely changed the way the universe is viewed.

Newton’s First Law of Motion (The Law of Inertia) Every object continues in a state of rest, or uniform velocity in a straight line, as long as no net force acts on it. Every object continues in a state of rest, or uniform velocity in a straight line, as long as no net force acts on it. Inertia: Tendency of an object to maintain its state of rest or of uniform velocity in a straight line (resists changes in motion) Inertia: Tendency of an object to maintain its state of rest or of uniform velocity in a straight line (resists changes in motion)

Inertial Reference Frame One in which Newton’s 1 st Law is valid. One in which Newton’s 1 st Law is valid.  Reference frames fixed on the Earth are inertial frames.  Any reference frame that moves with a constant velocity relative to an inertial frame is also an inertial frame. Noninertial frames are ones in which the 1 st law does not hold (accelerated frames) Noninertial frames are ones in which the 1 st 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 inertia.  Inertia is a property of an object (quantity of matter)  Standard is the kilogram.

Net force (  F) Sum of all of the forces that act on an object. It is a vector sum. Sum of all of the forces that act on an object. It is a vector sum.  When  F = 0, There is no change in the motion of the object. This is also called equilibrium.  When  F  0, the object accelerates.

Newton’s 2 nd Law Of Motion The acceleration of an object is directly proportional to the net force acting on it, and is inversely proportional to its mass. The direction of the acceleration is in the direction of the net force The acceleration of an object is directly proportional to the net force acting on it, and is inversely proportional to its mass. The direction of the acceleration is in the direction of the net force a =  F/m a =  F/m  F = ma

Newton’s 2 nd Law Of Motion Units: F = kg∙m/s 2 = Newtons (N) g∙cm/s 2 = dyne (1 dyne = N) slug∙ft/s 2 = pound 1 N = lbs 1 lb = 4.45 N

Newton’s 3 rd Law Of Motion Whenever an object exerts a force on a second object, the second object exerts an equal force in the opposite direction on the first. Whenever an object exerts a force on a second object, the second object exerts an equal force in the opposite direction on the first.  Forces are exerted on objects by other objects  Forces act on different objects  Notation: F on, by

Examples of Newton’s 3 rd Law