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Forces Chapter 6 Pages: 116-147

Force A force is a push or pull upon an object resulting from the object's interaction with another object. Contact Forces Contact Forces Long-Range Forces Long-Range Forces

Contact Forces Contact forces are types of forces in which the two interacting objects are physically in contact with each other.

Friction

Tension

Air Resistance

Long-Range Forces Long-Range Forces are types of forces in which the two interacting objects are not in physical contact with each other, but are able to exert a push or pull despite the physical separation.

Gravity

Magnetism

Electrical

Force

Force F for use in equations. Newton is the unit for Force. N abbreviation for Newton. Net Force can accelerate. N = kg m/s 2

Force is a Vector Quantity Magnitude Magnitude Direction Direction 4000lb

Read Pages 118-119 Answer Question 1 in Notebook

Homework Page: 124 Questions: 7-11

Free Body Diagrams The purpose of a free-body force diagram is to assist you in trying to determine the net force acting on a body.

Net Force The purpose of a free-body force diagram is to assist you in trying to determine the net force acting on a body.

Free Body Diagrams The net force is the vector sum of all the individual forces acting on a system.vector sumforces F net = F 1 ± F 2 ± F 3 ± F 4 …

Constructing “free-body force diagram” 1. Identify the object(s) you will draw a diagram for. 1. Identify the object(s) you will draw a diagram for.

Constructing “free-body force diagram” 2. Identify all the forces acting directly on the object and the object exerting them. Gravity Gravity Table Table

Constructing “free-body force diagram” 3.Draw a dot to represent the object of interest.

Constructing “free-body force diagram” 4. Draw a vector to represent each force. Gravity Table

Constructing “free-body force diagram” 5. If the object is stationary or is moving at a constant velocity, the vectors should graphically add up to zero. 5. If the object is stationary or is moving at a constant velocity, the vectors should graphically add up to zero.

Constructing “free-body force diagram” 5. If the object is accelerating, the sum of the vectors should produce a vector in the same direction as the acceleration.

Constructing “free-body force diagram” Gravity Floor Standing on Floor F floor =F gravity

Constructing “free-body force diagram” Gravity Muscle Jumping F muscle >F gravity

Constructing “free-body force diagram” Gravity In the Air F gravity

Types of Motion No Motion No Motion Gravity Road Brakes Motor F Road =F Gravity F Motor =F Brakes No Net Force

Types of Motion Constant Velocity Constant Velocity Gravity Road Friction Motor F Road =F Gravity F Motor =F Friction No Net Force

Types of Motion Speeding Up Speeding Up Gravity Road Friction Motor F Road =F Gravity F Motor >F Friction Net Force

Types of Motion Slowing Down Slowing Down Gravity Road Friction Motor F Road =F Gravity F Motor <F Friction Net Force

Finding Net Force F G =4000N F R =4000N F F =400N F M =400N Vertical F net = F R - F G F net =4000N–4000N F net = 0N Horizontal F net = F F - F M F net =400N–400N F net = 0N

Finding Net Force F G =4000N F R =4000N F F =400N F M =400N Not Moving Or Constant Velocity

Finding Net Force F G =4000N F R =4000N F F =100N F M =400N Vertical F net = F R - F G F net =4000N–4000N F net = 0N Horizontal F net = F F - F M F net =100N–400N F net = -300N

Finding Net Force F G =4000N F R =4000N F F =100N F M =400N Accelerating to the Left.

Newton’s Second Law of Motion LawofAcceleration

The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.

Newton’s Second Law of Motion F f = 40N F P = 41N F Net = 1N Accelerates

Newton’s Second Law of Motion F Net = ma F m a

Newton’s First Law of Motion LawofInertia

Inertia Inertia is the resistance an object has to a change in its state of motion. Mass

Newton’s First Law of Motion

Seat Belt!!!!

Newton’s First Law of Motion An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

Mass vs. Weight Mass is the amount of stuff you are made up of. (kg or slugs) Does not change!!!!

Mass vs. Weight Weight depends on how much gravity is acting on you at the moment; you'd weigh less on the moon than on Earth. (newtons or pounds)

Mass vs. Weight Weight

Mass

Weight To calculate weight use the acceleration due to gravity (9.8m/s 2 ). This will be called g. F=ma F g =mg

Weight Problems Mr. Clune has a mass of 110kg. How much does he weight? Given: m=110kg g=9.8m/s 2 Find: F g =? Equation: F g =mg =(110kg)(9.8m/s 2 ) =(110kg)(9.8m/s 2 ) F g =1078N

F=ma Problems A boy pulls a sled that has a mass of 5kg across the snow. The sled accelerates at a rate of 0.5m/s 2. What is the net force of on the sled? a=0.5m/s 2 F net

Equation: F net =mg =(5kg)(0.5m/s 2 ) =(5kg)(0.5m/s 2 ) Given: m=5kg a=0.5m/s 2 Find: F net =? F net =2.5N F net =2.5N

F=ma Problems A rock with a mass of 10kg fell off a cliff. At a specific time during its’ fall it had an acceleration of 3m/s 2, due to air resistance. What is the force of air on this rock at this time?

