Download Dynamics - Forces - hrsbstaff.ednet.ns.ca

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 à 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
force on it and inversely proportional to its mass"
Fnet
m
= m•a
a=
Fnet
-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:
"when one object exerts a force on a 2nd object, the 2nd object exerts a force on the 1st 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
-in a negative direction g = −9.81m / s
-minus sign means “down”
2
PDF created with pdfFactory Pro trial version www.pdffactory.com
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 = µFN
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,
FN
railway cars on incline railway tracks (like a tram car)
m1
F⊥
2) Incline Plane
m2
FII
θ
Fw
PDF created with pdfFactory Pro trial version www.pdffactory.com