Forces the laws of motion
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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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Forces & the Laws of Motion

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Forces the laws of motion

Forces & the Laws of Motion

Chapter 4


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


Example of a free body diagram

Example of a Free-Body Diagram


Common forces in force diagrams

Common Forces in Force Diagrams

  • Applied forceFapp

  • WeightFg(mg)

  • Normal forceFN┴ to surface

  • FrictionFf

  • Air resistanceFair

  • TensionFtens

  • Spring forceFspring


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 = mgW = mgFw = 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


Static vs kinetic friction

Static vs. Kinetic Friction


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


Coefficients of friction approximate

Coefficients of Friction(Approximate)


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


Frictional forces applied force

Frictional Forces & Applied Force


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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