Forces & the Laws of Motion

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Forces & the Laws of Motion - PowerPoint PPT Presentation

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

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?
• 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
• 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
• 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
• Example of a field force
• Stream of ethanol is attracted to an electrically charged probe
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
• Force is a vector
• Free-body diagrams illustrate forces acting on an object isolated from its surroundings
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
• 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
• 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
• 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 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
• 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 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
• 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 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
• 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 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 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

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 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
• 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
• 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
• 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
• 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
• 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 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
• 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
• Static friction increases with increasing force until overcome
• Kinetic friction is less than the maximum static friction
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 Resistance
• At low speeds FR is proportional to v
• At higher speeds FR is proportional to v2
• When FR = FA, constant speed
• Terminal speed