Newton's First Law • Newton's first law of motion: An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
The Meaning of Force • A force is a push or pull upon an object resulting from the object's interactionwith another object. • Force is a quantity that is measured using the standard metric unit known as the Newton.
All forces (interactions) between objects can be placed into two broad categories • Contact forces - that result when the two interacting objects are perceived to be physically touching each other. • Field forces - that result even when the two interacting objects are not in physical contact with each other, yet are able to exert a push or pull despite their physical separation.
Gravity Force (Weight) Fgrav • The force of gravity is the force with which the earth, moon, or other massively large object attracts another object towards itself. By definition, this is the weight of the object. All objects upon earth experience a force of gravity that is directed "downward" towards the center of the earth. The force of gravity on earth is always equal to the weight of the object as found by the equation: • Fgrav = m • g • where g = 9.81 N/kg (on Earth) and m = mass (in kg) • Note: g is different at different locations
Practice- indicate Fg on each box with an arrow Fg Fg Fg Fg Fg Fg
Comparing Mass and Weight Mass • The mass of an object refers to the amount of matter that is contained by the object; • Scalar, has no direction • The mass of an object (measured in kg) will be the same no matter where in the universe that object is located. Weight • The force of gravity. • Vector, its direction is downward. • W = mg • The weight of an object (measured in Newton) will vary according to where in the universe the object is.
Normal Force (FN ) • The normal force is the support force exerted upon an object that is in contact with another stable object (usually a surface). The direction of the normal force is perpendicular to the surface, from the surface toward the object and on the object.
Practice- indicate FN on each box with an arrow FN FN FN Fg Fg Fg FN FN FN Fg Fg Fg
Friction Force (Ff) • The friction force is the force exerted by a surface as an object moves across itor makes an effort to move across it. The friction force often opposes the motion of an object. • Friction results from the two surfaces being pressed together closely, causing intermolecular attractive forces between molecules of different surfaces. Friction depends upon the nature of the two surfaces and upon the degree to which they are pressed together. Ff = μFN
Practice- indicate Ff on each box with an arrow v FN FN FN Ff Ff v Ff v Fg Fg Fg Ff FN Ff Ff Ff FN FN v v v Fg Fg Fg
Air Resistance Force (Fair ) • The air resistance is a special type of frictional force that acts upon objects as they travel through the air. The force of air resistance is often observed to oppose the motion of an object. This force will frequently be neglected due to its negligible magnitude.
Tension Force (FT ) • The tension force is the force that is transmitted through a string, rope, cable or wire when it is pulled tight by forces acting from opposite ends. The tension force is directed along the length of the wire and pulls equally on the objects on the opposite ends of the wire.
Spring Force (Fspring ) • The spring force is the force exerted by a compressed or stretched spring upon any object that is attached to it. An object that compresses or stretches a spring is always acted upon by a force that restores the object to its rest or equilibrium position – directed toward equilibrium position.
Balanced and Unbalanced Forces If two individual forces are of equal magnitude and opposite direction, then the forces are said to be balanced. When only balanced forces act on an object, the object is said to be at equilibrium. Unbalanced forces
State of Motion • The state of motion of an object is defined by its velocity - the speed with a direction. • Inertia: tendency of an object to resist changes in its velocity. • Inertia: tendency of an object to resist accelerations.
Newton’s First Law Also known as the “Law of Inertia” Inertia Tendency of an object tomaintain its STATE OF MOTION Forces Don't Keep Objects Moving
Blood rushes from your head to your feet while quickly stopping when riding on a descending elevator. • The head of a hammer can be tightened onto the wooden handle by banging the bottom of the handle against a hard surface. • A brick is painlessly broken over the hand of a physics teacher by slamming it with a hammer. (CAUTION: do not attempt this at home!) • To dislodge ketchup from the bottom of a ketchup bottle, it is often turned upside down and thrusted downward at high speeds and then abruptly halted. • Headrests are placed in cars to prevent whiplash injuries during rear-end collisions. • While riding a skateboard (or wagon or bicycle), you fly forward off the board when hitting a curb or rock or other object that abruptly halts the motion of the skateboard.
