# Forces & the Laws of Motion - PowerPoint PPT Presentation

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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

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 forceFapp

• WeightFg(mg)

• Normal forceFN┴ to surface

• FrictionFf

• Air resistanceFair

• TensionFtens

• Spring forceFspring

### 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 = 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

• 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

• When FR up = Fgdown

• Fnet = 0

### Four Fundamental Forces

• All are field forces

• Strong nuclear force

• Holds nucleus together

• Weak nuclear force

• Electromagnetic force

• Gravitational force

• weakest