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Forces & the Laws of Motion. Chapter 4. 4.1 Changes in Motion. Objectives : Explain how force affects the motion of an object Distinguish between contact forces and field forces Interpret and construct free-body diagrams. Force. What is a force?

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4 1 changes in motion
4.1 Changes in Motion
  • Objectives:
  • Explain how force affects the motion of an object
  • Distinguish between contact forces and field forces
  • Interpret and construct free-body diagrams
force
Force
  • What is a force?
  • A push or pull that can change the motion of an object
  • SI unit is the newton (N)
  • One newton is the force required to accelerate a 1-kg mass at 1 m/s2
  • 1N = 1 kg·m/s2 1N = 0.225 lbf
  • 1lbf = 4.448 N
forces act through contact or at a distance
Forces act through contact or at a distance
  • Contact forces:
  • Forces that affect an object through physical contact with another object
  • Example: a baseball bat hitting a baseball
  • Field forces:
  • Forces that affect an object without physical contact
  • Examples: gravitational, magnetic, and electrostatic forces
field theory
Field Theory
  • Explains how forces can affect an object without physical contact
  • Explanation of field forces…
  • An object affects the space surrounding it so that a force is exerted on other objects in that space.
  • The “field” is the region of space in which the force is exerted
  • Example: magnetic field
electrostatic forces
Electrostatic Forces
  • Example of a field force
  • Stream of ethanol is attracted to an electrically charged probe
force diagrams
Force Diagrams
  • Force is a vector
  • Force diagrams:
    • Diagram the objects involved in a situation and the forces acting on the objects
  • Free-body diagrams:
    • Diagram the forces acting on a single object
    • i.e. diagram the object “free” from influence of other objects and their forces
representing forces
Representing Forces
  • Force is a vector
  • Free-body diagrams illustrate forces acting on an object isolated from its surroundings
free body diagrams
Free-body Diagrams
  • Free-body diagrams are diagrams used to show the relative magnitude and direction of all forces acting upon an object in a given situation
  • Represent object as a box with forces originating from center of box
  • Types of forces: Fapp, Fg, Ff, FT, FN
common forces in force diagrams
Common Forces in Force Diagrams
  • Applied force Fapp
  • Weight Fg (mg)
  • Normal force FN ┴ to surface
  • Friction Ff
  • Air resistance Fair
  • Tension Ftens
  • Spring force Fspring
4 2 newton s first law law of inertia
4.2 Newton’s First Law:Law of Inertia
  • Galileo noted that things tend to slide further on smoother surfaces
  • Concluded that an object would slide forever on a perfectly smooth surface in the absence of any applied force
  • This led to Newton’s First Law of Motion
newton s first law of motion
Newton’s First Law of Motion
  • An object at rest remains at rest, and an object in motion continues in motion in a straight line, with a constant velocity, unless acted upon by a net external force
  • Inertia: the tendency of an object to maintain its state of uniform linear motion
  • When net force on an object is zero, acceleration is zero (∆v/∆t= 0)
newton s first law of motion1
Newton’s First Law of Motion
  • An object at rest remains at rest, and an object in motion continues in motion with a constant velocity unless acted upon by a net external force
  • A net force is required to change the state of motion of an object
  • Net external force
    • Resultant force produced from combination of all forces acting on an object
net force
Net Force
  • A net force is the resultant force of two or more forces
  • Since forces are vectors, the net (resultant) force is determined as any other resultant vector.
  • Example: A student pushes a book across a table with a force of 5 N
net force1
Net Force
  • Example: A student pushes a book across a table with a force of 5 N. Frictional forces of 2 N act in the opposite direction. What is the net force acting on the book?
forces acting on inclined planes
Forces Acting on Inclined Planes
  • FN, normal force, surface acting on object
  • Fg, weight = mg
  • Fgx, component of g, ║ to surface
  • Fgy, component of g ┴ surface
  • Ff, friction
inertia
Inertia
  • Inertia is tendency of an object to maintain its state of motion unless acted upon by a net force
  • Mass is a measurement of inertia
  • ↑ mass → ↑ inertia
  • As the same speed, a rolling car is more difficult to stop than a rolling basketball
equilibrium
Equilibrium
  • The state of a body in which there is no change in motion
  • Net force acting on a body is zero
4 3 newton s 2 nd 3 rd laws learning objectives
4.3 Newton’s 2nd & 3rd LawsLearning objectives
  • Describe acceleration of an object in terms of its mass and the net external force acting on it
  • Predict direction & magnitude of acceleration caused by a known net external force
  • Identify action-reaction force pairs
  • Explain why action-reaction pairs do not result in equilibrium
newtons 2 nd law
Newtons 2nd Law
  • The acceleration of an object is directly proportional to the net external force acting on the object and inversely proportional to the mass of the object
  • a = ΣF /m , where Σ means “sum of”
  • ΣF = ma
conceptual question
Conceptual Question

