Forces and Newton’s Laws of Motion

Force… A push or a pull. Force is not a thing in itself, but rather an interaction between two objects.

Unit of Force The metric unit of force is the Newton (N) 1 Newton = 1 kilogram of mass being accelerated at 1 m/s2 Fnet = ma

Examples A Flag being blown by the wind A jet engine propelling an airplane

Forces Push or a pull Vector quantity Contact forces- arise from physical contact with each other Field (Noncontact) forces- force exerted even when not in direct contact (ex. Gravity, electrical force, magnetic forces)

Weight VS Mass http://www.pioneernet.net/curtis/wile_e/inline/wile_gravity.gif

Mass the amount of matter in an object a constant property a measure of the inertia measured in kilograms (kg)

Weight the force upon an object due to gravity measured in Newtons (N) weight = mass  accel. due to gravity Fw = mg measured in Newtons (N)

Practice  The weight of a 10 kg brick is... A) 98 N B) 10 kg C) 9.8 kg D) 10 N E) 98 kg

Mass and Weight On the Moon, the force of gravity is only 1/6 as strong as on the Earth. In space you are practically weightless but your mass remains unchanged. Your mass does not depend on where your are. e.g. Earth, Moon, or space

Normal Force Normal Force (Fn)- contact force that always acts perpendicular to the surfaces that are touching

Tension Tension (T)- force that acts through a solid object such as a rope or chain; directed along the rope and pulls equally on the objects on either end. .

Net Force Determined by combining ALL forces acting on an object. Balanced = No Net Force = Fnet = 0 N Determined by combining ALL forces acting on an object. Zero net force = zero acceleration If there is a net force, there will be an acceleration. Unbalanced = Net Force = Fnet ≠ 0 N http://4.bp.blogspot.com/_fDHmeCjB-bk/SS4hRNbLwaI/AAAAAAAAAHw/gx0p9HIDS1o/s1600-h/12.gif

Free-Body Diagram Free-body diagrams are drawings used to show the relative magnitude and direction of all forces acting upon an object in a given situation.

Basic Force Diagrams A Car on a Level Surface All forces on the car are vertical, so no horizontal force can be generated. A Car on a Banked Turn The normal force on the car due to the road is no longer vertical, so a component of the normal force acts in the horizontal direction. Normal force (N) - contact pressure that pushes surfaces together. It is easier to drag a light chair across a room than a heavy table. This is because the weight of the table exerts a higher normal force.

Fee Body Diagram (FBD) The next five slides include examples of FBDs Review each slide Make a list of any questions you have about the slides… be sure to sketch the diagrams in your notes 

A book is at rest on a table

A girl is suspended motionless from a bar which hangs from the ceiling by two ropes.

An egg is free-falling from a nest in a tree. Neglect air resistance.

A flying squirrel is gliding (no wing flaps) from a tree to the ground at constant velocity. Consider air resistance.

A rightward force is applied to a book in order to move it across a desk with a rightward acceleration. Consider frictional forces. Neglect air resistance.

Real Life Application

Balanced Forces (zero net)

Unbalanced Forces (non-zero net) Imagine pushing your text book so that it would slide across the desk…

Equilibrium Balance In regards to motion an object has constant velocity if it is in equilibrium. NO acceleration. Types: Static Equilibrium Dynamic Equilibrium

Equilibrium- net force is equal to zero

Friction The force that opposes the motion between two surfaces that are in contact. Friction is the "evil monster" of all motion. Regardless of which direction something moves in, friction pulls it the other way. Move something left, friction pulls right. Move something up, friction pulls down. It appears as if nature has given us friction to stop us from moving anything. Friction is actually a force that “appears” when there is relative motion between two objects. Although two objects might look smooth, microscopically, they are very rough and jagged.

Friction A force that opposes motion. Friction acts in a direction opposite to the object's direction in motion. Without friction, the object would continue to move at a constant speed forever Static Friction (Fs): when object is at rest Kinetic Friction (Fk): when object is moving Sliding Friction: when two surfaces slide one over the other Rolling Friction: when two surfaces slide one over the other

Static Equilibrium Velocity is zero Examples: Scales pushing up Weighing yourself on a set of scales Weight down Car parked on an incline Normal Friction Weight down

Dynamic Equilibrium Velocity is nonzero and constant Examples: Driving at constant velocity Normal up Friction Force from road Air resistance Weight down Terminal velocity when parachuting Weight down

Kinetic (sliding) Friction The force between surfaces in relative motion For the same object, why is the force of kinetic friction less than the force of starting friction?

Static (starting) Friction The force that opposes the start of the motion. Static means stationary ( not moving).

Science Friction Friction (Ff)- contact force between two surfaces that always opposes motion 2 types Static friction (Fs) = keeps an object from moving (must be overcome to move an object at rest Kinetic friction (Fk) =acts when an object is moving Kinetic friction is weaker than static friction Fk< Fs

Friction is not always a bad thing! Walking! Your foot pushes backwards and friction opposes it by pushing it forward

Coefficient of Friction A constant that depends on the two surfaces in contact Ff = FN (mu)….coefficient of friction Coefficient of Friction - Friction on an Incline

Acceleration << g Terminal Velocity Net Force Acceleration = g Velocity = 0 but motion is about to begin mg F Acceleration < g v increasing downward mg F Acceleration << g v still increasing downward just not as rapidly as before mg F Acceleration = 0 Terminal velocity mg

Falling with Air Resistance & Terminal Velocity When falling the force of air resistance becomes large enough to balance the force of gravity. At this instant in time, there is no net force — the object stops accelerating (see D below); terminal velocity has been reached.

More real life… http://www.ux1.eiu.edu/~cfadd/1150/04Nwtn/appl.html