 # Force Force is a push or pull on an object The object is called the System Force on a system in motion causes change in velocity = acceleration Force is.

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Force Force is a push or pull on an object The object is called the System Force on a system in motion causes change in velocity = acceleration Force is a vector, it has direction External World = everything around the system that exerts a force

Types of Forces Forces Contact When the external world is in contact with the system and exerts a force ex. Hand that pushes, Rope that pulls Field Forces A force exerted on a system without touching the system ex. Gravity, Magnetic force, Interaction between charged particles

Agent An agent is the item in the external world that causes the force to act on the system ex. The hand that holds The rope that pulls The earth The magnet

Free Body Diagram A diagram representing the System, Agent and the Forces acting on the system System is represented by a dot Forces represented by arrows pointing in the direction of the force, away from system Label each force with a subscript Choose direction of +ve usually towards stronger force

Ft gF Ball String (force of tension) (force of gravity) Positive Direction

Combining Forces As forces are vectors they can be added just as vectors ex.1 Fa + Fb = F net ex.2 Fa + Fb = Fnet Nonlinear non perpendicular forces are added by adding x and y components of the force vectors

Newton’s First Law An object at rest will remain at rest and an object in motion will remain in motion unless acted on by an external net force Also known as the Law of Inertia Inertia is the tendency of an object to resist change in motion Equilibrium = if the net force on a system is zero the system is in equilibrium speed and direction is unchanged.

Common Types of Forces Friction F f = Contact force opposing motion of two surfaces, parallel to the surface and opposite to the direction of motion Normal F N = Contact force exerted by system’s surface perpendicular to and away from surface Spring F sp = A restoring force- push or pull of spring exerted on system opposite to displacement

Tension F T = pull exerted by a rope on a system, away from the system and parallel to the rope Thrust F THRUST = a force that moves a system in the same direction as the acceleration Weight F g = a field force on a system due to gravity directed to the center of the earth

Newton’s Second Law The acceleration of an object is equal to the net force acting on it divided by the mass of the object a = F/m or F = ma The larger the force the greater the acceleration The greater the mass with the same force the lower the acceleration Force of 1N = 1kg * 1m/s.s

Newton’s Third Law All forces come in pairs equal and opposite in direction ex. A ball on a table and the table on the earth. Forces-ball on table / table on ball -table on earth/ earth on table - ball on earth/ earth on ball

Net free body diagram ball table earth F table on ball F earth on ball

Drag Force and Terminal Velocity Drag Force is the force exerted on an object as it moves the a fluid ex. Air and water as the speed of the object through the fluid increases so does the drag Drag is effected by the fluids viscosity and temperature Terminal velocity is when the drag force equals the force due to gravity no acceleration constant velocity app.(60m/s)

Tension Is a force exerted by a string or rope on a system It is assumed the rope has no mass Tension within all points of the rope is equal and opposite to the force exerted by the system’s weight

Example of bucket on rope FTFT F g =mg

Normal Force Is a contact force exerted by a surface on an object. This force is perpendicular to the surface of contact FgFg N

Gravitational Force Gravitational force is the mutual attraction between any two bodies in the universe Newton’s Law of Universal Gravitation =every particle in the universe attracts every other particle with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them F g = Gm 1 m 2 /r 2 where G = the universal gravitational constant = 6.67*10 -11 N.m 2 /kg 2

Gravity As weight = mg then w = GM E m/r 2 Then g = GM E /r 2 Therefore the larger the mass of an object the larger the effective gravity it generates

Forces of Friction f s and f k An object moving on a surface or passing through a fluid experiences resistance to motion = friction F f s = forces of static friction = the force that prevents movement of an object that is being subjected to an external force When movement is about to occur f s is at max.

f s and f k When a force F exceeds F f s max then movement will occur and the new friction force is less. This new friction force is called the Force of kinetic friction f k When F-f k = positive value there is acceleration Both f s and f k are proportional to the normal force; f s =u s n and f k =u k n u s is the coefficient of static friction U k is the coefficient of kinetic friction

Solving Friction Problems Draw a free body diagram making sure to label all forces

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