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Forces Ch. 6. Milbank High School. Sec 6.1 Force and Motion. Objectives Define a force and differentiate between contact forces and long-range forces Recognize the significance of Newton’s second law of motion and use it to solve motion problems

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Forces Ch. 6

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forces ch 6

ForcesCh. 6

Milbank High School

sec 6 1 force and motion
Sec 6.1Force and Motion
  • Objectives
    • Define a force and differentiate between contact forces and long-range forces
    • Recognize the significance of Newton’s second law of motion and use it to solve motion problems
    • Explain the meaning of Newton’s first law and describe an object in equilibrium
the forces of nature

I. Gravitational:

  • attraction bet. masses
  • tides, gravity, weight
  • III. Weak Nuclear:
  • helps to explain atomic collisions
  • II. Electromagnetic:
  • friction •tension
  • adhesion •lift
  • electrostatic •drag
  • buoyant •magnetic
  • IV. Strong Nuclear:
  • binds atomic nuclei
The Forces of Nature
  • How do forces influence motion?
  • Force– a push or pull exerted on an object having magnitude and direction
  • System—object that experiences the force
  • Environment—world around the system that exerts the force
two categories of forces
Two Categories of Forces…
  • Contact Force
    • Acts on an object only by touching it
  • Long-Range Force
    • Exerted without contact
      • Magnets
      • Gravity

Agent: a specific, identifiable, immediate cause of a force

types of forces
Types of Forces
  • Ff - - Friction (opposes sliding)
  • FN - - Normal (surface)
  • Fsp - - Spring (push or pull of a spring)
  • FT - - Tension (spring, rope, cable)
  • Fthrust - - Thrust (rockets, planes, cars)
  • Fg - - Weight (force due to gravity)
representing forces
Representing Forces...
  • Forces are vectors
    • Forces are drawn as arrows (vectors)
    • forces add like vectors.
    • the sum of all the forces is called the net force.
  • A picture of a body with arrows drawn representing all the forces acting upon it is called a FREE BODY DIAGRAM.
try it
Try it...

Draw a picture of your book sitting on the desk. Identify all the forces acting on it.

free body diagrams10
Free Body Diagrams...

T (table)


W (weight)


Free Body Diagrams...

What forces are acting on a

skier as she races down a hill?


The Answer...


f and d


draw free body diagrams for the following
Draw free body diagrams for the following
  • An egg is free-falling from a nest in a tree. Neglect air resistance.
  • A skydiver is descending with a constant velocity. Consider air resistance.
  • A car is coasting to the right and slowing down.
newton s second law
Newton’s Second Law
  • F = ma
  • a = Fnet / m
  • Expressed in Newtons (N)
    • Force required to give 1kg mass a 1m/s2 acceleration
  • A race car has a mass of 710 kg. It starts from rest and travels 40.0 m in 3.0 s. The car is uniformly accelerated during the entire time. What net force is exerted on it?
newton s first law of motion
Newton’s First Law of Motion
  • “An object that is at rest will remain at rest or an object that is moving will continue to move in a straight line with constant speed, if and only if the net force acting on that object is zero.”
newton s first con t
Newton’s First Con’t
  • Inertia—the tendency of an object to resist change.
  • Equilibrium—object at rest or moving at a constant velocity
finally misconceptions about forces
Finally…Misconceptions about forces
  • When a ball has been throw, the force of the hand that threw it remains on it.
  • A force is needed to keep an object moving
  • Inertia is a force
  • Air does not exert a force
  • The quantity ma is a force
sec 6 2 using newton s laws
Sec. 6.2Using Newton’s Laws
  • Objectives
    • Describe how the weight and the mass of an object are related
    • Differentiate between the gravitational force weight and what is experienced as apparent weight
    • Define the friction force and distinguish between static and kinetic friction
    • Describe simple harmonic motion and explain how the acceleration due to gravity influences such motion.
mass and weight
Mass and Weight
  • The weight force, Fg , is used to find the downward force of an object.
  • Both the net force and acceleration are downward.

Fg = mg

example problems
Example Problems
  • Pg. 128
  • Practice Problem 12.
    • Pg. 129
  • Static friction force
    • The force that opposes the start of relative motion between the two surfaces in contact
      • Friction force with object isn’t in motion
  • Kinetic Friction Force
    • The force that opposes relative motion between surfaces in contact
      • Friction force when object is in motion
calculating friction
Calculating Friction

Kinetic Friction Force

Ff ,kinetic = µkFn

Static Friction Force

0< Ff, static < µsFN

typical coefficients of friction
Typical Coefficients of Friction

Surface µs µk

Rubber on concrete 0.80 0.65

Rubber on wet concrete 0.60 0.40

Wood on wood 0.50 0.20

Steel on steel (dry) 0.78 0.58

Steet on steel (with oil) 0.15 0.06

Teflon on steel 0.04 0.04

example problems26
Example Problems
  • Pg. 131-133
    • Balanced Friction Forces
    • Unbalanced Friction Forces
terminal velocity
Terminal Velocity
  • The constant velocity that is reached when the drag force equals the force of gravity
  • Objects can only fall so fast due to their size and shape and density of the air/fluid
    • Ping-pong ball – 9 m/s
    • Basketball – 20 m/s
    • Baseball – 42 m/s
    • Skydiver: >62 m/s w/o chute

5 m/s w/ chute

periodic motion
Periodic Motion
  • Pendulums, springs, strings
  • Simple Harmonic Motion
    • Motion that returns an object to its equilibrium position as a result of a restoring force that is directly proportional to the object’s displacement
  • Period (T)
    • Time needed to repeat one complete cycle of motion
  • Amplitude
    • Maximum distance the object moves from equilibrium
amplitude frequency period
Amplitude, Frequency, Period

The Amplitude is the displacement.

The Frequencyis the number of cycles/sec.

The Period is the time for one cycle T = 1/f

  • Pg. 136
  • 17-19
sec 6 3 interaction forces
Sec. 6.3Interaction Forces
  • Objectives
    • Explain the meaning of interaction pairs of forces and how they are related by Newton’s third law
    • List the four fundamental forces and illustrate the environment in which each can be observed.
    • Explain the tension in ropes and strings in terms of Newton’s third law
interaction forces
Interaction forces
  • Two forces that are in opposite directions and have equal magnitude
  • Newton’s Third Law—all forces come in pairs
  • FA on B = -FB on A