"Dynamics - Forces"
Dynamics - Forces Definition: Dynamics - the study of why objects move -causes of acceleration were first studied by Sir Isaac Newton & he developed Newton's Laws of Motion Definition: Force -a push or a pull -an agent that results in accelerating or deforming an object 4 Types of Forces: 1. Gravitational Force -an attractive force that exists between objects 2. Electromagnetic Force -force due to electric charges, both static & moving 3. Strong Nuclear Force -force that holds the particles in the nucleus together -stronger than electromagnetic force -acts over distances the size of a nucleus 4. Weak Nuclear Force -form of electromagnetic force -involved in radioactive decay of certain elements -physicists try to form GUT (Grand Unified Theory) of all forces (maybe String Theory?) ***All forces are vectors have both magnitude & direction Newton's 1st Law of Motion (Law of Inertia): "an object with no force acting on it remains at rest or moves with a constant velocity in a straight line" -objects at rest tend to stay at rest ("rest" is a special case of v = 0 m/s) Newton's 2nd Law of Motion: "the acceleration of a body is directly proportional to the net Fnet a force on it and inversely proportional to its mass" m Fnet ma -as force increases, rate of velocity increases, therefore acceleration increases -acceleration depends on mass -as mass increases, acceleration increases (if net force is constant) -acceleration is always in the same direction as the net force causing it Definition: Inertia -tendency of an object not to change its motion -mass is a measure of inertia -unit of Force: F m a (kg)(m / s 2 ) Newton or N -method for finding net force ---> vector sum of all forces, keeping tracks of signs Newton's 3rd Law: st "when one object exerts a force on a 2nd object, the 2nd object exerts a force on the 1 that is equal in magnitude and opposite in direction" -action-reaction forces *Remember, net force and action-reaction forces are not the same thing! Mass & Weight: Weight is defined as... Fw m g 2 -in a negative direction g 9.81m / s -minus sign means “down” Two Kinds of Mass: Definition: Inertial Mass -the ratio of net force on an object and its acceleration Fnet m a Definition: Gravitational Mass -ratio of gravitational force to an object's acceleration Fnet m g -both "masses" are valid ways of describing mass Friction: Definition: Friction (Ff)-force that opposes motion between two surfaces that are in contact Definition: Static Friction -force that opposes the start of motion Definition: Sliding (Kinetic) Friction -force between surfaces in relative motion -sliding friction < static friction -to keep an object moving with constant velocity, one must apply a force equal & opposite to force of friction -friction depends on: force pushing the surfaces together (FN or 'normal' force) AND nature of contact surfaces ("µ"---> mu stands for coefficient of friction) Ff FN Problem-Solving Strategy for Problems Involving More Than One Force: 1. Always draw a picture of object. 2. Draw arrows representing all forces acting on object. (FBFD) 3. Label each force with its cause. Be specific. The Fall of Bodies in the Air: Definition: Air Resistance -force of air on objects moving through it -a.k.a. drag force -a friction-like force -depends on: size and shape of object, density of air, speed of motion Ex.: dropping a ping-pong ball -as v increases, drag force increases; after time, drag force = Fw (weight of ping-pong ball) -net force on ball is 0 N, no acceleration and velocity becomes constant ---> terminal velocity Definition: Terminal Velocity -velocity of a falling object reached when force of air resistance equals Fw (weight). _________________________________________________________________________________________________ Two common types of problems associated with dynamics: 1) Atwood’s Machine – invented in 1784 by George Atwood to analyze uniform accelerated motion -consists of two masses, m1 and m2, connected by an inelastic massless string over an ideal massless pulley -when m1 = m2, system is stationary (regardless of position of masses) -practical applications of Atwood’s Machine include counterbalance in elevator that relieves the motor from the load of holding the elevator car, railway cars on incline railway tracks (like a tram car) FN m1 F 2) Incline Plane m2 FII Fw