Inertia is proportional to MASS Do these guys have a lot of inertia? LOTS OF INERTIA hard to… GET MOVING or STOP MORE MASS means MORE INERTIA
Drawing Free-Body Diagrams • Free-body diagrams are used to show the relative magnitude and direction of all forces acting upon an object in a given situation. • The size of the arrow in a free-body diagram reflects the magnitude of the force. The arrow shows the direction that the force is acting. • Each force arrow in the diagram is labeled to indicate the exact type of force. • It is generally customary to draw the force arrow from the center of the box outward in the direction that the force is acting.
A block of wood is sitting motionless on a table. What forces are acting on it? Normal Force is a REACTION force that any object exerts when pushed on Normal FN Weight is the force of gravity pulling an object toward the CENTER OF THE EARTH Fg Weight
practice • A book is at rest on a tabletop. Diagram the forces acting on the book. FN Fg
30 N 40 N Determining the Net Force • The net force is the vector sum of all the forces that act upon an object. A B C 400 N up 200 N down 20 N left R2 = (30N)2 + (40N)2 θ = tan-1(30/40) = 53.1o Net force is 50 N at 53.1o West of North
Net Force If there is NONET FORCE on an object, then it is at EQUILIBRIUM and either: MOTIONLESS OR MOVING WITH CONSTANT VELOCITY So a “net” or “unbalanced” force will CHANGE AN OBJECT’S VELOCITY Changing velocity means ACCELERATION
A net force (an unbalanced force) causes an acceleration yes yes no no yes yes
Force Acceleration How much acceleration? Depends on: AMOUNT OF FORCE MORE FORCE = MORE ACCELERATION Acceleration is directly related to force MASS OF OBJECT MORE MASS = LESS ACCELERATION Acceleration is inversely related to mass
Newton’s Second 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.” Unit of force is theNEWTON (N)
a a F m Relationships: a ~ F; a ~ 1/m
If mass is held constant, • doubling of the net force results in … • a doubling of the acceleration, • halving of the net force results in … • a halving of the acceleration. • If force is held constant, • doubling of the mass results in … • a halving of the acceleration • halving of the mass results in … • a doubling of the acceleration.
Example A 2 kilogram box is pushed with a net, unbalanced force of 10 newtons. What is the acceleration experienced by the box? a = Fnet / m a = (10 N) / (2 kg) a = 5 m/s2
The Big Misconception • The most common misconception is one that dates back for ages; it is the idea that sustaining motion requires a continued force. • Newton's laws declare loudly that a net force (an unbalanced force) causes an acceleration;
Are You Infected with the Misconception? • Two students discussing an object that is being acted upon by two individual forces as shown. During the discussion, Anna Litical suggests to Noah Formula that the object under discussion could be moving. • Noah Formula objects, arguing that the object could not have any horizontal motion if there are only vertical forces acting upon it. • Who do you agree with?
Friction A force that causes surfaces to stick together and opposes motion. Ways to minimize friction SMOOTH SURFACES LUBRICATION At the MICROSCOPIC level, most surfaces are very BUMPY and IRREGULAR
Coefficient of Friction (μ) How much materials STICK TOGETHER DIMENSIONLESS (no units) The greater the coefficient, the greater the tendency to STICK TOGETHER The coefficient is lowered if surfaces are SLIDING past each other
Friction Force Static Friction STATIONARY OBJECTS – cancels out applied force - KEEPS OBJECTS IN PLACE CAN CHANGE – increases as the applied force increases until it reaches the maximum quantity for that specific surface. ROLLING OBJECTS Kinetic Friction SLIDING OBJECTS OPPOSES MOTION
Calculating Friction Force Amount of friction depends on: Coefficient of friction Static – the object is motionless, rolling, or pushing off from a surface Kinetic – the object is sliding across a surface Normal Force Greater normal force HIGHER friction force
Kinetic versus Static Friction • Static friction results when the surfaces of two objects are at rest relative to one another and a force exists on one of the objects to set it into motion relative to the other object. • The static friction force balances the force that you exert on the box such that the stationary box remains at rest. Ffrict-static≤ μfrict-static• Fnorm • kinetic friction results when an object moves across a surface. Ffrict = μ • Fnorm • The symbol μ represents the coefficient of kinetic friction between the two surfaces. The coefficient value is dependent primarily upon the nature of the surfaces that are in contact with each other. It does not depends on area of contact, the angle of the area, or the temperature, etc.
