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Dynamics Notes Part 1: Common Forces
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November 02, 2017
Mechanics
Unit 2: Dynamics (Chapters 4 & 5)
Predicting and describing an object's motion in terms of its displacement, velocity, and acceleration, are aspects of a branch of physics called Kinematics. This is what you were doing in Chapters 2 & 3.
The branch of physics that explains why objects move the way they do is called Dynamics.
Together, these two branches form the field of study called Mechanics.
Mechanics
Unit 2: Dynamics (Chapters 4 & 5)
Kinematics: HOW things moveaccelerations, displacements, velocities, & times...
Dynamics: WHY things moveForces that cause accelerations, displacements, velocities, & times...
Dynamics Notes Part 1: Common Forces
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November 02, 2017
Common Forces
Unit 2: Dynamics (Chapters 4 & 5)
The study of dynamics involves forces, which can be thought of as pushes and pulls on an object.
Forces cause objects to accelerate. F = ma Some common examples of forces you encounter on a daily basis are...• Earth's gravitational force keeping your feet on the ground• Sun's gravitational force holding the planets in orbit• Frictional force between blades & ice when you skate• Frictional force between you and air when you bike• Electromagnetic force pulling magnets together• Applied force from your car's engine moving you forward
Proportionality
Inertia & Proportionality
F = ma
Dynamics Notes Part 1: Common Forces
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November 02, 2017
Proportionality
Inertia & Proportionality
F = maWhat happens if you double the force on an object whose mass remains the same?
How big a force must you apply to an object to maintain its acceleration if its mass is doubled?
What happens if you reduce an object's mass by half but continue to apply the same force?
Units: Newtons
Units of ForceNewtons are a compound unit made up of basic units we've used before. Newtons were "created" to simplify what we write at the end of forces.
Can you figure out what Newtons are made of using the basic formula for a force? F = ma
Dynamics Notes Part 1: Common Forces
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November 02, 2017
Mass
Mass vs Weight
Be careful not to mix up MASS and WEIGHT.• Mass depends on the number of particles of which an object is made, and nothing else.• Weight is a measure of the force of gravity acting on an object. It depends on the object's mass and its position/motion in the universe.
Weight
Mass vs Weight
All we need to calculate an object's weight (Fg) is its mass in kg and the acceleration due to gravity:
Fg = mag
Remember weight is the force with which gravity pulls on a mass, so the symbol Fg is used.
Dynamics Notes Part 1: Common Forces
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November 02, 2017
Inertia
Inertia
Forces sometimes cause objects to move and sometimes they resist the object's movement.
Inertia is the natural tendency of an object to remain in its current state of motion in the absence of unbalanced forces. An object's inertia directly relates to its mass.
Inertia
Inertia
An easy way to remember Inertia is to say:
When all forces are balanced, an object at rest stays at rest
and an object in motion stays in motion.
Dynamics Notes Part 1: Common Forces
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November 02, 2017
Weight & Inertia
Mass vs Weight
Find the weight of the following objects on Earth's surface, then indicate which has more inertia:
Object 1: A cannon ball with mass 150kg
Object 2: A box containing 23kg of books
p. 137 #14
Practice Problems: p.137 #1‑4
Dynamics Notes Part 1: Common Forces
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November 02, 2017
p. 137 #14
Fg & FN
Gravitational & Normal Forces Fg = magAny object with mass, has weight (Fg).
FN : NormalAny object in contact with a surface has a Normal Force (FN). It's the force that keeps the object from falling through that surface.
Fg
FN
Fg
FN
Horizontal surfaces: Fg = ‑ FN (same #, opposite direction) so long as the object isn't accelerating up or down.
Sloped surfaces: Use Trig(Physics 122/121)
Dynamics Notes Part 1: Common Forces
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November 02, 2017
Friction
Ff• Friction is a contact force, and...
Two types of friction:• Static frictional force – exists when you start to move an object from rest• Kinetic frictional force – exists while the object is moving
• Static friction > kinetic friction (figure 4.6 page 138)
Mythbusters: Phonebook Friction Driving up an icy hill
• inhibits relative motion between objects in contact• strength depends on the 2 surfaces in contact. All surfaces create some friction (no such thing as a smooth surface)• is an electromagnetic force acting between the surface atoms of one object and those of another
Coeff of Friction
Frictional force depends on two things:1. Type of atoms and molecules making up the materials passing over each other results in the coefficient of friction (µ) ‑ the "stick factor"
2. The magnitude of the forces that are pressing the two surfaces together can be represented by the normal force (FN), which is perpendicular to the surface of contact. For horizontal surfaces, FN is equal and opposite to weight.
• Surprisingly friction is independent of velocity and area of contact (surface area) ‑ three assumptions are necessary for this to be true (details on p. 141)
Dynamics Notes Part 1: Common Forces
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November 02, 2017
Static & Kinetic Friction
Types of Friction• When two surfaces are at rest, the surface atoms interact to form relatively strong attractive forces. A push on one object creates a static frictional force that pushes back with exactly the same magnitude of force. If you continue to increase the applied force, you will eventually be able to break those attractive forces between the surface atoms.
FA Ff
v = 0 m/sa = 0 m/s2
FA Ff
v = # m/sa = # m/s2
• Once an object is moving, static friction is no longer an issue. New bonds are continually being formed and broken as one object slides over another. Kinetic friction is the force that opposes the motion of moving objects.
FA Ff
v = # m/sa = 0 m/s2
FA Ff FA Ff FA Ff
Static & Kinetic Friction
Types of Friction
• Which frictional force would you expect to be larger?
• How do these concepts relate to the concept of inertia?
Dynamics Notes Part 1: Common Forces
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November 02, 2017
Friction is Fun
uni
Static & kinetic friction example Why is Static Friction > Kinetic Friction?
Symbol Variable UnitFf force of friction (N)μ coefficient of friction (none)FN normal force (N)
Ff = μFN
"mew"
Example p. 141 #1
Sample Problem p. 141 #1During the winter, owners of pickup trucks often place sandbags in the rear of their vehicles. Calculate the increased static force of friction between the rubber tires and wet concrete resulting from the addition of 200kg of sandbags in the back of the truck (µs = 0.70).
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November 02, 2017
p. 144 #8
Free Body Diagrams
Free Body Diagrams (FBD)(Your BFF in Dynamics)
F
F F
F
A VERY simplified sketch of the scenario described in the question that shows the forces involved, including their direction.
You must include FBDs as part of the "Analysis/Artwork" portion of GRASS for this entire unit.