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Chapter 5: Forces and Newton’s Laws of Motion

Chapter 5: Forces and Newton’s Laws of Motion. Previously, we have studied kinematics, which - describes the motion of an object (x, v, a) - does not explain the cause of the motion

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Chapter 5: Forces and Newton’s Laws of Motion

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  1. Chapter 5: Forces and Newton’s Laws of Motion • Previously, we have studied kinematics, which - describes the motion of an object (x, v, a) - does not explain the cause of the motion • Now, we begin the study of the second part of mechanics – dynamics - which does address the cause of motion - that cause is a force, a push or pull • Force, F, is a vector, has magnitude and direction • How forces affect the motion of an object is described by Newton’s Laws of Motion (Newtonian Mechanics) • Objects are treated as point particles; in Chapter 10 we will consider the shape of an object

  2. Newton’s First Law of Motion • An object at rest will remain at rest • An object moving at a constant velocity will continue to move at the constant velocity, unless acted upon by a netforce • What does it mean? - tendency for an object’s motion not to change • Net force = the sum of all applied forces F1 F3 F2 - No effect on the motion

  3. According to the 1st law, zero velocity (at rest) is equivalent to constant velocity • An object with a constant velocity does not require a force to maintain its velocity - forces act to change motion, not sustain (e.g., the space shuttle) - seems contrary to everyday experience • Inertia – tendency for an object to remain at rest, or to remain in motion with a constant velocity - all objects have inertia • Mass – a quantitative measure of inertia (a scalar) - use symbol m - unit is kg (SI) or slug (British) - more mass, means more inertia - not equivalent to weight (a force)

  4. Newton’s Second Law of Motion • If there is a net force, there is a change in velocity (an acceleration) • 1st law implies the 2nd law • Meaning: if a net external force acts on an object of mass m, it will be accelerated and the direction of the acceleration will be in the same direction as the net force F1 F F2 F3 a

  5. The Free Body Diagram (FBD) • A schematic representation of an object and all the external forces that act upon it • Always draw in every problem • From Newton’s 2nd law: Book at rest on the table

  6. Newton’s Third Law of Motion • The first two laws deal with a single object and the net forces applied to it - but not what is applying the force(s) • The third law deals with how two objects interact with each other • Whenever one object exerts a force on a second object, the second object exerts a force of the same magnitude, but opposite direction, on the first object

  7. Astronaut, ma Space station, ms Third law says: force astronaut applies to space station, Fs, must be equal, but opposite to force space station applies to astronaut, Fa FBD Since

  8. Fundamental Types of Forces • Gravitational • Electromagnetic – (electric and magnetic) • Weak Nuclear • Strong Nuclear Electroweak We will only consider the first two Gravitational Force From our studies of free-fall motion and projectile motion  gravity causes an object to accelerate in the negative y-direction

  9. y y m  Apply the second law • This is only an approximation which holds only near the surface of the Earth (as g is only constant near the surface). But a good approximation! • We would like a more fundamental description of gravity - g is an empirical number - physicists don’t like empirical numbers • This lead Newton to devise his Law of Universal Gravitation

  10. Chap. 12. Law of Universal Gravitation (12.1,12.2) • Every object in the Universe exerts an attractive force on all other objects • The force is directed along the line separating two objects • Because of the 3rd law, the force exerted by object 1 on 2, has the same magnitude, but opposite direction, as the force exerted on 2 by 1 m1 m2 By 3rd law r

  11. where • And G  Universal Gravitational Constant = 6.67259 x 10-11 N m2/kg2 • G is a constant everywhere in the Universe, therefore it is a fundamental constant • g is not a fundamental constant, but we can calculate it. Compare: and

  12. Let m1 = ME = mass of the Earth, m2 = m = mass of an object which is << ME, r = RE , object is at surface of the Earth, Set the forces equal to each other: m RE ME

  13. Weight  mass • Weight - the force exerted on an object by the Earth’s gravity F = mg = W • Mass is intrinsic to an object, weight is not • From previous page, W=m(GME/RE2) - your weight would be different on the moon • Gravity is a very weak force, need massive objects • Units of force F = ma [M][L/T2] in SI - kg m/s2 = Newton, N in BE - slug ft/s2 = pound, lb

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