+ FgFgFgFg F air a F net F net = F air + F g F net = ma F g = mg

F air = F net - F g F air = ma - mg F air = m(a – g) F air = 10kg{(-3m/s 2 )–(-9.8m/s 2 )} F air = 10kg{(-3m/s 2 )+(9.8m/s 2 )} F air = 10kg(6.8m/s 2 ) F air = 68N

Homework Page: 147 Questions: 22, 27,29 Due: 10/25/06

Factors that determine Friction Weight Moving Stationary Surface

Friction Forces FgFg FTFT FPFP FfFf FNFN

F N – Normal Force: This force which will affect frictional resistance is the component of applied force which acts perpendicular or "normal" to the surfaces which are in contact and is typically referred to as the normal force. F N – Normal Force: This force which will affect frictional resistance is the component of applied force which acts perpendicular or "normal" to the surfaces which are in contact and is typically referred to as the normal force. Friction Forces

F T – Surface Force: This force opposite the normal force which is equal to this force. F T – Surface Force: This force opposite the normal force which is equal to this force. Friction Forces

F P – Push or Pull Force: This force is pushing or pulling the object. F P – Push or Pull Force: This force is pushing or pulling the object. Friction Forces

F f – Friction Force: Frictional resistance to the relative motion of two solid objects. F f – Friction Force: Frictional resistance to the relative motion of two solid objects. Friction Forces

F fs – Static Friction Force: Static frictional forces are non- moving forces between two surfaces. It will increase to prevent any relative motion up until some limit where motion occurs. F fs – Static Friction Force: Static frictional forces are non- moving forces between two surfaces. It will increase to prevent any relative motion up until some limit where motion occurs. Friction Forces

F fk – Kinetic Friction Force: The force between two surfaces that are moving with respect to one another, the frictional resistance is almost constant over a wide range of low speeds. F fk – Kinetic Friction Force: The force between two surfaces that are moving with respect to one another, the frictional resistance is almost constant over a wide range of low speeds. Friction Forces

μ – Coefficient of Friction: The ratio of the force of friction (F f ) between two bodies and the force pressing them together (F N ). μ – Coefficient of Friction: The ratio of the force of friction (F f ) between two bodies and the force pressing them together (F N ). Friction Forces

Coefficient of Friction FfFf FNFN μs=μs= F fs FNFN

Coefficient of Friction FfFf FNFN μk=μk= F fk FNFN

Friction Problem A refrigerator of total weight 400N is pushed at a constant speed across a room by pushing horizontally on one side with a force of 160N. What is the coefficient of kinetic friction? F fk FNFN F fk = 160N F N = 400N μ k = ?

μk=μk= F fk FNFN μk=μk= 140N 400N μ k = 0.35

If the coefficient of static friction between the floor and the refrigerator was 0.6, how much force would be needed to start the refrigerator moving? F fs = ? F N = 400N μ s = 0.6 μs=μs= F fs FNFN = μ k F fs FNFN

= (0.6) F fs (400N) = μ k F fs FNFN = 240N F fs

Homework Page: 133 Questions: 14,15 Page: 145 Questions: 33-35 Due: 11/2/06

Newton’s Third Law "For every action, there is an equal and opposite reaction."

While driving, Anna Litical observed a bug striking the windshield of her car. Obviously, a case of Newton's third law of motion. The bug hit the windshield and the windshield hit the bug. Which of the two forces is greater: the force on the bug or the force on the windshield?

Rockets are unable to accelerate in space because... There is no air in space for the rockets to push off of. There is no gravity is in space. There is no air resistance in space.... nonsense! Rockets do accelerate in space.

A gun recoils when it is fired. The recoil is the result of action-reaction force pairs. As the gases from the gunpowder explosion expand, the gun pushes the bullet forwards and the bullet pushes the gun backwards. The acceleration of the recoiling gun is... a.greater than the acceleration of the bullet. b.smaller than the acceleration of the bullet. c.the same size as the acceleration of the bullet.

In the top picture, a physics student is pulling upon a rope which is attached to a wall. In the bottom picture, the physics student is pulling upon a rope which is held by the Strongman. In each case, the force scale reads 500 Newtons. The physics student is pulling…

with more force when the rope is attached to the wall. with more force when the rope is attached to the Strongman. the same force in each case.

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