Chapter 5 The Laws of Motions

5.1 The Concept of Force Force (F) : “A push or a pull”. F is a VECTOR!!! Forces are what cause any change in the velocity of an object Force is that which causes an acceleration Force: Does not always cause motion. Sitting in this class a gravitational force acts on you and you are still on rest.

The Concept of Force, 2 The NET FORCE is the vector sum of all the forces acting on an object Also called total force, resultant force, or unbalanced force Forces: Behave as a Vectors. We must use the rules of Vector Addition to obtain The Net Force on an object

Zero Net Force When the net force is equal to zero: The acceleration is equal to zero The velocity is constant Equilibrium occurs when the net force is equal to zero The object, if at rest, will remain at rest!! If the object is moving, it will continue to move at a constant velocity!!

Classes of Forces Contact forces involve physical contact between two objects Field forces act through empty space No physical contact is required

Fundamental Forces Gravitational force Electromagnetic forces Between two objects Electromagnetic forces Between two charges Nuclear force Between subatomic particles Weak forces Arise in certain radioactive decay processes

More About Forces A spring can be used to calibrate the magnitude of a force Forces are vectors, so you must use the rules for vector addition to find the net force acting on an object

5.2 Newton’s First Law A common MISCONCEPTION in the 21st Century The ancient ( & wrong! ) view (of Aristotle): Need a force to keep an object in motion!! The “natural” state of an object is at rest!! CORRECT VIEW (of Galileo & Newton): Its just as natural for an object to be in motion at constant speed in a straight line as to be at rest. At first, imagine the case of NO FRICTION

Newton’s First Law, 2 Experiment: (proven by Galileo in 1620’s) If NO FORCE is applied to an object moving at a constant speed in straight line, it will continue moving at the same speed in a straight line! If I succeed in having you overcome the wrong ancient misconception & understand the correct view, then: I will be the Happiest Physics Teacher in the World!!!

Newton’s First Law, 3 If an object does not interact with other objects, it is possible to identify a reference frame in which the object has zero acceleration This is also called the law of inertia It defines a special set of reference frames called inertial frames, We call this an inertial frame of reference

Newton’s First Law, 4 Inertial Frames: Any reference frame that moves with constant velocity relative to an inertial frame is itself an inertial frame A reference frame that moves with constant velocity relative to the distant stars is the best approximation of an inertial frame We can consider the Earth to be such an inertial frame although it has a small centripetal acceleration associated with its motion In the absence of external forces, when viewed from an inertial reference frame, an object at rest remains at rest and an object in motion continues in motion with a constant velocity

Newton’s First Law , final Mathematical Statement of Newton’s 1st Law: If v = constant  F = 0 OR If v ≠ 0  F ≠ 0 Newton’s 1st Law DOES NOT say what happens to an object with NET FORCE = 0 . Newton’s First Law describes what happens in the absence of a force. Also tells us that when no force acts on an object, the acceleration of the object is zero.

5.3 Inertia and Mass Inertia: The tendency of an object to resist any attempt to change its velocity. Inertia : The tendency of an object to maintain its state of rest or motion. Mass is that property of an object that specifies how much resistance an object exhibits to changes in its velocity

More About Mass Mass is an inherent property of an object Mass is independent of the object’s surroundings Mass is independent of the method used to measure it Mass is a scalar quantity The SI unit of mass is kg Other unit = gram (g) = 10-3 kg

Mass vs. Weight Mass and weight are two different quantities Weight is equal to the magnitude of the gravitational force exerted on the object Weight will vary with location

5.4 Newton’s Second Law 1st Law : If no net force acts, object remains at rest or in uniform motion in straight line. Experiments: If the net force F  0  the velocity changes (in magnitude or direction or both). A change in the velocity is Δv  an acceleration: a = (Δv/Δt) A net force acting on a body produces an acceleration! F  a HOW? Answer by EXPERIMENTS!

Newton’s Second Law, 2 a  SF & a  1/m (proportionality) a  SF/m When viewed from an inertial frame, the acceleration of an object is directly proportional to the NET FORCE acting on it and inversely proportional to its MASS a  SF & a  1/m (proportionality) Force is the cause of change in motion, as measured by the acceleration We choose the units of force so that this is not just a proportionality but an Equation: a  SF/m Algebraically: SF = m a (5.2)

Newton’s Second Law, final ∑ ≡ Sum  SF ≡ Net force: Is the vector sum of all the forces acting on the object m ≡ The mass (inertia) of the object. a ≡ Acceleration of object. Description of the effect of F. F is the cause of a. Newton’s Second Law can be expressed in terms of components: SFx ≡ m ax SFy ≡ m ay SFz ≡ m az (5.3) A more precise definition of force: An action capable of accelerating an object.

Units of Force Units of force: SI unit = The Newton (N) 1N = 1 kg m/s2 (5.4)

Material for the Midterm Examples to be solved in Class!!! Example 5.1 (Page 118) Homework to be solved in Class!!! Problem: 3