Finding the unknowns • Fnet is the vector sum of all the individual forces. The three major equations that will be useful are • Fnet = m•a, • Fg = m•g, • Ff = μ•FN
Example #1 A man pushes a 50 kilogram crate across a frictionless surface with a constant force of 100 Newtons. FN FA Fg What is the normal force that pushes on the crate? Draw a free-body diagram of the crate. What is the net force on the crate? What is the crate’s acceleration? What is the weight of the crate? Fnet will only be the 100N horizontal force Fg = mg Fg = (50 kg)(9.81 m/s2) Fg = 490.5 N FN = Fg FN = 490.5 N a = Fnet / m a = (100 N) / (50 kg) a = 2 m/s2
Example #2 A horse pulls a 500 kilogram sled with a constant force of 3,000 Newtons. The force of friction between the sled and the ground is 500 Newtons. FN Ff FA Fg What is the normal force that pushes on the sled? Draw a free-body diagram of the sled. What is the net force on the sled? What is the sled’s acceleration? What is the weight of the sled? Fnet = ΣFx Fnet = 3000 N – 500 N Fnet = 2500 N Fg = mg Fg = (500 kg)(9.81 m/s2) Fg = 4905 N FN = Fg FN = 4905 N a = Fnet / m a = (2500 N) / (500 kg) a = 5 m/s2
the object is moving horizontally. Use the diagram to determine the normal force, the net force, the mass, and the acceleration of the object. Example #3 80 N 8 kg 5 m/s2 right 40 N right
Example #4 • Edwardo applies a 4.25-N rightward force to a 0.765-kg book to accelerate it across a tabletop. The coefficient of friction between the book and the tabletop is 0.410. Determine the acceleration of the book.
Example #5 • Lee Mealone is sledding with his friends when he becomes disgruntled by one of his friend's comments. He exerts a rightward force of 9.13 N on his 4.68-kg sled to accelerate it across the snow. If the acceleration of the sled is 0.815 m/s/s, then what is the coefficient of friction between the sled and the snow?
Free Fall and Air Resistance Falling with air resistance • As an object falls through air, it usually encounters some degree of air resistance - the result of collisions of the object's leading surface with air molecules. • The two most common factors that have a direct affect upon the amount of air resistance are • the speed of the object: Increased speeds result in an increased amount of air resistance. • the cross-sectional area of the object: Increased cross-sectional areas result in an increased amount of air resistance. Free Fall • Objects that are said to be undergoing free fall, are • not encountering air resistance; • falling under the sole influence of gravity. All objects will fall with the same rate of acceleration, regardless of their mass. This is due to that the acceleration is The ratio of force to mass (Fnet/m)
Falling with air resistance – terminal velocity • As an object falls, it picks up speed. The increase in speed leads to an increase in the amount of air resistance. Eventually, the force of air resistance becomes large enough to balances the force of gravity. At this instant in time, the net force is 0 Newton; the object will stop accelerating. The object is said to have reached a terminal velocity.
Newton's Third Law • For every action, there is an equal and opposite reaction. • Forces always come in pairs - equal and opposite action-reaction force pairs. • Examples: • The propulsion of a fish through the water. • The flying motion of birds. • The motion of a car on the way to school.
Third Law Examples A firefighter directs a stream of water from a hose to the east. In what direction is the force on the hose? A man getting out of a rowboat jumps north onto the dock. What happens to the boat? There will be a force on the hose to the WEST The boat will move to the SOUTH
Identifying Action and Reaction Force Pairs • Identifying and describing action-reaction force pairs is a simple matter of identifying the two interacting objects and making two statements describing who is pushing on whom and in what direction.