Mechanics Kinematics Dynamics • Describes motion. • The forces that cause motion.
Forces Elementary Definition: Force – a push or a pull Not totally accurate though.
Forces True Physics Definition: Force – any action that can cause a change of motion in an object
Forces • Vector quantity – Forces have magnitude and direction • May be a contact force or a field force • Contact forces result from physical contact between two objects • Field forces act between objects that are not in physical contact
Type of Forces Contact Forces Field Forces • Elastic Force (Felas) • Frictional Force (Ffric) • Applied Force (FA) • Tensional Force (Ftens) • Normal Force (FN ) • Strong Nuclear Force • Electromagnetic Force • Gravitational Force (Fg) • Electric Force (Felec) • Magnetic Force (Fmag)
Contact Forces Physical contact must be present during the exertion of the force. Elastic Force (Felas)– due to stretching or compression of an elastic object (rubber band, springs, basketball bouncing) Tensional Force (Ftens) - due to objects pulled by a rope or cable-like object Frictional Force (FFr) – any force that opposes motion due to two types of matter coming in contact (air resistance, cleats on a grass field)
Contact Forces (cont.) Physical contact must be present during the exertion of the force. Applied Force (FA ) – generic term for any other type of force (boxer strikes another fighter, wrecking ball hits the building) Normal Force (FN ) – force that acts perpendicularly when an object is on a surface
Field Forces From strongest to weakest … Strong nuclear force – holds particles in nucleus together Electromagnetic force – electric and magnetic fields. Holds particles together so that they can bend, stretch or shatter. Gravitational – attractive force that exists between all objects
Which of the following is a field force? • Normal • Frictional • Tensional • Electromagnetic • Applied
Which of the following is a contact force? • Strong Nuclear • Magnetic • Frictional • Gravitational • Electromagnetic
What type of force always opposes motion? • Normal • Applied • Frictional • Tensional • Gravitational
What type of force takes the sum of all the forces present? • Normal • Net • Frictional • Tensional • Summation
What type of force is always due to pulling and never due to pushing? • Normal • Magnetic • Frictional • Tensional • Applied
What type of force is present when an object lies on a surface? • Normal • Electromagnetic • Frictional • Tensional • Applied
Free Body Diagram • Must identify all the forces acting on the object of interest. • Choose an appropriate coordinate system. • If the free body diagram is incorrect, the solution will likely be incorrect.
Free Body Diagram – rope pulling a box • The force is the tension acting on the box • The tension is the same at all points along the rope • are the forces exerted by the earth and the ground
Newton – Dead but still somehow important… • 1642 – 1727 • Formulated basic concepts and laws of mechanics • Universal Gravitation • Calculus • Light and optics
Newton’s First Law (Law of Inertia)– An object moves with a constant velocity unless acted upon by a net force (nonzero). Net Force – vector sum of all external forces acting on an object
Newton’s First Law • Also referred to as the Law of Inertia • Inertia – the tendency of an object to remain in its present state (at rest or in motion at constant velocity)
Newton’s Second Law • The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.
Units of Force • SI unit of force is a Newton (N) • US Customary unit of force is a pound (lb) • 1 N = 0.225 lb
Weight Weight is considered a force Fg = m x 9.8 because a = 9.8 m/s2 Object has weight whether it is falling, resting, or being lifted. Gravity is considered a constant This does not mean that our weight is constant, however.
Some Notes About Forces • Forces cause changes in motion • Motion can occur in the absence of forces (if they are already in motion) • All the forces acting on an object are added as vectors to find the net force acting on the object • m is not a force itself • Newton’s Second Law is a vector equation
Newton’s Third Law – when one object exerts a force on a second object, the second object exerts a force on the first that is equal in magnitude, but opposite in direction Forces always occur in pairs. (action / reaction pairs) (ex: push on a door, we feel door push back)
Action Reaction pairs Action-reaction forces – equal in magnitude, opposite in direction. Also, action-reaction force pairs ALWAYS act on DIFFERENT objects. A single isolated force cannot exist.
Friction Where in real-life do we try to reduce weight to reduce required force?
Friction – the force that opposes the motion between two surfaces that are in contact.
Two Main Types of Friction There are two main kinds of friction: static friction – the force that opposes the start of motion sliding or kinetic friction – the force between surfaces while in motion (Also rolling friction – but we won’t deal with this one.)
To calculate force of friction (FF), use the equation: FFr = μFN or μ = FFr/F N μ = coefficient of friction, constant for any two types of matter μ = ratio of the frictional force to the normal force Frictional force is directly proportional to the μ.
Ffris always parallel to the surface and opposite the motion of the object. FN is force normal, or force perpendicular to surface. μ represents coefficient of friction, and changes with surface
Air Resistance Air resistance is the net force of the air molecules striking a moving object - another source of friction - without air resistance (like in a vacuum, all objects would fall unimpeded at the acceleration of gravity.
When force of air resistance equals the force of gravity, terminal velocity is reached. - velocity becomes constant For example, terminal velocities: ping pong ball – 6 m/s skydiver - 60 m/s skydiver w/chute – 5 m/s
How does a parachute work? How else could a diver change his/her terminal velocity?