A grain truck filled with soy beans accelerates along the highway at 0.50 m/s2. If the driving force on the truck remains the same, what happens to the acceleration of the truck if soybeans leak from it at a constant rate?

Answer: The loss of soy beans is a decrease in mass. Since a = ΣFnet /m , acceleration increases.

newton s 3 rd law
Newton’s 3rd Law
  • "For every action, there is an equal and opposite reaction." equal magnitude and opposite direction
  • In every interaction, there is a pair of forces acting on the two interacting objects.
  • Action-reaction force pairs: equal in magnitude, but opposite in direction.
action reaction force pairs
Action-Reaction Force Pairs
  • Since force pairs are equal in magnitude, but opposite in direction, why do they not result in equilibrium?
  • Because they act on different objects.
  • If equal but opposite forces acted on the same object, there would be equilibrium, i.e. no net force.
4 4 everyday forces
4.4 Everyday Forces
  • Weight

Force of gravity acting on a mass

Fg = mg W = mg Fw = mg

  • Normal Force

contact force exerted by one object on another in a direction ┴ surface of contact

  • Friction

contact force that opposes motion….

opposes applied force

weight normal force
Weight & Normal Force
  • Fg = mg
  • Always ┴surface of earth
  • Directed toward center of earth
  • FN = Fgcos (θ)
  • Always ┴surface of contact
  • Always opposes Fg
identify forces acting on inclined planes
Identify Forces Acting on Inclined Planes
  • FN, normal force, surface acting on object
  • Fg, weight = mg
  • Fgx, component of g, ║ to surface
  • Fgy, component of g ┴ surface
  • Ff, friction
force of friction
Force of Friction
  • Ff opposes applied force
  • Static friction Ffs ….

force exerted by environment on motionless body to resist applied force

  • Kinetic friction Ffk ….

force exerted by environment on moving object to resist applied force

  • Ffs > Ffk
  • Depends on surfaces in contact….

Types and smoothness

  • Proportional to FN
relationship of f f and f n
Relationship of Ff and Fn
  • Ff is proportional to FN
  • Proportionality constant is the coefficient of friction, μ
  • μ = Ff/ FN
  • Depends on types of surfaces in contact
  • Depends on static or kinetic friction

μs = Fs / FN μk = Fk/ FN

problem 4d
Problem 4D
  • A crate of mass 24 kg is set in motion on a horizontal surface with a horizontal force of 75 N. Find the coefficient of static friction, μs
  • μs = Fs / FN
  • = Fs / mg
  • = 75 N / (24 kg x 9.81 m/s2)
  • = 0.32
role of surface in friction
Role of Surface in Friction
  • Static friction increases with increasing force until overcome
  • Kinetic friction is less than the maximum static friction
air resistance
Air Resistance
  • When an object passes through a fluid….
  • The fluid has to be pushed out of the way for the object to pass through it
  • i.e., Motion of objects through a fluid is hindered by the fluid
air resistance1
Air Resistance
  • At low speeds FR is proportional to v
  • At higher speeds FR is proportional to v2
  • When FR = FA, constant speed
  • Terminal speed
    • For free falling object
    • When FR up = Fgdown
    • Fnet = 0
four fundamental forces
Four Fundamental Forces
  • All are field forces
  • Strong nuclear force
    • Holds nucleus together
  • Weak nuclear force
    • Involved in radioactive decay
  • Electromagnetic force
  • Gravitational force
    • weakest
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